Download Stem and Root Anatomy and Functions. Vegetative Propagation

Stem and Root Anatomy and Functions. Vegetative Propagation
What are root's functions?
The three universal functions of all roots are anchorage, absorption and translocation of water with
dissolved mineral nutrients. In many perennial and biennial species, roots are also sites for food
storage. These food reserves keep the plant alive through the non-growing season, and are used to
resume growth in spring or after cutting or grazing. Some species that store food in their roots are
yams, alfalfa and red clover. Food storage organs of some vegetables (carrots, beets, and radishes) are
actually a combination of root and stem tissues.
Types of root systems
There are two major types of root systems: fibrous and taproot (left). Grasses have fibrous
root system. Their roots are adventitious, arising from the lowest nodes of the stems.
Species with a fibrous system are more shallowly rooted than plants with a persistent
taproot.
Most dicots have a taproot system. The taproot originates from the primary root (radicle)
of the seed. The taproot may have many branches originating from it. Roots of legumes
may also have root nodules, which are sites for nitrogen fixation .
Zones of the root
A root can be divided into the mature zone, zone of maturation, zone of cell elongation,
and the zone of cell division (the apical meristem) protected by the root cap (right).
All of the root cells originate from the divisions of the cells of the apical meristem. These
cells are small, thin-walled, and contain large nuclei. Root meristem is protected by a
root cap. The root cap is a dynamic, multifunctioning organ. For many years it was
believed that the root cap functioned solely to protect the apical meristem of the root.
Recently, it was shown that the cells of root cap percieve both light and gravity. Root
caps of both dicots and monocots produce large numbers of metabolically active root
"border" cells, which are programmed to separate from the root into the surrounding soil.
In soil, border cells play important roles in protecting the roots from the soil-borne
diseases (Hawes et al, 1998).
What are the root tissues?
The primary root tissues are the epidermis, the outermost layer of cells covering the root surface, the
cortex that surrounds the stele, and the vascular tissue or stele, which occupies a central position.
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Stem and Root Anatomy and Functions. Vegetative Propagation
The root epidermis (1 on the cross-sections below) is usually a single cell layer that
protects the root. The cells of epidermis can elongate to produce root hairs. These root
hairs have larger surface area and are more efficient in absorbing water. Root hairs are
also the sites of Rhizobium invasion of the legumes.
Right: scanning electron micrograph of soybean root hairs.
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Legume-Rhizobium Symbiosis
Why are legumes important?
Each year legume-Rhizobium symbiosis generates more useful nitrogen for plants than all the nitrogen
fertilizers produced industrially -- and the symbiosis provides just the right amounts of nitrogen at the
right time at virtually no cost to the farmer. This symbiotic nitrogen fixation is very beneficial for two
reasons:
● it supplies the legume with nitrogen,
● it can significantly decrease spending on N-containing fertilizers for the subsequent crops.
Symbiosis is defined as a mutually beneficial relationship between two organisms.
In case of legume Rhizobium symbiosis, a legume provides the bacteria with
energy-rich carbohydrates and some other compounds, while Rhizobium supplies the
host legume with nitrogen in the form of ammonia. Unlike any plant, rhizobia (and
some other microorganisms) can fix inert N2 gas from the atmosphere and supply it to
the plant as NH4+ which can be utilized by the plant. Compare images on the left: a
soybean plant inoculated with Bradyrhizobium japonicum (left), and a plant that
wasn't (right). Un-inoculated plant shows signs of Nitrogen deficiency.
Adding nitrogen fertilizer, on the other hand, suppresses N2 fixing symbiosis because the plants
encounter enough nitrogen in the soil and don't need to expand energy to form the nodules and "feed"
rhizobia inside the nodules.
Let's briefly review the sequence of events leading to establishing a successful symbiosis.
Rhizobial inoculum is usually added at planting as seed coating. Commercial
formulations of inoculum, like the one we will use in today's lab, contain live bacteria.
On the right is a scanning electron microscope image of the free-living cells of
Bradyrhizobium japonicum which can form symbiosis with soybeans. You'll notice that
the bacterial cells have flagella, thread-like organs that allow bacteria to swim and
move in soils toward the host plants.
Roots of legumes produce flavonoids, - chemicals that attract rhizobia.
Different legumes produce different flavonoids to attract different
rhizobia.
On the left is a scanning electron microscop image of root hairs on
soybean roots. Root hairs are extentsions of the root epidermal cells, they
are the sites of rhizobial attachment and infection. When a plant senses
Nod-factors (chemicals produced by rhizobia), a root hair curls (right).
Rhizobium then invades the root cells.
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Legume-Rhizobium Symbiosis
Inside the root, rhizobia invade expanded cells of cortex, and then bacteria differentiate
into Nitrogen-fixing "bacteroids". On the left is microscopic picture of dissected
nodules on the root of a cow pea. The effectiveness of a given nodule may be checked
by cutting it open: an effective nodule should be pink (or purple) in color, while
immature or ineffective ones are either green or white inside.
Rhizobia inside the nodules, differentiated into "bacteroids", fix inert atmospheric N2
for the plants, and supply it in the water-soluble form for the plants.
There are 12,000-14,000 species in the Legume family (alfalfa, clovers, soybean, lupin, vetch, and
many other crops ). One should remember, though, that only certain species of Rhizobia can form
effective symbiotic nodules with specific legumes. In other words, Rhizobia used to inoculate peas will
not be effective in inoculating soybeans or alfalfa.
What is the Nitrogen Cycle?
The Nitrogen Cycle is a microorganism-aided recycling of different forms of nitrogen in nature. Let's
briefly review these biochemical conversions.
Nitrogen gas (N2) is the most abundant gas in the atmosphere. However, it is inert and cannot be
readily used by plants or animals. Symbiotic and non-symbiotic microorganisms have the ability to fix
N2 and convert it into NH4+, a form that can be easily absorbed by plants. Nitrogen can also be fixed
by industrial N2-fixation which requires high temperatures and catalysts.
This "fixed" nitrogen, now in the soluble form, when applied to soil can be either absorbed by plants,
lost with rainfall (leaching) or converted back to gaseous oxides of nitrogen or to N2 (denitrification).
Ammonia (NH4+) and nitrate (NO3-) are the nitrogen forms that can be readily taken up by plants and
used to build plants' own biological molecules (DNA, proteins, chlorophyll, vitamins, etc.). Animals
and humans can thereby utilize plants as sources of nitrogen-containing protein and vitamins.
As the plants and other soil inhabitants die, soil microbes break down decomposing organic matter
and convert the nitrogen from the biological molecules into ammonia and nitrates. Some denitrifying
microbes can sequentially convert various forms of reduced nitrogen back to gaseous forms, and
nitrogen is therefore lost into the atmosphere.
The sequence of events briefly discussed is usually called Nitrogen cycle. This is the way Nitrogen
(and many other nutrients) cycles in nature.
Your group will have a choice of doing either Experiment A or Experiemnt B. Read below for
instructions.
Experimental Design for Experiment A.
(Legume-Rhizobium Symbiosis)
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Legume-Rhizobium Symbiosis
The hypothesis to be tested in this experiment is:
Under the greenhouse conditions, Rhizobia can supply plants with sufficient available Nitrogen, and
this will result in higher yield of green mass and higher chlorophyll content as compared with
uninoculated plants.
●
●
The dependent variables in this experiment are yield (fresh weight, and number of seeds), and
chlorophyll content.
The independent variable (the variable that elicits the response) is the inoculation (infection) of
the plants with Bradyrhizobium japonicum.
We will use several controls in this experiment:
1) No nitrogen fertilizer, no inoculum. This treatment should have no effect on yield.
2) The application of urea CO(NH2)2 fertilizer (1/8 of a teaspoon per pot every other week) should
cause plants to yield more green mass and have higher chlorophyll content as compared to "No urea, No
inoculum" control.
To randomize the treatments, place your pots in random order on the bench of the greenhouse
(nevertheless, keep +INOCULUM treatments away from the other treatments to avoid contamination
with Bradyrhizobium japonicum). Treatments set up by other teams will serve as replications. At the
end of the experiment, we will compare the data obtained by different teams.
Protocol A. (Legume-Rhizobium Symbiosis)
1. Label each pot with the treatment, date, and your team number. Fill the pots with soil.
2. Apply 1/8 teaspoon of a fertilizer (0-26-26) to all pots.
3. Place five seeds of each species on the soil surface in the appropriate pots.
4. Moisten (do not saturate) the soil. Cover the seeds with soil except for +INOCULUM treatment.
5. Add 1/8 of a teaspoon of nitrogen fertilizer urea (46-0-0) to +UREA treatments.
6. Designate one person to inoculate +INOCULUM treatments.
Inoculator: Bring your +INOCULUM pots to the inoculation area. Take a pinch of dry inoculum
and sprinkle a little onto each seed. Then cover the seeds with soil. Wash your hands with soap
immediately.
Inoculum is safe to work with, but you MUST NOT allow it contaminate all of your treatments.
Inoculate your "+Inoculum" treatments last. Inoculate in the designated area only.
7. Place +INOCULUM pots on a separate bench in the greenhouse.
Observations and Data Collection
Measure heights and chlorophyll contents of each of your treatments according to the class calendar.
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Once you have collected the data, you will need to calculate variance and standard deviation. Every
team member should record all data.
1. When the seedlings appear, thin the plants to two per pot. Keep the largest healthiest looking plants.
2. Add 1/8 teaspoon of urea to the +UREA treatments every other week.
use only marked meter sticks to measure heights of the +INOCULUM treatments to avoid
contamination of the "No inoculum" treatments with Bradyrhizobium japonicum. Contamination of
other treatments with inoculum will make the collected data useless!
3. To measure height of soybeans, measure the distance from the soil surface to the apical meristem (the
topmost bud) of the plant.
4. Measure the chlorophyll content of the first true leaf and the newest fully developed leaf of all your
plants with the Minolta SPAD meter. In soybeans, the unifoliate leaf (not the cotyledon) is the first true
leaf.
At harvest (at the end of the quarter):
1. Carefully uproot the plants from their pots. Shake the roots. Rinse the soil from the roots. Briefly let
the excess water dry off the plants by placing them for a moment on a dry paper towel.
2. Measure: the fresh mass of the plants, number of branches per plant, the number of pods per plant.
3. Calculate average fresh weight of above ground parts of the plants from each treatment, variability
(s2) and standard deviation (SD). Complete the Data Sheets.
Experimental Design for Experiment B (Fertilizer Trial)
The hypothesis to be tested in this experiment is:
Under greenhouse conditions, vermicompost can supply adequate nutrition to plants and will result in a
similar yield of green mass and chlorophyll content as compared to those plants receiving traditional
garden fertilizer (12-12-12).
Your group can decide which plant you prefer to use. You will be using seedlings of either sorghum, or
sunflower.
●
●
The dependent variables in this experiment are yield (fresh weight, height) and chlorophyll
content
The independent (the variable that elecits the response) is the fertilizer treatment (vermicompost
or (12-12-12)-traditional garden fertilizer)
The treatments and controls include:
1. + vermicompost (15% of total volume)-treatment D
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2. + traditional fertilizer (12-12-12)-treatment E
3. control- no fertilizer- treatment F
. There will be three replications of each treatment for a total of nine pots
Protocol B. (Fertilizer Trials)
1. Label each pot with the treatment, replication, date, and your team number. There will be a total of
nine pots for each group (i.e. D1, D2, D3; E1, E2, E3; and F1, F2, F3.)
2. For the pots labeled D1, D2, and D3, mix in 15% vermicompost with the soil provided. Your group
will need to figure out the volume of the pots first before adding the appropriate amount of
vermicompost. Use your hands to mix thoroughly
3. Fill the remaining pots (treatments E and F) with the soil provided. Do not add vermicompost to these
treatments.
4. Moisten each pot with water, do not saturate the soil.
5. Transplant one seedlings of the plant that your group chose to work with into each pot of all
treatments. Be GENTLE and careful not do break the root system while transplanting.
6. Add the traditional garden fertilizer to those pots labeled E (ask your instructor about the correct
application rate).
7. Do not add anything to those pots labeled F.
8. Place in random order on the bench of greenhouse.
9. Fertilize your E treatments every week until the end of the quarter
Observations and Data Collection
Measure heights and chlorophyll contents of each of your treatments according to the class calendar.
Once you have collected the data, you will need to calculate variability (s2) and standard deviation
(SD).. Every team member should record all data.
●
To measure height of sunflower, measure the distance from the soil surface to the apical meristem
(the topmost bud) of the plant. To measure height of sorghum, measure the distance from the soil
surface to the end of the longest leaf blade.
●
Measure the chlorophyll content of the first true leaf and the newest fully developed leaf of all
your plants with the Minolta SPAD meter. In soybeans, the unifoliate leaf (not the cotyledon) is
the first true leaf.
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At harvest (at the end of the quarter):
1. Carefully uproot the plants from their pots. Shake the roots. Rinse the soil from the roots. Briefly let
the excess water dry off the plants by placing them for a moment on a dry paper towel.
2. Measure: the fresh mass of the plants, number of branches/nodes per plant,
3. Calculate average fresh weight of above ground parts of the plants from each treatment, variability
(s2) and standard deviation (SD) Complete the Data Sheets.
Guidelines for writing the Lab Report
Total 100 points
For Experiment A. (Legume-Rhizobium)
Your groups may chose to write either a report on Nutrient Deficiencies or on Legume-Rhizobium
symbiosis. This is a group report, contributions of every team member will be evaluated by peers. The
report should be typed, double-spaced and should contain the following sections:
Introduction This section is usually 2-3 paragraphs long. It introduces the topic and provides
background information on why the study was undertaken. Make sure you include objectives and
hypotheses. Clearly define what symbiosis is and discuss the importance of legume-Rhizobium
symbiosis in nature and agriculture (15 points).
Materials and Methods Briefly (in one paragraph) summarize the protocol you followed. What tools
did you use for measurements? Explain, why Bradyrhizobium japonicum, and not another Rhizobium
species was used in this experiment (5 points).
Results and Discussion This section is the "heart" of any report. It should be the longest part (2-3 pages
+ figures) of your report. Present data from the experiment in tables or graphs to support your
conclusions. Title your figures. Titles are usually put at the top of tables and the bottom of figures in
written documents. Refrain from using the laboratory data sheets to present data in your report (these
data sheets are only guides for collecting information and lack the appropriate organization for a report).
These questions will guide you in writing this section of the lab report.
1. Did you see any nodulation on the "NO INOCULUM" treatments? If yes, what happened? What did
they look like?(5 points)
2. Which treatment(s) developed plants with the highest chlorophyll content, the most branches and
pods, and highest mass? How variable were the results between replications What can you conclude
from these observations?(15 points)
3. Using the data you collected, discuss chlorophyll content in old and younger leaves of the treatments:
● Is there a difference in chlorophyll content between older and younger leaves? What can you
conclude from this finding? (10 points)
● Is there a difference in chlorophyll content between the treatment and the controls? What can you
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Legume-Rhizobium Symbiosis
●
attribute it to? (10 points)
Did you expect inoculation with Rhizobium to have an effect on the chlorophyll content? Explain
(5 points)
4. Do you think that chlorophyll content provide an accurate estimate of Nitrogen-status of the plant? (5
points).
5. Were there any confounding variables that might have interfered with the experiment? Would you set
up the experiment differently? (5 points).
6. Attach Xerox copies of the completed data tables (10 points).
8.Include clearly labeled Figures and Tables (10 points)
Literature Cited Include the list of the reference materials that you used to prepare your report. Cite only
the materials that you have actually read (5 points)
On a separate sheet of paper, evaluate contribution of each team member (including your
self-evaluation) to this project. Evaluate contributions as percentages, rather then letter grades, i.e. if
each member contributed equally, than each one gets 25%. Sign your name on the evaluation sheet.
Turn in the evaluation individually. These evaluations will be confidential and will not be returned.
For Experiment B. (Fertilizer Trials)
This is a group report, contributions of every team member will be evaluated by the peers. The report
should be typed, double-spaced and should contain the following sections: Use the following guidlines:
Introduction This section is usually 2-3 paragraphs long. It introduces the topic and provides
background information on why the study was undertaken. Make sure you include objectives and
hypotheses. Describe what is meant by inorganic and organic fertilizers. What is vermicompost and
how is it produced? Briefly describe findings of other studies which have incorporated the use of
vermicompost (15 points).
Materials and Methods Briefly (in one paragraph) summarize the protocol you followed. What tools
did you use for measurements? How did you fertilize your treatments? What was the experimental
design?(5 points).
Results and Discussion This section is the "heart" of any report. It should be the longest part (2-3 pages
+ figures) of your report. Present data from the experiment in tables or graphs to support your
conclusions. Title your figures. Titles are usually put at the top of tables and the bottom of figures in
written documents. Refrain from using the laboratory data sheets to present data in your report (these
data sheets are only guides for collecting information and lack the appropriate organization for a report).
These questions will guide you in writing this section of the lab report:
1. Which plants overall responded better to treatments? Which treatments developed plants with the
highest cholorophyll content? (10 points)
2. Is there a difference in growth responses between treatments/controls? How can you account for
these diferences?(10 points)
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Legume-Rhizobium Symbiosis
3. Are there signs of deficiecies in any of the treatments. If so, can you narrow them down to specific
nutrient deficiencies? Describe the symptoms. (10 points)
4. Do you think that chlorophyll content provides an accurate estimate of Nitrogen-status of the plant?
(5 points).
5. How variable were your results betwen treatments? Were there any confounding variables that might
have interfered with the experiment? Would you set up the experiment differently?(10 points)
6. In conclusion, which fertilizer out of the two would you recomend for other growers and why? Do
you think the application rate of either the vermicompost or traditional fertilizer was effective or should
a different recomendation be made? (10 points)
6. Attach Xerox copies of the completed data tables (10 points)
7.Include clearly labeled Figures and Tables (10 points)
Literature Cited Include the list of the reference materials that you used to prepare your report. Cite only
the materials that you have actually read (5 points)
On a separate sheet of paper, evaluate contribution of each team member (including your
self-evaluation) to this project. Evaluate contributions as percentages, rather then letter grades, i.e. if
each member contributed equally, than each one gets 25%. Sign your name on the evaluation sheet.
Turn in the evaluation individually. These evaluations will be confidential and will not be returned.
Guidelines for group Oral Presentations
presentations will be given on the last day of lab
(50 points)
Each group will be expected to make an oral presentation to their lab section that lasts no longer than
15-20 minutes including questions and discussion. During this presentation, the group should present an
introduction including objectives and hypothesis, materials and methods, results, and discussion. Visual
aids should be used. Data should be presented in a visual form and be explained thoroughly. The
discussion should include interpretations of data. If results did not comply with the original hypothesis,
other possible explanations need to be addressed. Your grade is not contingent on whether your results
complied with the hypothesis, but rather on the reasoning and explanations your group is able give to
support or reject the hypothesis. All members of each group are encouraged to participate in the oral
presentation, but the main presentation can be made by one or two persons as long as each has
contributed equally. Contributions of every team member will be evaluated by the peers and will be
incorporated into the final grade.
All materials on this website are for personal use only. Pictures, text or files cannot be legally
reproduced or duplicated in any form. For commercial or instructional use of this website or materials
from it, please contact Dr. P. McMahon or Max Teplitski.
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©Copyright by M.Teplitski and P.McMahon, 1999
For more information, email us at [email protected], [email protected].
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Nutrient Deficiency Symptoms
Law of Minimum
An important concept to remember is that one has to "feed" plants before the plants can
provide us with food. As you have learned in the previous exercises, plant "food"
consists of carbon dioxide and water (sources of C, H, O), and 16 elements (N, P, K, S,
Mg, Ca, Fe, Mn, B, Cu, Zn, Mo, Na, Ni, Si and Cl). The 16 elements should be present in
a water-soluble form so that a plant can take them up. The16 nutrients are divided into
primary nutrients (N, P, K), secondary nutrients (S, Ca, and Mg) and micronutrients (Fe,
Mn, B, Zn, Cu, Mo, Na, Ni, Si and Cl).
Even if only one nutrient is missing from the soil (or hydroponic) solution the plant will
not develop and produce normally. This notion was postulated by Justus von Liebig in
his Law of the Minimum. The Law of Minimum maintains that yield is proportional to
the amount of the most limiting growth resource. As you recall, such growth resources
are nutrients, light, temperature, water and space.
Justus von
Liebig
Deficiency Symptoms
The corn plant on the left is nitrogen-deficient. It developed deficiency symptoms which
include stunted growth, chlorosis (yellowing), and necrosis (death).
Nitrogen is part of a chlorophyll molecule (right, below). As you recall, chlorophyll is
the green pigment that plays an important part in photosynthesis. If nitrogen is limiting,
chlorophyll molecules cannot be synthesized. The plant loses its green color, and can't
photosynthesize As a result, the N-deficient plant does not produce required
carbohydrates. Older leaves develop deficiency symptoms earlier, because N is
translocated inside the plant from the older leaves to the younger ones.
Below are several examples of nutrient deficiencies. Some of these minerals are
involved in the formation of biologically active molecules, such as pigments
(chlorophyll, carotenoids, etc.), nucleic acids (DNA and RNA), energy molecules
(ATP, NADPH) and enzymes. All of these molecules have different important
functions within a plant cell. Nucleic acids, for example, carry an organism's genetic
information, ATP provides energy for the reactions within a cell, while enzymes
catalyze the reactions.
chrolophyll
Let's briefly talk about enzymes. An enzyme is a protein (sometimes RNA) that functions as a
biological catalyst.
●
Enzymes are encoded by genes. Sequence of DNA in the genes codes for a sequence of
aminoacids. Aminoacids are assembled together by ribosomes. When this amino acid chain is
released from a ribosome, interactions between aminoacids cause unique folding of the protein.
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This uniquely folded protein, sometimes associated with co-enzymes and metals, functions as a
biological catalyst.
● Enzymes are very selective in the substrates they act upon and in the kinds of the reactions they
catalyze. Rubisco, an enzyme involved in photosynthesis, catalyzes the conversion of
ribulose1,5-biphosphate to two molecules of 3-phosphoglycerate.
Considering how many biological reactions take place inside an organism (bacteria, plant or human),
you can only imagine how many different enzymes are present! The product of one enzymatic reaction
is usually a substrate for another enzyme. This sequence of enzymic reactions in an organism is known
as metabolism. If an enzyme (or any other important biological molecule) is not produced inside the
cell due to a mineral deficiency, then the biological reactions catalyzed by this enzyme do not take
place, the organism's metabolism is severely impaired, and deficiency symptoms develop.
Nitrogen (N) deficiency
N-deficiency is the most common nutrient deficiency. N is
Leaf of
a part of a chlorophyll molecule, aminoacids, proteins, and
N-deficient
many other important bioldogical molecules. Older leaves
corn (top);
of nitrogen- deficient plants are yellow from the tip
N-deficient
outside, plant is light green. Stalks of the N-deficient plants
barley leaves,
short and slender. Leaves drop.
healthy leaf
Excess N may cause K deficiencies. Potato, carrot, beet
on the
grown with excessive N, show prolific shoot growth with
bottom
small underground organs. Excess N leads to splitting of
tomato fruits as they ripen.
P-deficient
and healthy
lettuce
Phosporus (P) deficiency
Second to N, P is often the limiting element in soils. Older leaves of
P-deficient plants are purple or dark green. Stalks short and thin. New
growth is weak and stunted. Poor flowering and fruiting. Phosphorus is
important in nucleic acids, and in energy molecules (ATP, NADP).
Potassium (K) deficiency
Potassium is imporntant in many enzymes that are
essential for photosynthesis. Like N and P,
potassium is freely translocated inside the plant, so
the deficiency symptoms first occur on the older
leaves. Lack of potassium causes leaf margin
chlorosis, followed by necrosis from outside to the
midvein. K-deficient grasses are more prone to root
K-deficient
infections, and are easily bent to the ground
(lodged) by rain or wind. Researchers from U. of corn (above), K-deficient
Georgia suggest that K-deficient cotton plants are cucumber (right)
more susceptible to fungal infections. They suggest
split K applications (half at planting, half as
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Nutrient Deficiency Symptoms
side-dressing), and use foliar fertilization if the
deficiency occurs.
Calcium (Ca) deficiency
Calcium is often limited in acidic soils that recieve abundant rainfall.
When calcium is deficient, terminal bud dies, young leaves are hooked,
because Ca++ is not easily translocated inside the plant. Dying back
Ca-deficient occurs at tips and margins, foliage may become distorted. Stalk dies off at
the terminal bud. Root systems may be damaged by the root tip death.
tomato
Calcium is bound to enzymes, it also participates in cell wall formation.
Calcium is required for cell division and is required for normal membrane
functions.
Excess Ca may cause boron or magnesium deficiencies.
Sulfur (S) deficiency
Because enough sulfate is present in most soils,
sulfur deficiency is fairly uncommon. S is not
easily translolcated inside the plant, so
sulfur-deficient plants develop interveinal
chlorosis on younger leaves first. Necrotic spots
are usually not present.
Sulfur is essential for protein structure, it also
occurs in vitamins.
S-deficient
corn (right)
S-deficient
cotton plant
and healthy
plants (left)
Magnesium (Mg) deficiency
Mg is a part of the chlorophyll molecule, it is also important for
activating some enzymes. Plants lacking magnesium have leaves with
interveinal chlorosis. Leaves may redden, develop dead (necrotic) spots;
tips and margins sometimes cup upward. Stalks are usually slender.
Magnesium deficiency is rarely a problem in most soils. Excessive
magnesium, on the other hand, can induce potassium deficiency due to
interference with K uptake and utilization.
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Mg-deficient
cucumber
plants (right)
Nutrient Deficiency Symptoms
Iron (Fe) deficiency
Iron often becomes poorly soluble and therefore limited in soils with
neutral or basic pH. Fe-deficient plants develop interveinal chlorosis
occuring first on younger leaves. In severe cases, younger leaves
become white with necrotic lesions.
Iron is important because it is a part of some enzymes. Its ability to
undergo oxidations and reductions (Fe2+ <->Fe3+) is essential for
electron transport in many biochemical reactions inside the plant.
A leaf of
Fe-deficient
peanut plant
Deficiencies making front pages...
Here is how a recent journal PLANT PHYSIOLOGY desecribed its recent
cover(right): the interveinal chlorotic sunflower leaves shown in the photograph suffer
from Fe chlorosis. Fe chlorosis occurs mainly on calcareous soils with nitrate as the
exclusive N form, and leaves are frequently chlorotic in spite of abundant Fe
concentrations. Kosegarten et al. (pp. 1069-1079) have shown that pH of the
intercellular space ("apoplast") regulates Fe3+ reduction and thus Fe2+ transport
across the cell membrane. Microscope imaging combined with the fluorescence ratio
technique revealed high apoplastic pH at cellular sites in the interveinal area of young
leaves due to nitrate nutrition (see inset of the interveinal area). In the interveinal area,
Fe3+ reduction was depressed at sites of high apoplastic pH, thus inducing leaf
yellowing. In contrast, apoplastic pH in the xylem vessels (see related inset) was low
even with nitrate nutrition, and, due to high rates of Fe3+ reduction at low apoplastic
pH, the tissue around the leaf xylem remained green.
Deficiency symptoms could be sometimes confused with herbicide injuries. Refer to the following web
pages for an illustrated list of some herbicide injuries on common crops:
http://www.btny.purdue.edu/Extension/Weeds/HerbInj/InjuryHerb1.html
For more information on plant mineral nutrition and role of various nutrients, visit:
http://maine.maine.edu/~thomascb/nutri.html
Why do deficiency symptoms differ?
The deficiency symptoms for any nutrient depend on two factors:
● the role of the element in the plant;
● whether or not the element is translocated from older leaves to younger ones.
Ability of a nutrient to be translocated depends upon its mobility in the phloem. The mobility is
determined by solubility of the chemical form of the element. Symptoms vary somewhat between
species, and according to the severity of the problem, the growth stage, and complexities resulting from
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Nutrient Deficiency Symptoms
deficiencies of two or more elements.
Hydroponic production
The first hydroponic systems were developed in France and England during the 17th century.
Hydroponics is the technology of growing plants in a nutrient solution with or without the use of an
artificial medium (vermiculite, sand, gravel, etc.) to provide mechanical support. Hydroponic systems
are classified as liquid or aggregate, respectively. The vast majority of hydroponic systems are
enclosed in greenhouses to provide temperature monitoring, reduce evaporation, and to protect the
systems from unfavorable weather conditions.
Several hydroponic techniques have been developed in the recent years:
● Nutrient Film Technique. A thin film of nutrient solution is driven by gravity through
plastic-lined channels. The roots grow inside the channels and form a tangled mat.
● Floating hydroponics. Usually used to germinate seeds in beds floating on top of a nutrient
solution. Lettuce is grown in this manner in 2.5 cm-thick plastic floats for 4-6 weeks.
● Aeroponics. Plants are grown in holes of expanded polystyrene panels. Plant roots are suspended
in midair beneath the panel and enclosed in a spraying box. Aeroponics is valuable for the rooting
of stem cuttings and in the production of leafy vegetables. Space is used more efficiently in this
system.
● Aggregate hydroponics systems. A solid, inert medium provides support for the plants. As in
liquid systems, the nutrient solution is delivered directly to the plant roots.
Click here to obtain some practical advice on hydroponic production from Cornell scientists.
About the experimental setup
In this exercise we will use an aggregate/wick hydroponics system. Solid medium (vermiculite) will
provide support for the growing plants, nutrient solution will be driven into the medium by the capillary
action. You will replace the mineral solution every week to compensate for the removal of the nutrients
by the plants and pH changes resulting from this removal.
Mineral solutions were prepared based on the Hoagland Mineral Solution No2 for Hydroponic Culture.
The medium contains phosphates, which act as a buffer to prevent rapid pH changes in the solution.
Chelating agents are added to the solution to prevent ions (mostly divalent metals) from precipitating.
Click here to learn more about chelating agents and buffers.
You may choose from -N, -P, -K, -Ca, -S and control solutions. You may also decide to work with tall
fescue, lettuce, cucumber or a corn plant.
Protocol
1. Decide which crop and which deficiency your group would like to work with in this exercise.
2. Dilute the stock solution 5 times (i.e. 1 part of the stock per 4 parts of distilled water).
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Nutrient Deficiency Symptoms
3. Add the prepared mineral solution to the white bucket so that there is approximately 9 cm (3.5
inches) of liquid in the white bucket.
4. Place 2 sheets of cheesecloth inside the green pot. Pull the cheesecloth through the orifices in the
green pot, so that when the green pot is inserted into the white bucket, cheesecloth is immersed into the
mineral solution.
5. Fill the green pot with vermiculite. Wet vermiculite with the appropriate mineral solution.
6. Plant the seedling into vermiculite.
7. Place the green pot inside the white bucket with the mineral solution.
8. Clearly label the pot with your group number, date and treatment. Move the hydroponic assembly
into the designated part of the greenhouse.
All materials on this website are for personal use only. Pictures, text or files cannot be legally
reproduced or duplicated in any form. For commercial or instructional use of this website or materials
from it, please contact Dr. P. McMahon or Max Teplitski.
©Copyright by M.Teplitski and P.McMahon, 1999
For more information, email us at [email protected], [email protected].
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http://www.hcs.ohio-state.edu/hcs200/images/deficiencies/NDEFCORN.JPG
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Discussion questions
Preparing for the class, let's all think about the social aspects of plant biotechnology. Here are some
questions that have been igniting political debates recently:
1. Who owns the genes? Represenatives of Western companies travel to the developing countries to
collect seeds of the local crop varieties. Commercial breeders work with these varieties and eventually
protect them by their patents. Who do you think owns the rights to the crop varieties based on local
"landraces".
2. Should you keep the "designer" genes from the wild relatives? Cultivated grasses easily and
freely outbreed with their wild weedy relatives. Some breeders try to introduce herbicide-resistance
genes into turf grasses using techniques described in this lab. What effects, do you think, introduction of
herbicide-resistance genes into turf grasses will have on control of grass weeds?
3. As you'll learn in a couple of minutes, Russian botanist Nickolai Vavilov developed one of the first
theories of crop origin. In the beginning of the 20th century, his studies of genetics and crop evolution
clashed with the government's ideology. He was arrested and later died in Stalin's concentration camps.
In your opinion, can government, society or interest groups impose their ideals on scientists? Can
you think of other examples when different groups try to dictate their values to the scientists?
Frankenstein Foods or Crops for the Future?
Genetic engineering more and more often becomes a front-page news in popular
magazines. Crop breeders come up with new more productive crops that are
stress-tolerant, disease-resistant, have higher qulatity yields and other superior
traits. Corn and cotton plants were engineeried to carry genes of a bacterium
Bacillus thuringiensis allowing the plants to fight off insects. RoundUp Ready
soybeans are not destroyed by the herbicide which allows less expensive weed
control. High-starch potatoes have higher starch content in their tubers and
therefore are more nutritious. FlavrSavr tomatoes stay firm as they are stored and
transported. Tobacco plants can synthesize vaccines and biodegradable plastics.
Cotton, with genes from indigo plant, produce blue cotton fibers for natural,
environment friendly denim. A variety of "decaf" coffee has been developed to
produce naturally caffeine-free product.
Rice, genetically engineered to synthesize β-carotene, made a front-page in TIME
magazine (left).
Click on the image (left) to read the article in TIME.
According to a recent article in "Trends in Plant Science", scientists at Monsanto inserted a gene for
b-carotene production in canola plants. Oil from this new canola variety contains b-carotene, which
human body converts into vitamin A. One teaspoon of the oil could provide the daily recommended
intake for an adult. In the same journal, they report that a Spanish scientist, Jesus Fernandez, has
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developed a variety of artichoke that grows ~12 feet (3 meters) high. Artichokes grow well in the
infertile dry soils. The biomass of genetically modified artichoke is harvested and used for fuel. A
factory was built to use up to 105, 000 tons of artichokes to produce 91.2 GW of electricity.
Crops: where did they come from?
Crop domestication, a process of selection and adaptation of a wild species to cultivated
environments, started ~9,000 years ago. People grew barley, wheat, bean, flax, yam,
lentil, peas, and peppers as early as 7000-5000 b.c.e. Clover, forage grasses, oil palm,
sugar beet, and strawberry were domesticated relatively recently (1750 c.e.-present).
Planting, growing and harvesting the crops led to selection of types suitable to cultivation.
Selection of plants with desirable traits was - for centuries - the only form of crop
breeding.
According to the Russian scientist Nickolai Vavilov, there were 12 centers of crop
domestication around the world. Vavilov's theory has been modified since. Visit this nifty
Crop Evolution Website with TONS of images and cool stories to lean about the
revisions of the Vavilov's theory. What are the origins of the 10 crops we have mentioned
during this quarter?
N. Vavilov
For additional information about the crops, click on the highlighted text above.
Click here to learn more about Crop Genetic Diversity. You will also find out why the British are tea
drinkers and why Boston basketball team is called Celtics.
To learn more about life of N.Vavilov, click on his photograph (above right)
Brief history of crop breeding
Crop breeding changed significantly since the discovery of inheritance and development of genetics.
Gregor Mendel (left), in the 1850s made the first
observations that plant traits are inherited. Mendel noticed
that when green and yellow peas were crossed, all
progeny seeds were yellow. When plants of this first
hybrid generation (F1) were allowed to self-pollinate, the
progeny (F2) segregated with one green seed per three
yellow (right).
Mendel experimented further, and cross-pollinated plants
with green wrinkled and yellow smooth seeds (at the time,
Gregor Mendel the talented scientist did not know that texture and color
Click here
of pea seeds are inherited independentenly from each
other). In the first hybrid generation, F1, all seeds
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to visit MendelWebappeared yellow smooth (diagram below).When allowed
to self-pollinate, F1 plants produced segregating F2
progeny, with one green wrinkled seed, three yellow
smooth seeds, three yellow wrinkled and nine yellow
smooth seeds per each 16.
Based on the appearance of the seeds from F1 generation, one
can conclude that the allele coding for yellow seed color is
dominant over the allele coding for the green seed color; and
smooth or round allele is dominant over the allele coding for
wrinkled seed coat. In F2 generation, therefore some of the seeds
that appear yellow and smooth still carry alleles coding for green
wrinkled seeds.
Note that in the F2 generation there are green smooth and yellow
wrinkled seeds, a combination of traits that is different from
both parents. These new traits arose due to an independent
assortment of the alleles in meiosis. "Mendelian inheritance"
assumes that genes are inherited independently from each other.
In many cases, however, genes located close to each other on the chromosome are
inherited together, and the simple segregation discussed above does not take
place. Genes located on the same chromosome can be inherited separately due to
an event known at "crossing-over". Crossing-over can occur during the first
meiotic division. Crossing-over is the exchange of some of the corresponding
parts of homologous chromosomes. Crossing-over leads to recombination of the
traits.
Barbara McClintock was one of the first people to study chromosome
crossing-over in maize, she was awarded Nobel Prize for her studies and the
discovery of the mobile elements in maize chromosomes.
Click here to read an essay by D. Ardell on the fascinating life of B. McClintock
Brief review of genetic principles
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Barbara
McClintock
Diversity among individuals is the raw material of genetics. Variation among crop
plants is observed by breeders and farmers and allows them to select for the
individuals with desired traits. Genetics studies the mechanisms by which traits are
passed from one organism to another and how they are expressed. Here is a brief
overview of some genetic principles.
Francis Crick
Each cell of an organism contains at least one set of basic genetic information. This
set is called a genome. In a diploid organism, there is one set of chromosomes
derived from one parent and one chromosome set derived from the other parent
(that explains yellow wrinkled and green smooth seeds in Mendel's experiments).
A chromosome is one long double-stranded molecule of DNA. The double helical
structure of DNA was discovered by a British and an American scientists,
J.Watson and F.Crick (left). For them, a clue about the structure of DNA came
from X-ray photographs of DNA taken by Rosalind Franklin. Watson and Crick
were awarded Nobel Prize for their discovery.
James Watson
Genes are the regions of a DNA molecule. A gene specifies the structure of a
single protein. Each protein (enzyme) catalyzes a biochemical reaction within an
organism that leads to formation of other biological molecules.
Click here for an essay about life and Nobel Prize-winning discoveries of F.Crick
and J.Watson.
Another genetic discovery: decoding of human genome is probably the most
exciting scientific breakthrough of this year! What does it mean to the crop
scientists? Genome of Arabidopsis thaliana, a weedy plant from the Mustard
Family, is already sequenced. Genetic sequences of rice and Medicago truncatula
(a relative of alfalfa) are on their way.
Click on the image to the right to read the article in TIME magazine on
sequencing of human genome.
Haven't made up your mind on genetic engineering? Click on the highlighted
question to read a great compilation of pro and con arguments (including the story
on "crossing" tomato with cod)!
Need to refresh your Genetics? Click here to review a Glossary of Genetic
Terms.
Plant transformation
So, how did they genetically engineer rice to synthesize β-carotene?
Plant transformation (or genetic engineering) is the transfer of specific foreign DNA into a plant
species. Transformation involves several steps:
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●
●
●
●
●
isolation of a useful gene;
transfer of the gene into a plant cell;
integration of the gene into the plant genome;
regeneration of fertile plants through tissue culture;
transmission of the transgenic (transformed) from generation to generation through cross
pollination, as you will perform in today's lab.
Let's review our "yellow rice" example (left).
Scientist isolated a useful gene for β-carotene
production from daffodil, and excised that gene
(piece of DNA). The gene was then "glued" into a
carriers (small loops of DNA called plasmids).
Such plasmids are then introduced into plant cells, a
new transgenic organism is then re-generated from
a single cell.
There are several ways to introduce foreign DNA into a plant. In this Exercise, we will use the PIG,
Particle Inflow Gun improved by the OSU scientists (Dr. J.Finer and colleagues). The gun is used to
bombard (literally!) plant tissue with tungsten particles coated with DNA. DNA-coated particles are
accelerated inside a chamber under pressurized helium and partial vacuum. DNA is later integrated
into transformed cells' genome and transgenic plants are regenerated.
Review of flower anatomy and pollination
Flowers are highly specialized reproductive organs, adapted for the entire
range of reproductive functions: advertising, pollination, fertilization, seed
development, and dispersal of seeds. Flowers can be male, female or both.
By far the most common arrangement is having both male and female parts
within each flower, otherwise known as perfect flower. Imperfect flowers
have either male or female parts. Monoecious plants have male and female
parts on the same plant (e.g., corn, cucurbits, birch, walnut). Dioecious
plants have male and female flowers on separate plants (hemp, American
holly, hazel nut). Complete flowers have all four parts (sepals, petals,
stamen and pistil), while incomplete flowers are missing one or more of
these parts.
No two species of plants have identical floral anatomy, but the following
diagrams illustrate "typical" flowers with both male and female parts.
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Apple (above) has a perfect flower. Green sepals (6) protect the bud before the
flower opens. Petals (1) which people see as white, are highly visible to the
insect pollinators. Male parts of the flower are called stamens, and consist of a
filament (5) and anther (4). Pollen is produced in its anthers (4). When pollen
grains mature, they land on the stigma (2), which is a receptacle for the style (a
long tube that empties into the ovary (7)). The pollen grain then forms a pollen
tube that grows down the style (3) and reaches the ovary (7), where it releases
the male gamete. The gamete proceeds down the tube to fertilize an ovule in the
ovary. The fertilized ovule develops into a seed and the ovary typically develops
into the fruit.
Sepals and petals in flowers of tulip (right), and its monocot relatives (lilies,
daffodils, onions, etc) evolved into one organ, sometimes referred to as "tepal"
(8, right). Flower parts of tulip are labeled similarly to the flower parts of apple.
Grasses are also monocots. You'll notice that flowers of grasses are less
showy (eg. fescue flower, left). Grasses typically produce significant
amounts of pollen in their anthers (4). Carried by wind, pollen lands on
sticky feather-like stigma receptacles (2). Sepals and petals of grasses have
evolved into three layers of protective bracts -glume, palea, and lemma (9).
Lab Activities
1. Study flower anatomy. Identify flower parts.
2. Cross-polinate tomato flowers according to the protocol below.
Tomato cross-pollination
Tomato (Lycopersicon esculentum Mill.) is a highly self-pollinating species. Its
flower is perfect, having male (anthers) and female parts. Four to eight flowers are
borne on a compound inflorescence (right). A single tomato plant may produce up to
20 successive inflorescences during its life cycle.
The cultivated tomato forms a tight protective anther cone that surrounds the stigma.
Style elongation occurs within the anther cone and usually coincides with pollen
release. Outdoors, wind aids in release of pollen with subsequent fertilization, but
under greenhouse conditions, manual vibration of open flowers enhances effective
pollination and fruit set.
Protocol
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1. Select plump buds which are not yet open. The sepals can be separated but the petals
are still closed. The outside of the petals should appear creamy white in color.
2. With clean fine-pointed tweezers, peel open the sepals and petals. The anther cone
should be very pale yellowish-green and the petals should be pale yellowish-white. If
the anther cone is yellow or the petals are yellow the flower is too old. With tweezers,
remove all sepals. Take them off all the way down to the base of the bud.
3. Carefully remove all flower petals.
4. Completely remove anther cone by puncturing the base of the cone with tweezers,
gently lifting upwards and away. This exposes the style and stigma of the flower.
Emasculation for the purpose of cross pollination must be done approximately one day
prior to anthesis (flower opening) to avoid accidental self-pollination. At this time, the
sepals begin to change from light yellow-white to a dark-yellow. The stigma is fully
receptive which allows for pollination immediately after emasculation. However,
stigmas do remain receptive to pollen up to seven days. Under greenhouse conditions,
hand-pollinated stigmas require no protection to prevent uncontrolled crossing, as
would be the case under field conditions.
5. With your pollen source in hand, insert the stigma of the emasculate flowers into the
pollen, making sure the stigma receives ample pollen.
Gently snip off any immature flower buds located on the inflorescence.
Under greenhouse conditions, hand-pollinated stigmas require no protection. In 4-5
days, if the fertilization was successful, the ovary will begin to show signs of swelling
and enlargement as fruit development advances. Temperatures can influence the rate of
ripening with optimal temperature for fruit maturation and color development between
20oC and 24oC.
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Techniques of plant transformation
As you recall from our previous discussion of plant transformation, it is a rather time-consuming
process. We will not have time to carry a transformation experiment all the way through. We will,
however, learn some techniques of plant transformation.
1. Suppose we want to introduce a gene for RoundUp resistance into soybeans.
2. A gene for the herbicide resistance has already been isolated from another organism. This
herbicide-resistance gene has been excised with special enzymes, working as biological "scissors".
Another set of enzymes, working as biological "glue", inserted the gene of interest into the carrier
plasmids. Your TA has coated gold particles with these prepared plasmids.
3. Your TA will demonstrate how to "shoot" these DNA-coated particles into plant tissue. Now it's your
turn to play with the PIG.
4. Place bombarded tissue onto a regeneration tissue culture medium.
It will take time to grow a plant from this tissue. When the tissue gives rise to a plant, it is time to test
the plants for herbicide resistance.
5. Your instructor will spray seedlings of resistant and susceptible soybean seedlings with RoundUp.
Materials on this website are for personal use only. Text or files cannot be legally reproduced or
duplicated in any form. For commercial or instructional use of this website or materials from it, please
contact Dr. P. McMahon or Max Teplitski.
©Copyright by M.Teplitski and P.McMahon, 1999
For more information, email us at [email protected], [email protected].
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Untitled Document
SCIENCE
JULY 31, 2000 VOL. 156 NO.5
Grains Of Hope
Genetically engineered crops could
revolutionize farming. Protesters fear they
could also destroy the ecosystem. You
decide
BY J. MADELEINE NASH/ZURICH
A Grain of Hope--and
Fear: Ingo Potrykus
had a simple idea:
create genetically
modified rice to feed
the starving poor and
give it away. Now,
amid fresh protests
against
"Frankenfoods," his
golden grain is caught
in an increasingly
polarized public debate
Inside the Protest:
Taking It to Main
Street
How to Make Golden Rice
Click here for full diagram
At first, the grains of rice that Ingo Potrykus sifted
through his fingers did not seem at all special, but that
was because they were still encased in their dark,
crinkly husks. Once those drab coverings were
stripped away and the interiors polished to a glossy
sheen, Potrykus and his colleagues would behold the
seeds' golden secret. At their core, these grains were
not pearly white, as ordinary rice is, but a very pale
yellow--courtesy of beta-carotene, the nutrient that
serves as a building block for vitamin A.
Potrykus was elated. For more than a decade he had
dreamed of creating such a rice: a golden rice that
would improve the lives of millions of the poorest
people in the world. He'd visualized peasant farmers
wading into paddies to set out the tender seedlings
and winnowing the grain at harvest time in
handwoven baskets. He'd pictured small children
consuming the golden gruel their mothers would
make, knowing that it would sharpen their eyesight
and strengthen their resistance to infectious
diseases.
And he saw his rice as the first modest start of a new
green revolution, in which ancient food crops would
acquire all manner of useful properties: bananas that
wouldn't rot on the way to market; corn that could
supply its own fertilizer; wheat that could thrive in
drought-ridden soil.
But imagining a golden rice, Potrykus soon found,
was one thing and bringing one into existence quite
another. Year after year, he and his colleagues ran
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TIME.COM
COVERAGE
Search for TIME
stories about the
genetically modified
foods
POLL:Genetically
Modified Foods
Are you concerned
about consuming
genetically altered
fruits, vegetables and
grains?
NEWSFILE: The
Genetics Revolution
Coverage of the new
science set to
profoundly change our
lives
WEB FEATURE:
Visions of the 21st
Century
Find out if frankenfood
will feed the world
TIME ARCHIVES
Make Way for
Frankenfish!
What Happens To
These Ordinary
Untitled Document
into one unexpected obstacle after another, beginning
with the finicky growing habits of the rice they
transplanted to a greenhouse near the foothills of the
Swiss Alps. When success finally came, in the spring
of 1999, Potrykus was 65 and about to retire as a full
professor at the Swiss Federal Institute of Technology
in Zurich. At that point, he tackled an even more
formidable challenge.
Having created golden rice, Potrykus wanted to
make sure it reached those for whom it was intended:
malnourished children of the developing world. And
that, he knew, was not likely to be easy. Why?
Because in addition to a full complement of genes
from Oryza sativa--the Latin name for the most
commonly consumed species of rice--the golden
grains also contained snippets of DNA borrowed from
bacteria and daffodils. It was what some would call
Frankenfood, a product of genetic engineering. As
such, it was entangled in a web of hopes and fears
and political baggage, not to mention a fistful of
ironclad patents.
Salmon If The
Genetically Modified
Lunkers Ever Get
Loose?
MARCH 6, 2000
Who's Afraid of
Frankenfood?
So far, mostly just
Europeans. But thanks
to a little uncertainty
and a lot of agitprop,
that's changing
NOVEMBER 29, 1999
Of Corn and
Butterflies
U.S. farmers are
planting 20 million
acres of bioengineered
corn. Will it poison the
monarchs?
MAY 31, 1999
For about a year now--ever since Potrykus and his
chief collaborator, Peter Beyer of the University of
Freiburg in Germany, announced their achievement
--their golden grain has illuminated an increasingly
polarized public debate. At issue is the question of
what genetically engineered crops represent. Are
they, as their proponents argue, a technological leap
forward that will bestow incalculable benefits on the
world and its people? Or do they represent a perilous
step down a slippery slope that will lead to ecological
and agricultural ruin? Is genetic engineering just a
more efficient way to do the business of conventional
crossbreeding? Or does the ability to mix the genes of
any species--even plants and animals--give man
more power than he should have?
The debate erupted the moment genetically
engineered crops made their commercial debut in the
mid-1990s, and it has escalated ever since. First to
launch major protests against biotechnology were
European environmentalists and consumer-advocacy
groups. They were soon followed by their U.S.
counterparts, who made a big splash at last fall's
World Trade Organization meeting in Seattle and last
week launched an offensive designed to target one
company after another (see accompanying story).
Over the coming months, charges that transgenic
crops pose grave dangers will be raised in petitions,
editorials, mass mailings and protest marches. As a
result, golden rice, despite its humanitarian intent, will
probably be subjected to the same kind of hostile
scrutiny that has already led to curbs on the
commercialization of these crops in Britain, Germany,
Switzerland and Brazil.
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WEB RESOURCES
The Golden Age of
Agriculture ó Grains
A case study of golden
grains by the
Department of Natural
Resources and
Environment (NRE) of
Victoria, Australia
Future Foods
Science Museum
presentation about
genetically modified
foods, including real
video on how to extract
DNA from an onion
Waiter, there's a
Gene in my Food
Introduction to issues
and controversies
surrounding genetically
modified food by the
Australian
Broadcasting
Corporation
FDA Center for Food
Safety and Applied
Nutrition
Contains information
on biotechnology and
Untitled Document
The hostility is understandable. Most of the
genetically engineered crops introduced so far
represent minor variations on the same two themes:
resistance to insect pests and to herbicides used to
control the growth of weeds. And they are often
marketed by large, multinational corporations that
produce and sell the very agricultural chemicals
farmers are spraying on their fields. So while many
farmers have embraced such crops as Monsanto's
Roundup Ready soybeans, with their genetically
engineered resistance to Monsanto's Roundup-brand
herbicide, that let them spray weed killer without
harming crops, consumers have come to regard such
things with mounting suspicion. Why resort to a
strange new technology that might harm the
biosphere, they ask, when the benefits of doing so
seem small?
Indeed, the benefits have seemed small--until golden
rice came along to suggest otherwise. Golden rice is
clearly not the moral equivalent of Roundup Ready
beans. Quite the contrary, it is an example--the first
compelling example--of a genetically engineered crop
that may benefit not just the farmers who grow it but
also the consumers who eat it. In this case, the
consumers include at least a million children who die
every year because they are weakened by vitamin-A
deficiency and an additional 350,000 who go blind.
MORE>>
PAGE 1 | 2 | 3 | 4
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IMAGE CREDITS | TIME DIAGRAM BY JOE LERTOLA
SOURCE: DR. PERET BEYER, CENTER FOR APPLIED
BIOSCIENCES, UNIVERSITY OF FREIBURG
COPYRIGHT © 2000 TIME INC. I PRIVACY POLICY
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Nikolai I. Vavilov (1887-1943)
---------------------------------------------------------------------------From : http://www.dainet.de/genres/vir/ (Russian Institute of Plant
Industry )
Nikolai I. Vavilov was born into the family of a merchant in Moscow on
November 25, 1887. In 1911, having graduated from the Agricultural Institute,
Vavilov continued to work at the Department of Agriculture Proper headed by
Prof. Pryanishnikov. In 1911-1912 Vavilov did practical work at the Bureau for
Applied Botany and at the Bureau of Mycology and Phytopathology of the
Agricultural Scientific Committee. In 1913-1914, Vavilov traveled to Europe
where he studied plant immunity, mostly with Prof. W. Bateson, a co-founder
of the science of genetics.
In autumn 1917 the Head of the Bureau for Applied Botany Robert. E. Regel
(1867-1920) supported the nomination of N.I.Vavilov, a young professor from
the Saratov Higher Agricultural Courses, as Deputy Head of the Bureau. As
Regel wrote in his reference letter, "In the person of Vavilov we will employ ...
a talented young scientist who would become the pride of national
science". Regel's prediction turned out to be true. Since then, all Vavilov's life
and creative work have been inseparable from the world's largest crop research
institute, into which he transformed the Bureau in the1920-30's.
Vavilov continued his investigations in Saratov where he has awarded the title
of Professor of the Saratov University in 1918. During the Civil War, from
1918 to 1920, Saratov became the scientific stronghold for the Department of
Applied Botany (Bureau till 1917). In 1920 Vavilov was elected head of the
Department, and soon moved to Petrograd (St.Petersburg now) together with
his students and associates.
In 1924, the Department was transformed into the Institute of Applied Botany
and new Crops (VIR since 1930), and occupied the position of the central
nationwide institution responsible for collecting the world plant diversity and
studying it for the purposes of plant breeding.
Vavilov is recognized as the foremost plant geographer of contemporary times.
To explore the major agricultural centers in this country and abroad, Vavilov
organized and took part in over 100 collecting missions. His major foreign
expeditions included those to Iran (1916), the United States, Central and South
America (1921, 1930, 1932), the Mediterranean and Ethiopia (1926-1927). For
his expedition to Afghanistan in 1924 Vavilov was awarded the
N.M.Przhevalskii Gold Medal of the Russian Geographic Society. From 1931
to 1940 Vavilov was its president.
These missions and the determined search for plants were based on the
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Vavilov's concepts in the sphere of evolutionary genetics, i.e. the Law of
Homologous Series in Variation (1920) and the theory of the Centers of Origin
of Cultivated Plants (1926).
N.I.Vavilov was a prominent organizer of science. In the period from 1922 to
1929 he headed the Institute of Experimental Agronomy (the former ASC)
which developed in 1930 into the V.I.Lenin All-Union Academy of
Agriculture; from 1930 to 1935 Vavilov was its first president. From 1930 to
1940 he was director of the Institute of Genetics. Vavilov organized and
participated in significant home and international scientific meetings and
congresses on botany, genetics and plant breeding, agricultural economy, and
the history of science. All around the world N.I.Vavilov has gained respect and
renown; he was elected member of many academies of sciences and various
foreign scientific societies.
Vavilov, the symbol of glory of the national science, is at the same time the
symbol of its tragedy. As early as in the beginning of the 1930's his scientific
programs were being deprived of governmental support. In the stifling
atmosphere of a totalitarian state, the institute headed by Vavilov turned into a
resistance point to the pseudo-scientific concepts of Trofim D.Lysenco. As a
result of this controversy, Vavilov was arrested in August 1940, and his closest
associates were also sacked and imprisoned.
Vavilov's life ceased in the city where his star had once risen. He died in the
Saratov prison of dystrophia on 26 January 1943 and was buried in a common
prison grave.
Nevertheless, the memory of Vavilov has been preserved by his followers.
During that tragic period they kept on gathering Vavilov's manuscripts,
documents and pictures. Since mid-50's, after the official rehabilitation of
Vavilov, hundreds of books and articles devoted to his life and scientific
accomplishments have been published. Memorial displays have been opened in
Major N.I.Vavilov's Expeditions
---------------------------------------------------------------------------1916 Expedition to Iran (Hamadan and Khorasan) and Pamir (Shungan, Rushan
and
Khorog).
1921 Acquaintance trip to Canada (Ontario) and USA (New York,
Pennsylvania,
Maryland, Virginia, North and South Carolina, Kentucky, Indiana, Illinois,
Iowa, Wisconsin, Minnesota, North and South Dakota, Wyoming, Colorado,
Arizona,
California, Oregon, Maine).
1924 Expedition to Afghanistan (Herat, Afghan Turkestan, Gaimag, Bamian,
Hindu Kush,
Badakhshan, Kafiristan, Jalalabad, Kabul, Herat, Kandahar, Baquia, Helmand,
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Farakh, Sehistan), accompanied by D.D. Bukinich and V.N. Lebedev.
1925 Expedition to Khoresm (Khiva, Novyi Urgench, Gurlen, Tashauz).
1926-1927 Expedition to Mediterranean countries (France, Syria, Palestine,
Transjordan,
Algeria, Morocco, Tunisia, Greece, Sicily, Sardinia, Cyprus and Crete, Italy,
Spain, Portugal, and Egypt, where Gudzoni was explored by Vavilov's request)
and to Abyssinia (Djibouti, Addis Ababa, banks of Nile, Tsana Lake), Eritrea
(Massaua) and Yemen (Hodeida, Jidda, Hedjas).
1927 Exploration of mountainous regions in Wuertemberg (Bavaria, Germany).
1929 Expedition to China (Xinjiang - Kashgar, Uch-Turfan, Aksu, Kucha,
Urumchi,
Kulja, Yarkand, Hotan) together with M.G. Popov, then alone to Chine
(Taiwan),
Japan (Honshu, Kyushu and Hokkaido) and Korea.
1930 Expedition to USA (Florida, Louisiana, Arizona, Texas, California),
Mexico,
Guatemala and Honduras.
1932-1933 Trip to Canada (Ontario, Manitoba, Saskatchewan, Alberta, British
Columbia),
USA (Washington, Colorado, Montana, Kansas, Idaho, Louisiana, Arkansas,
Arizona, California, Nebraska, Nevada, New Mexico, North and South
Dakotas,
Oklahoma, Oregon, Texas, Utah);
Expedition to Cuba, Mexico (Yucatan), Ecuador (Cordilleras), Peru (Lake
Titicaca, Puno Mt., Cordilleras), Bolivia (Cordilleras), Chile (Panama River).
Brazil (Rio de Janeiro, Amazon), Argentina, Uruguay, Trinidad and Porto Rico.
1921-1940 Systematic explorations of the European part of Russia and the
whole regions of
the Caucasus and the Middle Asia.
---------------------------------------------------------------------------Major Collecting Missions Accomplished by N.I.Vavilov's Associates
---------------------------------------------------------------------------1922-1923 Expedition of V.E.Pisarev and V.P.Kuzmin to Mongolia.
1923 Expedition of E.I.Barulina to Crimea (Ukraine).
1924 Expedition of E.I.Sinskaya to Altai.
1925-1926 Expedition of S.M.Bukasov and Yu.N.Voronov to Mexico,
Guatemala and Colombia.
1925-1926 Expedition of E.N.Stoletova to Armenia.
1925-1927 Expedition of P.M.Zhukovsky to Turkey.
1926 Expedition of N.N.Kuleshov and V.V.Pashkevich to Azerbaijan.
1926 Expedition of K.A.Flyaksberger to Azerbaijan and Russia (Daghestan).
1926 Expedition of N.N.Kuleshov and V.K.Kobelev to Uzbekistan.
1926 Expedition of K.A.Flyaksberger to Far East of Russia.
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1926-1928 Expedition of V.V.Markovich to Palestine, Pakistan, India, Java and
Ceylon.
1926-1928 Expedition of S.V.Yuzepchuk to Peru, Bolivia and Chile.
1927 Expedition N.N.Kuleshov to Turkmenia.
1927 Expedition K.G.Kreier to Central and Western part of Siberia.
1928-1929 Expedition of E.N.Sinskaya to Japan.
1928-1932 Expedition of G.K.Kreier to Georgia and Azerbaijan.
1930 Expedition of E.A.Stoletova to Georgia (USSR).
1930 Expedition of G.K.Kreier to Kirgizia and Uzbekistan.
1933 Expedition of E.I.Barulina to Georgia (USSR).
----------------------------------------------------------------------------
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Barbara McClintock (1902 - 1992)
Essay by David Ardell
Until recently, scientific research was considered beyond most women's abilities,
despite notable historical exceptions - such as that of the great 19th century
co-discoverer of radioactivity, Marie Curie. If a woman displayed natural talent in
science and mathematics, the option to pursue her talents as a scientist was likely to
be closed off in favor of more traditional roles: mother, wife, and homemaker. Sadly,
this was true in America even as late as the 1950s. That is what makes Barbara
McClintock and her lifelong achievements in genetics all the more notable.
McClintock launched her scientific career at Cornell in1919 and, in the face of social
adversity and tremendous intellectual challenges, established herself among the great
geneticists of this century.
At the time McClintock started her career, scientists were just becoming aware of the
connection between heredity and events they could actually observe in cells under
the microscope. McClintock
pioneered the field of maize cytogenetics, or the cellular analysis of genetic
phenomena in corn, which for the first time provided a visual connection between
certain inheritable traits and their physical basis
in the chromosome.
McClintock rose to many challenges throughout her career - not only scientific but
personal - from other scientists who felt intimidated or threatened by what one of her
colleagues described as her
"independence, originality, and extraordinary accomplishment." In the most notable
case, Lowell Randolph, her advisor and colleague, became extremely irritated with
McClintock's success in solving a problem he had spent his entire life working on.
McClintock became the dominant member of his research team, and Randolph found
this intolerable. McClintock soon departed, going on to greater things.
For her ground-breaking work in the genetics of corn, Barbara McClintock earned a
place among the leaders in genetics. She was elected to the prestigious National
Academy of Sciences in 1944.
Despite this, she still met with social adversity in her department at the University of
Missouri and finally left there, too. She kept her next appointment at the Carnegie
Institute at Cold Spring Harbor for
the rest of her life.
In 1983, Barbara McClintock was awarded a Nobel Prize in Genetics. To this day,
her work is highly esteemed, still relevant despite the fact that much of it was
completed over half a century ago, before the
advent of the molecular era.
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Intrigued by B.McClintock? Click here to learn more about her discoveries.
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The Genetics Revolution
web resources
Monsanto Agriculture
Press releases and news
from the company on its
biotech program
The AgBio Forum
A quarterly online
magazine on the
management of
agricultural biotechnology
International Center for
Genetic Engineering
and Biotechnology
Italian institute promoting
the safe use of
biotechnology world-wide
interact
POLL
Genetically Modified
Foods: Are You Afraid of
Eating Them?
newsfile subjects
Research
The latest discoveries and
the Human Genome
Project
Cloning
Dolly was just the first.
How long until humans
follow?
Plant & Animal
Applications
Why the farm will never be
the same
Human Applications
Designer babies, maybe.
But also designer
A genetically engineered tomato on the vine
The Killer Tomatoes
Somewhere, someone is crossing a fish with a tomato.
Researchers are inserting an antifreeze gene from the winter
flounder to produce a cold-resistant love apple, one that
American consumers seem indifferent to but has Europeans
taking to the streets to keep off their shelves.
These are the front lines of the genetics revolution, the
practical applications of the truly amazing discoveries of the
past two decades. Here are miracles and wonders that could
help feed an ever-more crowded world: extra-starch
potatoes, coffee beans grown decaf right on the vine,
low-sugar strawberries. Wonder Bread-quality wheat
courtesy a plant with extra gluten built right in. Super
high-protein grains that could be a boon to the developing
world. And cotton and potatoes with herbicide-producing
genes that could eliminate the need for toxic sprays.
Here are dragons: Activists worry that plants with an innate
herbicide might breed a new generation of resistant "super
insects." Or that man-made seeds might cross-pollinate with
other plant species, with unknown and potentially
devastating results. Already, early studies show Monsanto's
highly popular Bt corn could prove devastating to Monarch
butterflies.
Then there's the matter of intellectual property. To protect
its billion-dollar investment, Monsanto hopes to introduce an
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The Genetics Revolution
treatments for your specific
ailments
Ethics
What to do with our
newfound knowledge
Business
The worth of the gene
Timeline
From discovery of the
double helix to deciphering
the human genome
elegantly malevolent technology, called "Terminator," that is
a set of genetic instructions that render a seed sterile after
just one planting -- thus enforcing the company's copyright.
From a biotech standpoint, this is a marvel, what one
scientist has called "the most intricate application of genetic
engineering to date." From a human standpoint, it's a
potential time bomb. The UN has already expressed concern
that Terminator seeds could force farmers into total
dependence on seed companies. Others are worried about
possible cross-pollination that could render other plants
sterile. Meanwhile, the U.S. Army War College is reportedly
intrigued about the possibilities of technologies that could
tell plants to commit suicide on demand. Which means the
only certain thing is that there's a crop dustup in our
future.
from TIME
Will Frankenfood Feed The World?
Genetically modified food has met fierce opposition among
well-fed Europeans, but it's the poor and the hungry who
need it most
JUNE 19, 2000
Make Way for Frankenfish!
What Happens To These Ordinary Salmon If The Genetically
Modified Lunkers Ever Get Loose?
MARCH 6, 2000
Who's Afraid of Frankenfood?
So far, mostly just Europeans. But thanks to a little
uncertainty and a lot of agitprop, that's changing
NOVEMBER 29, 1999
Of Corn and Butterflies
U.S. farmers are planting 20 million acres of bioengineered
corn. Will it poison the monarchs?
MAY 31, 1999
The Suicide Seeds
Terminator genes could mean big biotech bucks--but big
trouble too, as a grass-roots protest breaks out on the Net
JANUARY 19, 1999
Brave New Farm
Fears of "Frankenstein" food run deep, especially in Europe
PHOTO: GERRY GROPP/SIPA
JANUARY 11, 1999
Copyright © 1999 Time Inc. New Media. All Rights Reserved.
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MAPPING THE GENOME
JULY 3, 2000 VOL. 156 NO. 1
The Race Is Over
The great genome quest is officially a tie,
thanks to a round of pizza diplomacy. Yet
lead researcher Craig Venter still draws
few cheers from his colleagues
BY FREDERIC GOLDEN AND MICHAEL D. LEMONICK
One day last April, Aristides (Ari) Patrinos, a scientist
at the Department of Energy who directs that
agency's share of the Human Genome Project, got a
call from Francis Collins, director of the National
Institutes of Health's National Human Genome
Research Institute and the project's unofficial head.
"Let's try it," said Collins--and at those words Patrinos
knew that a longstanding scientific feud finally had a
chance of being resolved. For months, Collins had
been under pressure to hammer out his differences
with J. Craig Venter, the prickly CEO of Celera
Genomics, which was running its own independent
genome-sequencing project--differences over who
should get the credit for this scientific milestone; over
whose genome sequence was more complete, more
accurate, more useful; over the free exchange of
what may be mankind's most important data versus
the exploitation of what may also be its most
valuable.
The bickering had become downright nasty at times,
upstaging the enormous importance of the project
and threatening to slow the pace of scientific
discovery. Therefore Patrinos had been lobbying his
colleague to make love, not war, despite Venter's
uncanny ability to get under the skin of Collins and
other leaders of the U.S.-British genome project. So
had Collins' counterparts at other NIH institutes. And
so, most important, had President Clinton, who at
one point scribbled a note to science adviser Neal
Lane with the terse instruction: "Fix it...make these
guys work together."
Venter was clearly ready. His tactless rhetoric had
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lost him respect among his colleagues, and he
recognized that more controversy could overshadow
a historic moment in biomedicine. Beyond that, he'd
taken a beating in the marketplace. After a joint
declaration by Clinton and British Prime Minister
Tony Blair in March that all genomic information
should be free, the value of Celera stock plummeted
from $189 a share to $149.25.
So on May 7, over pizza and beer at Patrinos'
Rockville, Md., town house, the two wary antagonists
sat down in a deliberately casual setting to work out
their differences. In an exclusive conversation with
Collins, Venter and TIME correspondent Dick
Thompson last Thursday night, Patrinos recalled, "I
don't think I've ever seen them as tense as they were
that day." Yet despite mistrust on both sides, Collins
and Venter met a second time and a third.
And finally they came, if not to a meeting of the
minds, at least to a workable understanding--and a
framework for this week's joint announcement. After
more than a decade of dreaming, planning and heroic
number crunching, both groups have deciphered
essentially all the 3.1 billion biochemical "letters" of
human DNA, the coded instructions for building and
operating a fully functional human.
It's impossible to overstate the significance of this
achievement. Armed with the genetic code, scientists
can now start teasing out the secrets of human health
and disease at the molecular level--secrets that will
lead at the very least to a revolution in diagnosing
and treating everything from Alzheimer's to heart
disease to cancer, and more. In a matter of decades,
the world of medicine will be utterly transformed, and
history books will mark this week as the ceremonial
start of the genomic era.
But while the announcement has been exquisitely
choreographed to make the two scientists look like
equals, it's clear to insiders that Venter's project is a
lot further along. HGP scientists may have decoded
97% of the genome's letters--the remaining 3% are
generally considered unsequenceable and
irrelevant--but they know the order of only 53% of
them. It's as if they've got the pages in the so-called
book of life in the proper order but with the letters on
each page scrambled. "It's going to take us a couple
of years to put this together," Collins told TIME.
Celera, by contrast, has not only the pages but all
the words and letters as well--though neither side can
yet say what most of these words and letters mean.
And while the HGP boasts that it has done its
sequence nearly seven times over to guarantee
accuracy, Celera has gone over its own almost five
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times. Moreover, the company came up with a new
technique that made its sequencing rate, already the
fastest around, even faster. In addition, Venter claims
that by the end of the year, he'll have sequenced the
genome of the mouse--whose 2.3 billion letters
contain enough similarities to ours to make it vitally
important to scientists tracking down human gene
function.
Given this remarkable record, why are so few of
Venter's fellow scientists trumpeting his success? Or
talking him up for a Nobel Prize? Why, in fact, is this
cherubic-looking, blue-eyed ex-surfer hated by so
many colleagues, who have called him everything
from a greedy megalomaniac to a Hitler? Forget
about easy explanations, such as his outsize ego
(yes, one of the samples he is analyzing is rumored
to contain his own DNA) or his penchant for doing
science by press release (yes, he keeps his door
open to reporters) or his tendency to do not science
but, as pioneer DNA mapper James Watson sneered,
tedious assembly-line labor on machines that "could
be run by monkeys" (yes, most of Celera's analysis
was done by robot gene sequencers and high-speed
computers). MORE>>
PAGE 1 | 2 | 3 | 4
Get the Magazine - Try 4 Issues Free
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Biochemistry: Proteins
Plant Biochemistry: Proteins
Proteins are long chains of amino acids linked together by peptide bonds.
● All enzymes are proteins, not all proteins are enzymes.
● Some are parts of membranes (channels and gates).
● Some are structural and/or storage units.
There are 20 common amino acids.
All amino acids have a carboxyl (COOH) end and an amino (NH2) end. This is the first time we have
seen where N is a major component of a structure.
The peptide bonds form a backbone with the unique portion of the amino acid attached to the backbone.
These amino acids are arranged in very specific order for each different protein. There can be 100,000's
per molecule. They are what gives a protein it's specific role as an enzyme.
The aminoacid sequence causes the protein to coil up in a very specific, convoluted (folded) form. The
form is what determines if it is active or inactive many times.
Only a slight conformational change is enough to activate or deactivate a protein.
Adding or taking away even a single CH3 (methyl) from the entire molecule is enough to put it into or
take it out of action.
Other things can activate or deactivate.
● Kinases (enzymes that phosphorylate and de-phosphorylate a molecule using ATP as the P
donor) need Mg+ to work. Without Mg+ the Calvin and TCA cycles shut down. Not good.
Remember, all enzymes are proteins and an enzyme is needed for every single step of every
single biochemical pathway - including protein synthesis.
But not all enzymes are present in every cell all the time. They are synthesized as needed. Every
cell has a complete set of genes. Proteins are synthesized when a gene is 'turned on'.
Some proteins are soluble in H2O, some are not. The soluble ones can be transported to other
cells in the plant.
Remember, N is a mobile element, this is partly why.
Two amino acids, methionine and cysteine also contain sulfur.
Peanuts contain all the amino acids essential for human nutrition.
The problem with peanuts is they also contain high amounts of lipids so they are high calorie.
Some proteins help to make cellulose more rigid in the cell walls. These proteins are not mobile.
Some proteins are a storage compound in legume seeds such as soybean, chickpeas (garbanzos),
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Biochemistry: Proteins
and lentils. These seeds are an important nutrient source for people in developing areas where the
traditional primary diet is based on high carbohydrate seeds such as rice and corn.
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Index Page of HCS200
Class Schedule
Syllabus
Week
week 1
Lectures
Overview of Crop Science
Plant morphology and anatomy
week 2
week 3
Environmental factors affecting crop
growth and development. Light, water,
heat
Environmental factors cont'd. Soil/media,
nutrients, atmospheric gases
week 4
Plant physiology and biochemistry
week 5
Transpiration, photosynthesis and
respiration
week 6
Crop growth and development
week 7
Crop breeding (genetics, reproduction and
improvement)
week 8
Cropping systems, Agroecology
week 9
Group Reports
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Labs
Introduction
Index Page of HCS200
All materials on this website are for personal educational use only. We ask you not to reproduce any
files, texts or figures without our permission. We are gratefull to Dr. M.Knee, Dr. X.Wei, American
Society of Plant Pathology, PLANT PHYSIOLOGY journal, TIME magazine, Liebig Museum, Nobel
Prize Archives, and Vavilov Research Institute for allowing us to use their copyright images. We thank
Dr. D. Bauer, N. Cavender, G. Glaunsinger, T. Mangen, H. Brown, and J. Schmoll for contributing their
ideas and suggestions. We are indebted to our students and colleagues for their constructive criticism
and help in designing the website.
Development of this website was supported, in part, by the Faculty Innovator Grant (2000) to Dr.
P.McMahon.
Copyright by M.Teplitski and P.McMahon, 2000
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HCS 200 - Winter Quarter 2001
Syllabus for CROP SCIENCE
Horticulture and Crop Science 200
Winter 2001
Instructor: Dr. Joe Scheerens
Columbus Office: 232 Kottman Hall
Columbus Phone (work): 614-247-6859
Wooster Office: 213 Williams Hall
Wooster Phone (work): 5-3826 from Campus
E-mail Addresses:
[email protected]
[email protected]
Wooster Phone (work): 330-263-3826
Wooster Phone (home): 330-264-4930
Wooster Fax: 330-263-3887
Teaching Assistants:
Ms. Nicole Cavender
[email protected]
Kottman Greenhouse Supervisor:
Mr. Harold Brown
144 Kottman Hall
[email protected]
Ms. Gitta Glaunsinger
[email protected]
Administrative Assistant:
Ms. Regina Vann
216 Howlett Hall
Phone: 614-292-3866
Course Description: Study of environmental, genetic and cultural factors which influence plant productivity
Lecture:
Discussion:
Labs:
M,W
TBA
TBA
10:00 - 11:30 AM
TBA
TBA
Final Examination: Monday, March 12, 2001, 9:30 -11:18 AM.
References:
Required Text; Plant Science
Barden, Halfacre and Parish
McGraw Hill, Publ.
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164 Howlett Hall
TBA
334 Kottman Hall
HCS 200 - Winter Quarter 2001
Lab Manual; http://hcs.ohio-state.edu/hcs200
Lab Worksheets; Available at Cop-EZ
Lecture Notes: http://hcs.osu.edu/hcs200/notes1.html
Supplementary; Placed on reserve in Agric. Library
Purpose: This class give students interested in the production of plants and crops for food, fiber, ornamental,
and recreational use the basic understanding of how environmental, cultural and genetic factors influence crop
productivity. Students are introduced to contemporary issues surrounding plant agriculture and to the current
concepts and techniques for improving crop productivity. Students are encouraged through interactive
discussions and hands-on projects to develop skills needed to make informed decisions about the growing,
production, and utilization of plants and crops. In addition, students develop an appreciation for the
contribution that cultivated plants make to the environment and humanity.
Goals: Class goals for Winter 2001 will be determined by the class.
Lecture Schedule and Content: The majority of lecture topics that will be covered in H&CS 200 are listed
below. However, several class periods (exact number to be determined collectively by students) will be
devoted to exploring student-relevant topics or issues. Students will be involved in the development and
perhaps, the presentation of these issues.
Topic 1. The origins of crop agriculture, the history of crop improvement, the importance of genetic diversity
and the consequences of an agrarian society
Topic 2. Crop classification and diversity
Topic 3. Crop growth and development
Topic 3a - Crop growth and development (continued)
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HCS 200 - Winter Quarter 2001
Topic 4. Environmental factors (light, heat, soil/media, water, nutrients, atmospheric gasses) affecting crop
growth and development through modification of crop physiology and biochemistry
● Light
●
Heat
●
Water
●
Nutrients 1
●
Nutrients 2
Topic 5. Crop products and their relationship to our daily lives
Topic 6. Cropping systems at all levels of technology
Topics 7-? Additional topics to be determined by class
SPECIAL NOTES
NOTES 1 - Plant Cells
Lab Schedule and Content: See lab manual and accompanying materials
Discussion Section Schedule and Content: A portion of each discussion period will be devoted to
clarification of lecture material if necessary or for pre-examination reviews . However, most of the discussion
classes will be devoted to exploring individual crops (history, production, use, etc.) of importance to the world
or of particular interest to students. Assignments in discussion sections will accomplished individually or in
teams. For the most part, activities in discussion sections will be student-directed and interactive. This is your
chance to be creative, productive and to have some fun at the same time.
Evaluation Methods: The relative importance of class activities and how and in some instances, by whom
they are graded was determined by the class. The results are as follows
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HCS 200 - Winter Quarter 2001
Activity
Two Midterms + Final*
Labs
Discussion Projects
Percent of Grade
50
30
20
Evaluator
Instructor
TAs
To Be Determined
*Each test will be worth 25% of the class grade - the lowest score on the three tests will be dropped.
Grading Scale: A = 94-100 points C = 73-76 points
A- = 90-93 points C- = 70-72 points
B+ = 87-89 points D+ = 67-69 points
B = 83-86 points D = 60-66 points
B- = 80-82 points E < 60 points
C+ = 77-79 points
Midterm Examinations: Midterms will be given during lecture periods and will be announced at least 5 days
before being given. There will be ample opportunity for review prior to each examination. Examinations may
contain objectively graded questions (e.g., matching or one word answer), but most assuredly will contain short
essay questions. Students will always have a choice as to which short essays they write (i.e., students do not
answer all questions that are posed, only those for which they have the most complete understanding of the
subject).
Final Examination: A final examination will be given on the date/time listed previously. The structure (style)
of the final exam will be similar to that used for midterms.
Second Chance Examinations: Students will have the opportunity to turn into the instructor (within an
agreed-upon time frame) revised answers to any questions for which they lost points. The final score for each
question that has an improved answer will be the average of the old and new score for that question. Students
will not be penalized if they change an answer incorrectly. For the second chance exam, students can use any
source or reference (except the instructor, teaching assistants, guest lecturers or classmates) to determine the
appropriate answer. Second chance examinations for the final exam will be offered only if time permits.
Make-up Examinations: A full credit, written make-up midterm exam with a second chance will be given to
students who notify the instructor or TA's ahead of time of their absence from the exam. There must be a
verifiable, reasonable excuse (e.g., field trip, illness, transportation problems, family emergencies, etc). An
unacceptable excuse would be any excuse that indicates a lack of responsibility on the part of the student. A
student who has missed a midterm exam without an excuse has the option of taking a full-length, written exam
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HCS 200 - Winter Quarter 2001
worth 75% of the original points with no second chance exam option. Failure to attend the final exam will be
adjudicated on a case by case basis.
Attendance: Students are encouraged to attend class regularly. Material presented in lectures may or may not
be found in the text book. Detailed lecture notes will be posted upon the completion of each topic at the web
site indicated above. However, these notes are made available to students so as to provide best possible
opportunity for students to listen and think as lectures are being delivered. They are not be designed to act as
comprehensive web-based learning materials on their own. Because much of discussion and lab activities are
team-based and interactive, failure to attend either will result in the inconvenience of others and the loss of
experiences important to the educational process.
Class Participation: Although the instructor and TA's assume responsibility for most of the instruction in this
course, each student brings to class, relevant personal experience that will relate to the subject matter. Students
are asked to share this experience with their classmates, if they feel comfortable doing so. Students who enroll
in this course come from a diversity of backgrounds and personal skills. The sharing of knowledge or insight
with others is encouraged as a means to enrich the experience for all.
Effort: It is understood that individuals within the class will have other commitments (educational and
personal) that he/she must fulfill. Moreover, because students in this course are diverse, it is unlikely that all
will be able to devote equal amounts of time or effort to performing assignments in this class. However, the
instructor and TA's ask that students work as diligently as possible to complete the activities in this course to
the best of their abilities. A portion of the lab and discussion section grades will result from an assessment of
effort. If students are experiencing difficulty (i.e, a crisis has arisen) please let the instructor or TA's know as
soon as possible.
Code of Conduct: In H&CS 200, courtesy and respect for others will be given by all participants, including
the instructor, teaching assistants and guests, in the class at all times. An environment that fosters free,
non-confrontational expression of ideas will be maintained. When working on teams, each team member will
assume full responsibility for their role as a member of that team. Academic misconduct or suspected academic
misconduct will be handled according to policies of the Code of Student Conduct in the Student Handbook or
Faculty Rule 3335-5-487.
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #1: Crop origins, crop development and the effect of crops and crop science on human
life
References:
Brownowski, J. 1973. The harvest of the seasons. In: The ascent of man. Little, Brown Inc., Boston, MA.
Chandler, R.F. 1992. The role of the international agricultural research centers in increasing the world
food supply. Food Tech. 46(7):86.
Council for Agricultural Science and Technology. 1985. Plant germplasm preservation and
utilization in U.S. agriculture.
Goldblith, S.A. 1992. The legacy of Columbus, with particular reference to foods. Food Tech
46(10):62-85.
Hanson, H., N.E. Borlaug and R.G. Anderson. 1982. Wheat in the third world. Westview press,
Boulder, CO.
Harlan, J.R. 1992. Crops and man (2nd ed.) Amer. Soc. Agron., Madison, WI.
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant Science (2nd ed..).
Prentice-Hall, Englewood Cliffs, NJ.
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Hawkes, J.G. 1990. The potato: evolution biodiversity and genetic resources. Belhaven Press.
London.
International Potato Center. 1984. Potatoes for the developing world. International Potato Center, Lima,
Peru.
Metcalf, D.S., and D.M. Elkins. 1980. Crop production principles and practices (4th ed.) MacMillan
Publishing Co., New York, NY.
National Academy of Sciences. 1972. Genetic vulnerability of major crops. NAS, Washington, D.C.
Niderhauser, J.S. 1992. The role of the potato in the conquest of hunger and new strategies for
international cooperation. Food Tech. 46(7):91-95.
Zohary, D., J.R. Harlan and A. Vardi. 1969. The wild diploid progenitors of wheat and their breeding
value. Euphytica 18:58-65.
http://www.ars-grin.gov/npgs/
http://www.cgiar.org/
http://www.state.oh.us/agr/97AnnlRpt/97SUMMAR.HTM
Quotation:
"Man during his history in all parts of the world has used for food more than 3000 species of plants. Of
these, only some 150 parts have ever been extensively cultivated and only about dozen are important
from the standpoint of the energy which they contribute." (Paul S. Mangelsdorf)
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Outline:
1. Definitions for "agriculture" and "crop"
The field of agriculture is diverse and not easily defined. A crop could be defined as a cultivated plant
that yields an economically-valuable product (other definitions are valid). Crops are genetically distinct
from their wild relatives.
2. The change in human society from that of "hunter-gatherer" to "agriculturist" is recent and irreversible.
The neolithic revolution (the dawn of agriculture) began about 10,000 years ago, corresponding with the
end of the last ice age. There is evidence to suggest that plant agriculture was "invented" in various areas
of the world independently. The changes to both man, animals and plants resulting from this invention
were gradual and promoted a mutual dependency between man and agriculture.
Agriculture is a recent invention. About 90% of the humans who have ever lived made their living as
hunters-gatherers, 6% were agriculturists and only 4% were urban dwellers.
3. What happened?
A. Ecological prospective-biological vs. agricultural fitness- general characteristics of potential
domesticates.
A natural ecosystem includes the interrelated factors of: climate, soils, man, and other animals, and
plants.
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●
●
●
●
●
Natural ecosystems contain a diverse number of species.
The diversity of niches and organisms to fill them makes the system relatively stable and resistant
to change.
There is no net yield. All the energy arriving to the system via the sun is utilized by members of
the ecosystem.
Successful plants in the first ecosystem are those which are biologically fit (i.e. produce the
greatest number of offspring).
Plants in the community are subject to natural selection.
An agricultural ecosystem includes the interrelated factors of: climate, soils, man as the manipulator,
crop plants, and domestic animals.
●
●
●
●
●
Agricultural ecosystems contain a select number of species (few).
The number of niches in very finite so the system is not very stable (i.e. vulnerable to change).
There is no net yield (i.e. something to store or sell). The ecosystem outperforms the needs of its
members.
Successful plants in the second ecosystem are those which are agriculturally fit.
Plants in the community are subject to natural selection and to the active or passive selection
pressures for agricultural fitness by man.
The transition from biological fitness to agricultural fitness involves genetic changes in the
organism.
The characteristics of potential domesticates are that:
-they produce a useful product
-they are adapted to grow in disturbed soils (agricultural fields)
-they exhibit high levels of genetic diversity
Weedy species often exhibit these traits.
B. Genetic changes mechanisms for change
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Morphological and physical changes in species undergoing domestication include:
●
●
●
●
●
●
●
●
●
●
gigantism of horticultural/agronomic yield components (e.g. the development of corn ear from a
small terminal inflorescence bearing a few kernels).
loss of speed dispersal mechanisms (e.g. passive selection for non-shattering wheat rachis)
loss of delayed seed germination (e.g. hard seed coat in wild beans absent from domesticates)
loss of bitter or toxic substances (e.g. deleterious fats- canola: cyanogenic glycosides- canola and
cassava: antagonists to digestion- lima and other legume crops; bitter steroids- squashes and
melons; alkaloids-potato)
changes in photoperiodic responses (e.g. development of day neutrality in cotton)
changes in floral structures or pollination schemes (e.g. multiple petals- rose: increased
self-pollination - chili)
changes in flowering cycle (e.g. biennial bearing and development of tap root-carrot)
synchronous tillering (e.g. rice amenable to single harvests)
diversity of form (e.g. the multiple forms of Brassica oleracae - cabbage, cauliflower, broccoli,
kohlrabi, kale, Brussels sprouts, etc)
mechanisms to protect against predators (e.g. pendant rather than upright fruit in chili protecting
from predatory birds; the corn husk)
Genetic mechanisms effecting (causing) these changes include:
●
mutation- Mutations are sudden heritable changes in a gene. In wild populations, the natural
mutation rate is about 10-6 (a low frequency) and unless they offer a strong reproductive
advantage over the original gene, these mutant alleles remain at low frequency in the population.
However, under selection for agricultural fitness by and for man, the new trait may become fixed rapidly
(e.g. non-shattering rachis in wheat). Therefore, emerging crops and their wild and weedy relatives began
to diverge (separate).
●
human migration-As man moved into new areas, his crops went with him. Planting a crop in a new
area has several consequences including the passive or active selection pressure to adapt to a new
environment (e.g. short day cottons into northern areas with long summer days) and new contacts
with compatible weedy species.
●
introgression- Introgression is the transfer of small amounts of genetic information from one
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species to another. The initial step in this process is a chance interspecific hybridization between
an emerging crop (e.g., AA) and its relative (BB). Although the resulting hybrid (AB) is mostly
sterile (i.e., chromosomes from the crop fail to pair with those of the related species so the meiotic
process in the hybrid is hampered), sometimes a few viable gametes will be formed. With repeated
backcrossing to the crop, the fully-fertile crop type can be recovered. However, a small number of
useful genes from the related species will now be incorporated (AA B). "It is the genetic support of
their companion weed races" (Jack Harlan).
Stephens demonstrated the effects of introgression using two species of cotton -- upland cotton
(Gossypium hirsutum) and sea island cotton (G. barbadense). Sea island cotton, a short day plant (SDP)
that initiates flower buds as the days grow shorter, was crossed to an SDP segregate of upland cotton,
normally day-neutral (DN).
Gossypium hirsutum X Gossypium barbadense F1 Hybrid (genes are ½ Gh and ½ Gb)
The F1 hybrid was backcrossed to Gossypium barbadense for 11 generations. With each successive
generation, the percentage of Gh genes decreased and the percentage of Gb increased. After 11
backcrosses, there were few Gh genes left, but enough to demonstrate a high level of variability in
flowering times among BC11 progeny.
Weeks to flower among individuals in two Gossypium barbadense populations.
Weeks to flower
G. barbadense introgressed with G. hirsutum genes
G. barbadense (control)
11
2
0
12
4
3
13
17
2
14
33
0
15
48
0
No flowers
138
0
●
Polyploidization- Polyploidization is also initiated by the chance hybridization between an
emerging crop (AA) and its relative (BB). However, an additional event also occurred- a chance
doubling of chromosomes in the hybrid resulting in an AABB individual with twice as many genes
as either parent. Polyploidization may increase hybrid vigor through complementary gene action, a
dosage effect or due to greater tolerance for mutant alleles). Polyploidization, in some instances,
conferred new traits to crops also.
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All of these genetic mechanisms played major roles in the development of modern wheat.
●
wild einkorn- (AA, brittle rachis) to cultivated einkorn (AA, durable rachis)- a major step in this
process involved the incorporation of the mutation for non-shattering.
●
migration of einkorn from cultivation in the Zagaros Mtn. Foothills to the Fertile Crescent. Growth
of a crop innew environment (i.e., new selection pressures).
●
cultivated einkorn (AA) crossed with wild emmer (BB, wild relative with brittle rachis). Chance
doubling of chromosomes in the hybrid to form tetraploid cultivated emmer (AABB). Emmer was
adapted to a much broader range of soil types and environments than was einkorn. Emmer became
a crop of commerce and traded throughout the region. Emmer is the forerunner to the modern
durum or pasta wheats.
●
cultivated emmer brought to Iran (another migration) where chance crossing and polyploidization
with a third species (DD) resulted in formation of the hexaploid bread wheats (AABBDD). The
addition of the gene D genome did two important things-first, the seed storage proteins of emmer
were highly modified. New wheat seed storage proteins were high in gluten, a protein complex
with highly elastic properties which are responsible trapping yeast-derived CO2 in bread dough
causing it to rise. Second, the adaption range of wheat was greatly increased again allowing for its
cultivation in colder and drier climates. Eventually wheat culture was spread worldwide.
●
in addition, there is ample evidence that wheat gathered genetic material from many other species
as it evolved through the process of introgression, bringing disease resistance and additional useful
traits.
Why did human kind domesticate plants and animals? What conditions might have promoted the origin
of agriculture?
-Greek and Roman mythology suggest that agriculture was a gift from the gods to save the human race
from savagery.
-Judeo-Christians believe that man was forced to till the soil as punishment for sins committed in the
Garden of Eden.
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-Modern scholars (within the last 100 yrs or so) have put forth many theories concerning why
hunter-gatherer societies began to domesticate the plants and animals they used. Theories that supposed
that agriculture was "invented" out of desperation by starving people have been more or less dismissed as
inaccurate. There is ample evidence to evidence to suggest that hunter-gatherers had a relatively stable
economy and ecosystem, had a fair amount of leisure time (i.e., they supposedly worked about 15 hrs per
week) and enjoyed a well-developed society as evidenced by the art they left behind. The exact reasons
and methods of domestication are lost in antiquity. Moreover, as domestication occurred in many
different places throughout the world about the end of last ice age, these methods and reasons may have
varied from group to group. However, Carl Sauer (one of the domestication gurus) suggested some
conditions which were necessary for the domestication of plants to occur.
●
●
●
●
●
●
the domesticating society must already have a flourishing economic base- starving people are not
innovative and they can't afford to experiment.
the domesticating society must be oriented to food gathering as a way of existence (most likely
this was accomplished by females whereas males hunted)
the domesticating society must be partially sedentary
the domesticating society must live in areas (such as woodlands) where the soil is tillable using
crude implements
the society must be relatively safe from natural disasters which would discourage settlement. The
overall climatic situation was improving greatly at this time.
there must be a wide diversity of plants and animals to exploit. This condition is certainly true with
modern-day hunter-gatherers. African h-gs have been found to collect 60 species (ssp) of grains,
50 ssp of legumes, 90 species of root and tuber-bearing plants, 60 ssp of oil seeds, 500 species of
fruits and nuts , and 600 ssp of vegetables and spices. Their North American Indian counterparts
have been shown to collect over 1000 species of plants from 400 genera and 120 families of
plants.
In 1926, N.I. Vavilov published a teatise stating that diversity in plant species is not evenly distributed.
Crops were likely to have been developed in these Centers. See lecture outline for a map of the Centers
of Diversity (Fig. 4.1).
C. What happened to human society as a result of adopting an agricultural way of life?
Essentially, like the Greeks and the Romans believed, we domesticated ourselves as well.
1. Increased carrying capacity of the land
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Carrying capacity for human habitation under several cultural systems
Cultural system
Land capacity (people/mile2)
Food gatherers
Higher hunters and fishers
Simple cultivators
Pastoralists and nomads (possessing domesticated animals)
Advanced cultivators
21st Century man
2
20
50
100
150
???????
2. Formation of sedentary societies
3. Development of technology (e.g., serrated scythe, plough, wheel)
4. Development of crafts-job diversity
5. Development of new products
6. Emergence of the concept of property and ownership
7. Development of complex distribution system for goods and services
8. Formation of trade centers and trade routes
9. Development of a legal code-especially laws controlling water rights
10. Advent of architecture for storage and protection of property
11. Urbanization
The net results of the domestication process were
That plants and animals that were domesticated underwent significant and irreversible genetic change
(i.e. from biological or natural fitness to agricultural fitness) which makes them solely dependent upon
us for survival.
And, conversely, that humankind underwent significant and irreversible cultural changes (i.e., from
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hunter-gatherer to urbanized (civilized) society) which makes them solely dependent upon domesticated
plants and animals (i.e., agriculture) for survival.
6. Plant improvement and consequences
A. Formation of land races
Prior to modern breeding efforts crop varieties were land races. These land races were formed primarily
by the practice of saving seed for planting from year to year by farmers.
Land races of self-pollinating crops (individuals breed true) were mixtures of pure lines. Land races of
cross-pollinated crops (individuals do not breed true) were composed of heterogeneous populations with
each plant possessing a unique genotype.
Land races had the following characteristics:
●
●
●
they were endemic to a specific area or region
they were extremely well-adapted to the area because...
they were composed of a mixture of plant genotypes
DIVERSITY=STABILITY (remember our models of agricultural vs. natural ecosystems?)
The use of land races is relatively "safe" (for an ecological ecosystem) because the variability within the
variety buffers against biotic and abiotic stresses/hazzards thus, avoiding potential disasters. Land races
aren't particularly high yielding by today's standards, but when crop failure means starvation, a yield
every year is preferable to high yield one year and none the next
B. Modern plant breeding and genetic vulnerability
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The scientific age for plant breeding starts in the late 1800's. For most crops, this process resulted in the
gradual and systematic decrease in variablity (and genetic diversity). Many successful land races were
abandoned/lost.
For self-pollinated crops-methods included pure line selection and pedigree breeding (developing new
pure lines by inbreeding after crossing).
For cross-pollinated crops the method of choice was mass selection-formation of very successful
open-pollinated varieties such as Krugs Yellow Dent-improvement by mass selection is a slow process
with potentially diminished returns per cycle of selection.
To continue, the discussion will focus on corn (Zea mays), a cross-pollinated crop. However, before it
does, a few definitions must be given:
●
inbreeding depression - the loss of general adaptation and reproductive capacity associated with
the accumulation of homozygosity. Plants are weak, highly subject to environmental stress, and
are poor seed producers. Cross-pollinated crops suffer inbreeding depression, self-pollinated
crops do not!!!!
●
heterosis - the increased general adaptation and reproductive capacity associated with the
accumulation of heterozygous loci following the cross of two unrelated inbreds (i.e., the opposite
of inbreeding depression).
●
combining ability - a relative measure of heterotic response through the combination of any two
inbreds.
There are two theories why heterosis occurs:
●
The dominance theory suggests that a hybrid resulting from a cross between unrelated inbreds
displays heterosis because it possesses at least one dominant allele at a maximum number of loci.
●
The overdominance theory states that a hybrid resulting from a cross between unrelated inbreds
displays heterosis because it is heterozygous at a maximum number of loci. This explanation
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presumes that heterozygosity is superior to either homozygous condition.
Most cross-pollinated crops suffer inbreeding depression. Understand the consequences of inbreeding
(self pollinating) a cross-pollinated species by considering the fate of heterozygosity at a single locus.
Notice that ½ of the heterozygosity in the population is lost with each successive generation of self
pollination.
Decrease in heterozygosity at a single locus as affected by inbreeding.
Generation Frequency of Frequency of
Frequency of
Frequency of
parental
heterozygous
homozygous
homozygous
genotypes
offspring
dominant
recessive
offspring
offspring
0
1.0 Aa
1
2
3
4
5
6
1.0 Aa
0.50 Aa
0.25 AA
0.25 aa
0.25 Aa
0.375 AA
0.375 aa
0.125 Aa
0.4375 AA
0.4375 aa
0.063 Aa
0.469 AA
0.469 aa
0.031 Aa
0.484 AA
0.484 aa
0.50 Aa
0.25 Aa
0.125Aa
0.0625 Aa
0.031 Aa
0.016 Aa
0.25 AA
0.125 AA
0.25 AA
0.063 AA
0.375 AA
0.031 AA
0.438 AA
0.016 AA
0.469 AA
0.008 AA
0.484 AA
0.25 aa
0.125 aa
Frequency of
homozygous
individuals in
population
0
0.50
0.75
0.25 aa
0.063 aa
0.875
0.375 aa
0.031 aa
0.938
0.438 aa
0.016 aa
0.969
0.469 aa
0.008 aa
0.984
0.484 aa
The progeny of inbreds, i.e., F1, hybrids, usually display a great deal of heterosis, especially when they
are relatively unrelated. In other words, unrelated inbreds exhibit good combining ability because their
offspring possess a high level of dominant alleles or heterozygous loci (depending on what theory you
support.
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Even though the phenomenon of heterosis was known, it was not exploited in corn or other
cross-pollinated crops because F1 seed (the seed for sale) would have been produced on a very weak,
low-yielding inbred plants.
Until...D.F. Jones 1917 created the double cross below
Inbred A x Inbred B = Hybrid C
Inbred D x Inbred E = Hybrid F
Inbred C x Inbred F = Hybrid G
Seed of the Hybrid G born on Hybrid C (maternal parent) ears was what was sold to the farmer. Seed on
yield on Hybrid C plant exhibited heterosis.
Immediate increases in yield of over 25% were realized through the use of these double-cross hybrids!!
Later, as inbreds improved, F1 hybrids were produced directly which maximizes heterotic response.
.
The consequences of the use of F1 hybrids include:
●
●
●
●
yield and acreage increased (see original handout)
dependence on high yield
dependence on uniformity- increased mechanization, consumer demand for uniform product. Note
that the hybrid progeny of two inbreds are all genetically identical.
the loss of diversity. Land races and open-pollinated varieties which held considerable genetic
diversity were being abandoned. Within each field genetic diversity was essentially non-existent as
each plant is genetically identical.
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Scientists began to worry about the loss of diversity as early as the 1930's. Then in the 1960's the FAO
began cataloging variability in major crops, but no collections were made.
The triggering event in 1970 was an outbreak of Southern Corn Leaf Blight in the US caused by the
fungus Helmenthosporium maydis. About 15% crop losses, primarily in southern states. Because of the
method in which hybrid corn was produced, almost all corn in US was uniformly susceptible. This could
have been a disaster.
The epidemic and near disaster sparked a flurry of scientific and political interest to collect and preserve
plant germplasm, especially close crop relatives. In the 1980's combating the genetic vulnerability of
crops became a national priority.
C. Loss of diversity in natural communities
The loss of natural habitats for plants (and animals) due to human development is a process that has been
occurring throughout our history as cultivators. Ohio, after all, was not originally covered with
monocultures of corn (Zea mays) and soybeans (Glycine max), but rather with hardwood and white pine
forests. Clearing land for agricultural production and other uses, even if buffer zones of diversity are left
in tact, has major consequences on gene pools.
In our lifetimes, perhaps the most notable examples of wholesale habitat destruction is occurring in
Africa and South America. Deforestation is occurring in rainforests and in other areas as well at an
alarming rate. These events certainly have caused the loss of important germplasm and may be effecting
our global weather patterns (i.e., global warming through the greenhouse effect). It is beyond the scope
of this course to delve into this controversial subject deeply, but for those who are interested, I urge you
to explore the topic on your own.
See Fig. 1.1 and Table 1.2 in your lecture outline.
D. Efforts to preserve germplasm
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"To feel a world population growing by up to 160 people per minute, with >90% of them in developing
countries, will require an astonishing increase in food production...Access to a range of genetic diversity
is critical to the success of breeding programs. The global effort to assemble, document and utilize these
resources is enormous, and the genetic diversity in the collections is critical to the world's fight against
hunger." (Hoisington et al., 1999).
Potentially useful genetic resources for combating genetic vulnerability include:
● •current varieties
● •obsolete commercial varieties
● •breeding lines
● •induced or natural mutations in breeding lines
● •old land races
● •primitive forms of the crop
● •related weed races
● •related wild races
Collections for individual crops should focus upon material in their Center of crop origin/diversity
(especially for disease resistance genes).
In-situ vs. ex-situ collections and their maintenance.
"The conservation of germplasm can be managed according to two models: in situ, in its place of origin,
or ex situ, outside its place of origin, as in zoos, botanical gardens, and germplasm banks. In situ
conservation, clearly the more complex of the two attempts to protect species under the natural
conditions in which than are normally found, be they pristine or anthropogenic habitats. In contrast with
ex situ conservation, which saves germplasm under artificial conditions, in situ conservation seeks to
maintain the genetic diversity of the species under the conditions in which it evolved so as to allow the
process of adaption to continue". (B.F. Benz).
National/international efforts to combat the problem include:
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http://www.ars-grin.gov/npgs/
CGIAR -visit this web site to learn morehttp://www.cgiar.org/
FAO Seed and Plant Genocide Resources Service -AGPS - visit these web sites to learn morehttp://www.fao.org/ag/AGP/Welcome.htm
http://www.fao.org/ag/AGP/AGPS/prg/global.htm
7. Summary of crop domestication effects
●
●
●
●
●
●
●
●
•There was a gradual change in way of life from hunter-gatherer societies to agrarian societies.
•Cultivation brought about new selection pressures (active or passive) on emerging crops.
•Emerging crops underwent significant genetic changes to become "agriculturally" fit. In the
beginning, genetic variability increased as crop-weed complexes interacted.
•Domestication was irreversible.
•Domestication occurred most often in Centers of Diversity.
•Domesticators had broad-spectrum economy. They were not starving.
•Origin of agriculture caused an irreversible cultural revolution in human society. Sedentary life
greatly increased the carrying capacity of the earth and the complexity of civilization.
•Agriculture as it evolves continues to both solve problems and, at the same time, create others
such as the example given above with F1 hybrid corn.
8. Crop Science- its role and challenges in 2000 and beyond?
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●
●
●
●
●
•Solving problems
•Reducing chemical inputs
•Reducing risks for and instances of environmental degradation
•Devloping new commodities-value added
•Maintaining profitability in a global economy
•Feeding an ever-increasing world population
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●
•?????????????????
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #2: Crop classification
References:
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science growth,
development and utilization of cultivated plants. Prentice Hall,Inc., Englewood Cliffs, NJ.
Outline:
Botanical classification by binomial nomenclature
There are over 500K species of plants - classification and naming system is important to be able to
communicate about plants and to show relationship between them.
Classification of plants was first attempted by Theophrastus, the "father of botany", in the 3rd Century
BC. He based his classification schemes primarily on plant form, growth habit and differences in flower
structures.
Today, we actually use many different classification systems in everyday communication (see below),
but perhaps botanical classification (the binomial nomenclature) system used by scientists is the most "
information-rich" and least ambiguous of these naming systems.
This system was developed by Carlus Linnaeus in the 18th Century AD. His was also based primarialy on
flower morphology. Living organisms were divided into groupings (taxa) at many different levels of
complexity. The first division, " Kingdom" is the most general (refer to Table 3-1 from Hartmann et al in
handout); with each subdivision thereafter, the descriptions of members in the taxa are more specific and
the number of members within the taxa decreases until at the species level, an individual plant with
unique characteristics is identified. Today this system also considers other factors (e.g., genetic evidence)
to distinguish one species from another. Note: the classic definition of species presumes that members of
the species can freely interbreed and that member of different species can not, but in the case of plants,
this is not clearcut.
In terms of crops Members of the Plant Kingdom are thought to have some common characteristics: they are stationary,
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contain chlorophyll and photosynthesize, they have cells with rigid walls made of cellulose,and continue
to grow throughout their life cycle. However, there are organisms that are considered plants which do not
conform to one or more of these characteristics.
I can't think of any crops that are not considered to be in the Plant Kingdom. Although we do use
products derived from bacteria (genetically engineered or otherwise), we commonly don't think of
production of this type as "cropping". Almost all of the crops that we study will be in the Division of
Spermatophyta - seed - bearing. Mushrooms(edible) and ferns(ornamental) are two exceptions.
Moreover, most crops that are grown are in the Class Angiospermae - plants that produce seeds inside
ovaries. A notable exception to this statement, of course, is the production of conifers ( Members of the
Gymnospermae - plants that bear naked seeds) as ornamental or industrial(forest) crops. Order, Family,
Genus and Species will vary from crop to crop.
Genus and species names are either underlined of italicized to indicate that they are in Latin. The former
is capitalized while the latter is not e.g.,Agrostis stolonifera - creeping bent grass. Note that the species
name is often descriptive - ie.,stolonifera referring to the fact that the plant produces lateral above ground
stems called stolons.
Question: Can you identify some of the important families which contain crop plants? What are some
examples of crops (genus and species) that belong in these families? Do they have some characteristics in
common?
cultivar - a name derived from the term "cultivated variety". Cultivars describe a subject of plants within
a species which demonstrate some recognizable uniformity in traits. This uniformity results primarily
from mans efforts through breeding and selection, even though the genetic path by which uniformity is
achieved and maintained varies depending on whether the plant is cross pollinated, self pollinated or
asexually reproduced (discussed in detail in a later lecture). Cultivar identities are extremely
important to producers because indicate what to expect in term of performance in the field(growth rate,
flowering time , potential yield, etc.), resistance to diseases and other pests, and crop quality factors, etc.
Confusion about what cultivar is being planted may lead to cropping disasters - consider your chagrin
when you discover that your field of peppers is yielding chilies instead of bell peppers - the crop you
were contracted to grow. This mix - up actually happened and, of course, resulted in some serious
litigation.
Cultivar designations are made in one of two ways: E.g.,Jubilee sweet corn should be designated as Zea
mays ' Jubilee'. Note that the cultivar name is always capitalized and never italicized or underlined.
- botanical variety - Botanical varieties also describe subspecies with specific traits. The difference is that
botanical varieties are wild segregates. For instance in Aesculus parviflora,(a type of Buckeye tree) there
is a botanical variety called serotina, which blooms several weeks later that its partent species. It is
designated as Aesculus parviflora var. serotina.
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- group - Group is another subspecies classification. In a species with great diversity of form, the group
designation literally "groups" cultivars with characteristics. E.g.,Brassica oleraceae Italica = broccoli,
Gemmifera = Brussels sprouts, Capitata = cabbage, Botrytis = cauliflower, Gonglylodes = kohlrabi,
Acephala = kale and collards. Roses would be another example of a species with groups.
-hybrid (interspecific) - Interspecific hybrids are example of how the "species" designation can get
somewhat "muddy" when considering plants. Case in point, Fragaria chiloensis ( the beach strawberry)
was isolated from Fragaria virginiana ( the Virginia strawberry )in the wild, separated by 1500 miles of
prairie, mountains and deserts between the Mississippi River and the Pacific Coast. However, when
brought in close proximity to each other in the 18th century botanic gardens of Europe, these two "
species" hybridized readily to form Fragaria X ananassa the cultivated strawberry of commerce. The
latter is a "species" formed by a planned interspecific hybridization, therefore to designate it as such ,
there is an X between its genus and species names.
-the problem with common names
Consider a home owner entering a nursery and asking to jasmine (see list on handout). If you were an
employee of this nursery, would you know what to get him/her? You might be in trouble even if they
asked specifically for "star jasmine" because several species are commonly referred to by that name. In
this case a single common name is used to designate several species. Can you think of a similar example?
The other possibility is that a single species is known by several common names. Often these names are
specific to regions. My ex-father-in-law, a native Ohioan, called bell peppers "mangos" his entire life as
did many others in SW Ohio. The first time he referred to this, I presumed he meant the fleshy tropical
fruit that I know by that name. This problem is not confined to reference to horticultural crops - If you
refer to alfalfa in many other countries of the world, they won't know what you are talking about - they
call that crop "Lucerne". In many regions of Africa, ground nuts are an extremely important food crop
providing protein and energy. One of our ex-Presidents was a ground nut producer at one point in his life,
but he called them peanuts. Some of his fellow Southerners might also refer to this food as "goober
peas".
A complex example of this problem involves the term "corn". In this country, corn means the grain from
Zea mays. The same grain in England is called maize, and if you say corn, they think you are referring to
wheat or barely.
Classification by production - There are obviously many different ways to classify crop plants in addition
to their botanical classification using binomial nomenclature. For instance, plants could be grouped as
temperate or tropical, annual, biennial or perennial, etc. Some useful classifications for out purposes are
discussed below:
One very useful scheme is to consider our overall use of the plant/or in other words, how much of the
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plant do we produce. To develop such a scheme, one might consider data on either land area devoted to
producing each crop or to their overall yield (tonnage produced). Table 1.1 on your handout classifies
crops as either edible or industrial, then within each of these groups separates species by their overall
yields. Notice that of the five crops the world relies upon for food, four of them are grain crops (sort of
--- look at the footnotes in the Table --- one might argue over their definition of "food crop").
Classification by nutritive value = One might also consider classifying crops by the nutritive value they
deliver. Certainly, in the grand scheme, hunger in the world is a matter of protein-calories malnutrition
(See Table 1.2 on your handout). Note that there are some crops in this Table that are a fair source of
both. However, other crops are also important for nutritive reasons, especially those which provide
vitamins, minerals, dietary fiber, and other compounds that promote human health. Crops that produce
secondary plant products that improve or maintain health (i.e., anticancer compounds, compounds that
effect mood or performance) have been recently referred to as nutraceuticals. Documentation concerning
the effectiveness of these secondary products varies tremendously from crop to crop. However, in
general, these crops enjoy limited production and high profit margins (e.g.,ginseng).
4.Classification by crop use - perhaps the most useful (Class - derived list)
Grains-corn,wheat,sorghum,oats,rice,spelt,rye,tritical, millet
Pulses- (beans) common bean, soybean,cowpeas,chickpeas,fava beans,peas
Oil crops- canola,safflower,peanuts,olives,coconut, sunflower,flax
Forages - clovers,alfalfa, timothy,birdsfoot trefoil
Fiber crops-cotton,flax,coconut,hemp,sissal
Sugar crops-cane,sugar beet, corn(various corn syrups)
Medicinal crops-marijuana,Echinacea,ginseng,Ginko biloba
Herbs, spices and stimulants - tobacco,cinnamon, sage
Vegetables- cabbage, zucchini, peppers, tomato, pumpkin, watermelon
Fruits- strawberry, blueberry, peach
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Ornamental crops - red maple, dogwood, parsley, gourds, cabbage, pumpkins
Turf- tall fescue, Kentucky bluegrass, Zoyzia, Bermuda, bentgrass
Industrial crops- soybeans, pulpwood, rubber, southern yellow pine
Note that many crops can be fit in multiple categories.
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #3: Crop Plant Morphology
References:
Text = Chapter 3
Campbell, N.A. 1996. Biology. Benjamin Cummings Publ. Co., Menlo Park, CA = Chapter 34.
Copeland, L.O. and M.B. McDonald. 1985. Principles of Seed Science and Technology. Burgess, Publ.
Co., Minneapolis, MN.
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science - growth,
development and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ = Chapter 2.
Pollock, B.M. and V.K. Toole. After-ripening, rest period and dormancy. In. Seeds - 1961Yearbook of
Agriculture. U.S. Govt. Printing Off., Washington, DC.
Raven, P.H., R.F. Evert and S.E. Eichhorn. 1999. Biology of plants. W.H. Freeman and Co., New York,
NY = Chapters 23-27.
Toole, E.H. and V.K. Toole. 1961. Until time and place are suitable. In. Seeds - 1961Yearbook of
Agriculture, U.S. Govt. Printing Off., Washington, DC.
Quotation:
".......plants and their features can be identified and appreciated from their external structure, but their
internal structure and function are often overlooked. The beauty of an orchid blossom is greatly admired,
but just as impressive are the parts of a cell as recorded with a scanning electron microscope"......
Hartmann et al., 1988.
Outline:
1. Crop Life Cycles (in brief)
A crop's life cycle is determined by the seasonal pattern it exhibits between seed germination and the
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development of mature seed for the next generation. As you may guess, crops vary tremendously in this
respect and the definition of "crop" life cycle is confounded significantly when one considers crops that
are typically asexually reproduced (e.g., chrysanthemum). However, even in the latter case, we can still
consider the "seed to seed" model with respect to traditional crop breeding.
Most agronomic crops and quite a few horticultural crops are considered to be annuals. That is, they
complete the crop life cycle in a single season [i.e., it germinates, grows, flowers, fruits, matures seed
then senesces (dies)]. Almost all of them are herbaceous (non-woody). The annual life cycle begins with
the germination of the seed and emergence of the new plant (see diagram in handout) which is usually a
relatively rapid process. The rate of seed germination is species specific and can be highly influenced by
a number of environmental factors (see detailed discussion below). During germination and for the first
few days after emergence, the new plant is nourished by stored reserves present in the cotyledons or
endosperm of the seed. As these reserves are depleted, newly developed leaves begin the photsynthetic
process and newly developed roots begin to supply water and nutrients to the growing plant body. For a
period of time, the plants develop vegetatively (i.e., their "growing tips" and "buds" produce only new
leaves, stems or roots. The length of the vegetative phase of the annual life cycle varies per crop and may
be as short as a few weeks (e.g., certain members of the Brassicaceae or cabbage family) or may last up
to nearly a year (e.g., banana). However, at some point, one or more vegetative shoot growing tips or
buds undergoes a transformation (floral initiation) which re-programs it to produce flowers and fruit. To
ensure that flowering occurs at the most opportune period for plant survival and/or agronomic fitness,
floral initiation is often triggered by an environmental trigger, such as the day length (see discussion
below). Floral development progresses over time and when its development is visually obvious, it can be
said to have emerged. The floral development process continues for a period of time before the flower is
ready to open. During this period, pollen and egg cells are typically developing and are usually fully
formed (or nearly so) at the time of flower opening (anthesis). Flowers vary tremendously with respect to
the exact timing of pollen shed or stigma receptivity; in some species floral structure, pollen shed and
stigma receptivity favor self-pollination of the flower (e.g., pea) whereas with others, these phenomenon
are arranged to promote cross-pollination (e.g., corn). Pollen tube growth through the style is usually a
rapid process requiring from 1-3 days to complete. The period following fertilization is marked by
development of both fruit and seed structures in a set pattern which varies with species (see discussion
below). During this period, much of the energy produced by the plant and the mineral nutrients it
acquires from the soil/media are devoted to the fruit and seed maturation process. Therefore, fruit and
seed development accounts for much of the addition of dry weight to the plant during this period. Some
annuals have distinct vegetative and flowering phases (e.g., cereal grains) and are said to be determinant,
whereas others are indeterminant and continue to grow vegetatively while flowers are periodically
produced (e.g., some types of beans and tomatoes).
Biennial plants require two seasons to complete their life cycle (e.g., carrots and onions), although in
some cases we crop them for only one season because the yield component of interest is not the seed.
Biennials are typically plants with a rosette form (i.e., vegetative shoots are extremely short and leaves
appear to be in a whorl pattern). Floral initiation occurs during the first season of growth but is arrested
early in its development, resuming only after receiving an appropriate environmental signal. In temperate
crops, this signal is a period of chilling requiring temperatures from 1-7° C (34-45° F). The process of
chilling is called vernalization. Once vernalized, floral development advances with the formation of a
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blooming stalk (typically attaining heights much in excess of the leaf canopy) and a "seed head" or
flower. After seed is matured, the plant usually dies. In the case of carrots and onions, the root or bulb
that we eat serves as a storage of food reserves that are almost entirely consumed by the plant in order to
flower and mature seed.
Herbaceous perennials (e.g., tulips, chrysanthemums, etc.) annually produce aerial vegetation and
flowers from perennial plant parts [crowns (stem bases), rhizomes (underground stems) or bulbs]. The
patterns of above-ground growth, flowering, seed maturation and senescence vary dependent upon
species.
Woody perennials produce top growth over a period of years, adding new stems, leaves, flowers and
fruit every year while typically increasing the girth (thickness) of stems and roots produced in earlier
seasons. Growth therefore is cumulative. Annual growth often occurs in flushes followed by sessation
and the "setting" of a terminal bud. Many hardwoods (e.g., oaks) have only one growth flush, others
(e.g., some pines) exhibit recurrent growth flushes throughout a season, whereas others exhibit sustained
growth throughout the season. Optimum conditions must be maintained during the production of woody
perennials, because growth flushes can terminate prematurely if stressed by unfavorable environmental
conditions.
1. Seed Germination
"One for the buzzard, One for the crow, One to rot, and One to grow!" (Fay Yauger)
"In many ways, the seed is a microcosm of life itself. The seed is a neatly wrapped package containing a
living organism capable of exhibiting almost all of the processes found in the mature plant." (Copeland
and McDonald, 1985).
"A seed is essentially a young plant whose life activities are at a minimum" (Toole and Toole, 1961).
"The [biological] function of a seed is to carry its embryonic plant through the hazzards of time and
space to a time and place where the new plant can grow, flower, and in its turn, produce seeds" (Pollock
and Toole, 1961)
Crop life cycles often begin with the germination of seeds. Each seed contains the following: a) an
embryonic plant that has a radicle (embryonic root) and a plumule (embryonic shoot); one or two
cotyledons that are used as a food source by the embryonic plant until it is able to photosynthesize on its
own; (NOTE: in moncots, this function is also performed by the endosperm, a storage tissue for starch
and other compounds) and c) a method of protecting the embryonic plant (seed coat or fruit structures).
Shown in your original handout are the first phases of the lives of the quintessential monocot (corn) and
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the quintessential dicot (bean). Notice some of the similarities and the differences in their overall
structure.
As in all other phases of a crop plant's life, environmental conditions have a profound effect on plant
performance. The exact conditions necessary for optimum seed germination are specific to each crop
species. However, in general seeds often germinate best when the following requirements are met:
●
●
●
●
●
Adequate soil or media mosture content. Seeds will not germinate in dry soil or media. Likewise
they will not germinate under waterlogged conditions. Certain types of rice and aquatic species
represent notable exceptions to the latter for they will not germinate unless they are under water.
Proper temperature (15-26°C or 59-79°F common)
Adequate soil or media aeration
Soil or media free of diseases and other pests
Soil or media with low salt concentrations
In addition, some seeds require light where others require darkness to germinate.
A seed is said to be quiescent (resting) if it fails to germinate unless the above mentioned conditions are
met. The quiescent seed is physically and physiologically ready to germinate, but awaits the proper
conditions before doing so.
A seed is said to be dormant if it fails to germinate even though the above mentioned conditions are
met. Dormant seeds are not yet physically or physiologically ready to germinate no matter what
environmental conditions are present. There are several common forms (types) of seed dormancy; a few
are listed below:
●
Hard seed coat (a physical dormancy) - the seed coats of some species (most notable examples are
in the Fabaceae, the bean family) are either highly lignified or covered with waxy or oily
substances (cutin and/or suberin) so as to be impervious to water and or gasses. Germination can
only occur after the seed coat has been ruptured or breached. In nature, the disruption can result
from conditions brought about by heavy rains (abrasion), fire, or consumption by birds or other
animals (acid digestion).
Embryo dormancy (a physiological dormancy). In many temperate zone species, seeds physically
mature on the plant, but are physiologically unable to germinate until exposed to cold temperatures
over a prolonged period of time. There is evidence to suggest that during the cold period, levels of
a growth-inhibiting hormone (abscisic acid) decrease while levels of growth promoting hormones
(gibberellins and cytokinins) increase. Similarly, in some species (e.g., Cucurbitaceae, the squash
family) allowing physically mature seeds to "after-ripen" in detached fruit will increase their
germinability.
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Rudimentary embryos (a physical and perhaps physiological dormancy). Some species (e.g., holly,
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magnolia) shed their fruit before fully maturing the seed. The seed continue to develop in or
outside the fruit until competent to germinate.
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Chemical inhibition (physiological dormancy). Chemical inhibitors to germination (e.g., caffeic
acid, coumarin) can be present in the embryo itself, in the seed coat or in the fruit tissue
surrounding the seed. These compounds must be metabolically inactivated, leached, degraded or
removed in some other way before germination can occur. For instance, the gelatinous material
surrounding a tomato seed contains an inhibitor which prevents the seed from germinating inside
the fruit. This inhibitor must be removed prior to germination.
Under natural conditions, dormancy is an important phase of a seed's life as it often ensures that the seed
does not germinate inside the fruit (vivipary) or does not germinate when environmental conditions are
unfavorable. Delayed and staggered germination is a undoubtedly a selective advantage in a natural
ecosystem, but it is not a trait commonly associated with agricultural fitness. See discussion of the
difference between natural and agricultural fitness in Topic 1 notes.
As agriculturists, we often "process" seed in order to overcome dormancy using a variety of techniques.
Hard seededness is often alleviated through various scarification (scratching) methods using abrasives
and mechanical devises to move seed across abrasive surfaces. Hot water treatments are also sometimes
used as are acid scarification treatments using concentrated sulfuric or hydrochloric acids (these are
somewhat dangerous). The exact timing and conditions of these treatments necessary for optimum
success vary tremendously with species (of course).
Embryo dormancy can be overcome by stratification, a process which can be done outside under
"natural" conditions, but is more commonly practiced under controlled conditions. The critical factors in
the stratification process include:
●
chilling temperatures - (1-7°C or 34-45°F). The physiological processes necessary to alter
hormone levels progress under these conditions. If the temperature is below freezing or above
critical temperatures, the processes are delayed or halted
●
Moisture - Seeds are usually soaked prior to stratification and then placed in moist sand or paper to
keep them moist (but not immersed) throughout the stratification procedure.
●
Oxygen - Oxygen is necessary for continued respiration which keeps the seed viable and
physiologically active. Therefore, stratification procedures should not be conducted in air-tight
containers or under water-saturated conditions.
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Time - The period of time necessary to complete the stratification process varies tremendously
with species and must be determined experimentally. However, 30-90 days is a typical time frame.
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Chemical inhibition may be overcome in a variety of ways depending on the inhibitor and on whether or
not the inhibitor is internal or external to the seed. When extracted from the fruit, the gelatinous matrix
surrounding tomato seeds must be "fermented" (removed) in order to remove it and the inhibitor it
contains. In other species, inhibitors may be overcome by leaching (continued washing).
Seed viability vs. maturity
A seed which has reached physiological maturity when it has reached maximum dry wt (typically
5-20%). Other conditions which act as indicators of maturity include the sessation of nutrient importation
and the formation of abcission layer at hilum (area where seed is connected to ovary tissue by a "stalk"
called the funniculus).
Most seeds viable before they are mature. For example, weed seeds (colonizing species) especially adept
at producing new plants even when seeds are not fully ripened on the plant or under environmental
situations that are not suitable for the germination of seed from truly wild (feral) or cultivated crops.
For the seedsman, it is important to determine when maximum seed maturity is reached so that the seed
they sell will be highly vigorous.
The physiological steps in the seed germination process.
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The imbibition of H2O. Water is taken up by the seed throughout the germination process in three
distinct phases (see diagram in handout). The first "log" phase of imbibition is passive (i.e., it does
not require the espenditure of the seed's energy) and relatively rapid. The rate of water uptake is
determined by the difference between:
- The matric and osmotic potentials (pulling forces) in the soil/media as determined by the type of media
and the level of dissolved salts that it contains and.
- the osmotic potential (pulling force) and turgor pressure (pushing force) of the seed's cells.
In order for seeds to imbibe water, their osmotic potentials must be greater (more negative) that the soil
forces combined (i.e., the seed wins the tug-of-war for the water in the environment). If water in the soil
remains adequate, the seed will continue to imbibe water until cells are fully hydrated. At that point
turgor pressure will be great enough to prevent further movement of water into the cells (i.e., the
beginning of the lag phase shown in the handout diagram).
The rate of uptake in this phase is influenced by the composition of the seed (protein-rich seed usually
imbibes water faster than starchy seeds), and seed coat permeability
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An environmental trigger? As stated above, some seeds require light in order to germinate. The
compound in plants that senses light and the duration of light and dark periods is called
phytochrome. Phytochrome, in its active form, promotes membrane permeability, and stimulates
enzymatic and metabolic activity (see below). Phytochrome-mediated triggers often act in concert
with or can be complicated by temperature factors. For more on phytochrome, see discussion on its
effects upon the flower initiation process (daylength sensitivity).
●
Enzymatic and metabolic activity. As the first log phase of imbibition ends, a "lag" phase begins,
characterized by a greatly diminished rate of water uptake (see diagram in hand out). This period
corresponds with an acceleration of metabolic and enzymatic processes within the seed as it
prepares for growth. In this period, membrane-bound enzyme systems are activated and active
transport (ATP-requiring) transport of ions and solutes across membranes is promoted. Gene
expression occurs. Enzymes are also synthesized -- especially hydrolytic enzymes that control the
glycolytic process (metabolism of sugars for the production of energy). Hormonal contents of
seeds may also be altered so as to favor growth promoting substances over inhibitors.
Some aspects of cereal crop (e.g., corn, barley) seed germination illustrates how some of these processes
are coordinated within the germinating seed. In cereals, the first step in the process of enzymatic
stimulation is based upon the synthesis of gibberellic acid (GA3) in the scutellum (cotyledon) and its
subsequent transportation to the protein-rich cells of the aleurone layer cells (i.e., the outer layer of the
endosperm or storage tissue). The GA3 acts as a messenger to the aleurone cells, "informing" them that
the conditions are now adequate for germination to occur. Peleg and coworkers demonstrated the
necessity of the GA3 as a messenger in barley seeds (see figure in handout). If the embryo of barley is
removed (i.e., including the scutellum), hydrolysis of starch in the endosperm does not occur. However,
if it is exogenously-treated with GA3, the breakdown of carbohydrate progresses rapidly. Thus, the
identity of GA3 as the messenger is confirmed. Exogenous application of GA3 breaks dormancy and/or
stimulates germination in many species (both monocot and dicot) and many species have been shown to
produce GA3 during the lag phase of the germination process. In other species, including most dicots, the
stimulation of metabolic processes by hormones is much more complex and is based upon a balance of
promoting and inhibiting substances.
●
Initiation of growth. The cell division and elongation necessary to form the new plant begins
during the lag phase as metabolic and enzymatic activity progress. During this process, storage
tissues decrease in dry weight while tissues in the embryo increase in dry weight.
●
Protrusion of the radicle. In most instances, the protrusion of the radicle is the first physical sign of
germination and is an indication of the seed's viability. However, at this stage, many "slings and
arrows of outrageous fortune" (sorry Will) may still prevent the ultimate establishment of the new
plant. The protrusion of the radicle also signals the beginning of the second log phase of water
uptake. Rapid growth of both the root and shoot systems depends upon the rate of cell division and
cell elongation which, in turn, requires optimum cell turgor pressure (pushing forces on cell walls
and membranes caused by water uptake).
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Emergence and seedling establishment. Emergence is the process wherein the shoot of the new
plant breaks through the soil/media surface and becomes aerial. The factors which promote or
inhibit this process (e.g., whether or not the soil surface is crusted) are numerous and primarily
soil/media dependent. There are several classic categories of emergence based primarily upon the
relative position (above or below ground) of the cotyledons and other seed tissues. See any plant
science text book, if you are interested in learning about these - we won't consider them further
here).
As the embryonic plant begins to grow, it undergoes a planned (genetically programmed) sequence of
tissue development which it continues throughout its life. Although there are obviously some major
anatomical and physiological differences among higher plants, there is also substantial commonality in
the types of tissues that are present.
3. Meristems
All higher plants have meristems. Meristematic regions are composed of undifferntiated,
parenchymatous cells which, when active (not dormant) are rapidly undergoing cell elongation and cell
division. Meristematic tissues give rise to permanent tissues (containing mature, often specialized cells)
which comprise the bulk of the plant body. Meristems can be found in various places on the plant.
One might argue that the most obvious and logical place for a meristem to be is at the growing point
(tips) of shoots and roots. It is so, and these meristems are referred to as apical meristems. Apical
meristems of above-ground shoots give rise to cells that will eventually become the shoot epidermis,
cortex, primary xylem and phloem and central pith. Some shoot apical meristems remain vegetative
throughout their life while others undergo a transition to a flowering meristem in response to internal
dictates and/or environmental cues. Some shoot apical meristems experience several seasons of growth
punctuated by periods of dormancy. Apical meristems are also the source of axillary buds which, when
allowed to develop, form new shoots (branches), inflorescences or both. Axillary bud growth is often
suppressed by plant hormones produced by the apical meristem resulting in the phenomenon of apical
dominance. Root tips also have apical meristems that give rise to the various tissue systems of the root,
the stele, the root vascular system, the pericycle, the endodermis, the cortex and the epidermis.
Shoot apical meristems also give rise eventually to other meristematic regions such as subapical
(axillary) and lateral shoot meristems. Sub apical meristems are located just "underneath" the apical
meristem. Some plants grow vegetatively following a "rosette" growth habit where internodes are very
short and leaf petioles are in a compact bunch close to the crown of the plant. If such plants also have a
terminal flowering habit, like carrots, for instance, one might expect the flower to also be very close to
the ground. However, in these instances, after the terminal meristem has already transitioned to a
flowering meristem, the subapical meristem produces the cells necessary for the development of an
inflorescence or bloom stalk.
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Lateral meristems refer to cell layers in stems that give rise to new xylem and phloem members (the
vascular cambium) or to new bark (the cork cambium). The vascular cambium in woody plants is
cylindrical and the development of new xylem elements inward results in increased girth and the
development of annual growth rings due to differential rates and cell sizes of xylem production
throughout the season. Phloem gets produced outward from the cambium but as the season progresses,
older phloem members get crushed and obliterated. Herbaceous stems have vascular cambiums as well,
but these meristems are arranged with vascular tissue in "bundles" (i.e., not in a continuous cylinder).
Intercalary meristems are meristems that have been separated from the shoot terminal meristem by
intervening mature tissue. One good example of an intercalary meristem is that which is associated with
the lower regions of a grass leaf sheath. Grass blades grow from this meristem rather than from a
terminal meristem. I.e., they elongate from the base of the blade, not the tip of it.
4. Primary and secondary development of stems
Stems and branches are the scaffolds which support leaves, flowers, fruit and other above-ground plant
organs. A typical shoot (Figures 2-6, 26-3 and 26-7) has the following tissue groups: epidermis, cortex,
primary xylem and phloem and central pith. It is punctuated in its growth by the development of nodes.
Nodes typically contain a leaf supported by a petiole and an axillary bud which possesses a dormant,
flowering or vegetative meristem of its own. A repeating stem unit containing a leaf, node, internode and
axillary bud is called a phytomere. Each node (leaf axil) must be supplied with a vascular trace. In stems
that are young, the vascular system is arranged in bundles which are located just interior to the cortex.
The arrangement of these vascular bundles is well ordered but complex. Three typical arrangements are
shown in Figure 26-7. Many dicot stems (e.g., basswood) have vascular bundles that more or less form a
ring around the pith (Fig. 26-7 a), whereas others (e.g., elderberry) have more descrete vascular bundles
that have wide interfascicular regions (spaces) between them (Fig. 26-7 b). Monocots (e.g., corn) and
some herbaceous dicots have vascular bundles scattered throughout the central cylinder.
Secondary growth occurs primarily in woody perennials. It can be defined as an increase in girth
(thickness) of stems or roots in regions where individual cells are no longer dividing or elongating. In the
case of stems, this growth is characterized by presence and activity the vascular cambium, a lateral
meristem. The progression of from primary growth to secondary growth in elderberry is illustrated in
Fig. 27-6. Note that in secondary growth, the interfascicular regions disappear and the vascular cambium
forms a complete cylinder around the xylem and pith. The vascular cambium continues to produce xylem
and phloem tissues and also a system of vascular (fluid conducting) rays which connect (radially) the
various layers of xylem and phloem produced in successive growth phases. The vascular cambium
increases in girth by anticlinal (lateral) cell division.
What we commonly refer to as wood is secondary xylem. Secondary xylem that is still functional is
called sapwood whereas that which no longer conducts is called heartwood. The transition from sapwood
to heartwood often involves the loss of food reserves and the infiltration of oils gums and resins and
tannins which give woods their characteristic color and odor. In a given season, new xylem members that
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formed early are wider, thinner-walled and less densely packed than those which are formed in late
season. This pattern coupled with periodic yearly activity of the cambium results in "annual growth
rings" which give wood its characteristic grain and arguably, its beauty.
Any tissue located to the "outside" of the vascular cambium is considered bark. Bark includes operational
and defunct phloem, periderm if present and epidermal-derived tissues. As girth increases, older phloem
members are crushed and there is a tremendous stress on outer layers of bark resulting in its
characteristic irregular surface. This material eventually sloughs off as new phloem are formed in
subsequent seasons. Common products that we obtain from secondary phloem growth include bark
mulches, maple syrup (tapped from active phloem members nearest the cambium - for an instructional
website visit http://www.mi-maplesyrup.com/howto.html), and cork.
5. Primary and secondary development of roots
Root systems have five functions: to anchor the plant, to absorb water and nutrients, to conduct water and
nutrients to aerial portions of the plant, to synthesize plant hormones (primarily cytokinins) and to act as
a storage organ for carbohydrates. Like the shoot or stem, primary root growth is a function of the root
apical meristem and, in turn, tissues derived from this meristem develop into primary meristems, primary
tissues and eventually, secondary growth. In some species, a quiescent center with reduced cell division
forms within the meristematic region which may play a role in the organized development of root tissues.
The apical meristem is protected by a rootcap; root cap cells are scraped off as the root penetrates the
soil/media and mucigel, a "slimy" substance associated with the cap helps to lubricate the entire process.
The region of [cell] elongation, typically only a few mm in length, constitutes an area of the new root
where pith, vascular, cortical and epidermal tissues are beginning to mature. Water and nutrient uptake,
however, occur most readily in the region of maturation characterized by the development of root hairs
(see below). Tissue systems within this region are as follows:
●
●
vascular cylinder - composed of differentiating xylem and phloem and associated parenchyma
cells
pericycle - a root tissue system meristematic region that gives rise to new vascular tissue, new
endodermal tissues and adventitiously to branch roots.
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endodermis - the boundary layer between the vascular cylinder and thr root cortex. This tissue
system includes a specialized layer of cells which are surrounded by a suberized (oily) substance
called the casparian strip. The casparian strip serves a very important function - it prevents the
free movement of solutes (nutrients) from the soil solution (water + dissoved nutrients) to the
vascular cylinder. Therefore, in order to enter the vascular cylinder, solutes must first enter a living
cell which is bounded by a cell membrane. The cell membrane is selective allowing only certain
solutes to enter. It also prevents solutes that have entered from effluxing (escaping) back out into
the soil solution due to osmotic potential.
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cortex - The cortical tissue system is composed of apoplastic (non-living intercelluar spaces) and
symplastic (living cortical cell) regions. Movement of the soil solution is virtually unrestricted in
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this region. Cortical cells are interconnected by plasmodesmata and are a primary sites of nutrient
assimilation. Assimilation by cortical cells is affected by the environmental parameters of
temperature (assimilation generally increases up to 40 C, then declines) , soil aeration, light
(shading of leaves affects CHO movement to roots restricting their energy levels), pH (because it
affects membrane Ca content), and the relative conc. of ions in the soil solution.
●
epidermis - the primary functions of the epidermis are that of uptake and to protect cortical cells
from the soil environment. Root hairs develop from tricoblasts (specialized epidermal cells) in the
region of elongation. Root hairs are short-lived and are continually renewed. The function of root
hairs is to increase surface area of the root which increases contact with the soil solution. It was
estimated that a 4 month old rye plant root system contained 14 billion root hairs constituting 400
m2 of surface area and that if placed end to end, would stretch 10 thousand kilometers. Root hairs
are especially important for the acquisition of phosphorus.
Some species (a few of them are crops) do not develop root hairs. Often, these plants have co-evolved
with species of mycorrhizal fungi capable of forming a symbiotic relationship wherein fungal hyphae
function similarly to root hairs.
Secondary growth in roots is similar in its characteristics to those exhibited by stems. Although roots that
have undergone secondary growth are not actively absorbing water and nutrients, they are involved with
aeration and gas exchange through lenticels (pores).
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HCS 200 Winter Quarter 2001
Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #3a: Crop Plant Morphology (continued)
References:
Text = Chapter 3
Bernier, G., J.M. Kinet and R.M. Sachs. 1981. The physiology of flowering. Vols. 1-2. CRC Press, Boca
Raton, FL.
Buban, T. and M. Faust. 1982. Flower bud induction in apple trees. Hort Rev. 4:174-203.
Campbell, N.A. 1996. Biology. Benjamin Cummings Publ. Co., Menlo Park, CA = Chapter 34.
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science - growth, development
and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ = Chapter 2.
Pratt, C. 1988. Apple flower and fruit: morphology and anatomy. Hort Rev 10:273-308.
Raven, P.H., R.F. Evert and S.E. Eichhorn. 1999. Biology of plants. W.H. Freeman and Co., New York, NY =
Chapters 23-27.
Salisbury, F.B. and C.W. Ross. 1992. Plant physiology. 4th ed., Wadsworth Publ. Co., Belmont, CA.
Westwood, M.N. 1978. Temperate zone pomology. H.W. Freeman and Co., San Francisco, CA
Quotations:
"It has been said that an oak is an acorn's way of making more acorns. Indeed, in a Darwinian view of life, the
fitness of an organism is measured only by its ability to replace itself with healthy fertile offspring .... These
two developments, pollen and seeds, are among the most important adaptations of plants to life on land."
Campbell
"Due to its tremendous agricultural and economic importance, reproduction has perhaps been one of the most
studied processes in plant development." Scheerens (PhD Dissertation, 1985)
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HCS 200 Winter Quarter 2001
Outline: (Continued) Previously we discussed
1. Crop Life Cycles
2. Seed Germination
3. Meristems
4. Primary and Secondary Development of Stems
5. Primary and Secondary Developmetn of Roots
See Topic 3 Handout and Lecture Notes to review these concepts.
6. Leaves
Leaf structure
Leaf structures vary considerably with respect to size and shape. For instance, some species have simple leaves
consisting of a single leaf blade or lamina (e.g. oak, corn) whereas others develop compound leaves that are
comprised of several "leaflets" (e.g., tomato, rhododendron). Further, compound leaves are either palmately
compound where all leaflets are joined at a central point or pinnately compound where leaflets are attached to a
rachis in some organized fashion (usually two by two). Leaf size also varies greatly among species; consider
the size difference between a Blue Spruce "needle" and a banana leaf. Please review Lab 1 for some details.
Moreover, structural and morphological variations in leaves often can be understood in relation to differences
in environmental adaptation among species. For example, leaves on desert species are often small or in other
ways highly modified to protect against water loss. Cacti are, of course, an extreme example of modification
for environmental stress as leaves have been "replaced" by a photosynthesizing plant body.
Despite their differences, leaves typically share some general morphological features and functions (Figs 2-33
and 2-34).
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Epidermis - Upper and lower surfaces of the leaf blade are formed by a single layer of parenchymatous
cells which comprise the epidermis. The epidermis provides strength and some rigidity to the blade.
Often, epidermal cells are covered by a cuticle (waxy material) which further protects the leaf from
damage and water loss. The leaf epidermis also contains a group of specialized cells called Guard cells
which flank (surround) the stomates (pores) through which water vapor and atmospheric gasses enter
and exit the leaf. Because guard cell shape fluctuates over time in rapid response to environmental ques,
guard cells can act as gate keepers, allowing gaseous interchange between the interior of the leaf and the
surrounding atmosphere only when it is to the plant's advantage (see discussion below). Stomate size,
shape (Figs. 3.18 and 3.19), density, order (random or ordered in rows) and position varies substantially
among crop plants. Stomates are often more prevalent on the lower surface of the leaf, but this is by no
means, universal. The stomates of aquatic plants (e.g., water lily), for obvious reasons, are primarily
found in the upper epidermis. In some species, epidermal cells also give rise to trichomes or hairs
(collectively called pubescence) that perform a number of important functions (see below).
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Spongy mesophyll and palisade layer - The spongy mesophyll and palisade layers of the leaf are
composed of parenchymatous cells, both of which are present in many dicot leaves. As they contain
chloroplasts, the double-membraned organelles that house pigments and enzymes responsible for
photosynthetic processes, they are the "work horse" cells of the leaf. The cells in the spongy mesophyll
are loosely and irregularly arranged resulting in relatively large intercellular spaces. Especially evident
in some species are the large intercellular chambers that are associated with stomatal openings.
Intercellular spaces enhance the process of gas exchange (primarily CO2, O2 and H2O) with the
atmosphere. In contrast, palisade layer cells are more highly organized and contain minimal
intercellular spaces. However, because they have expanded surface areas that are 2 to 4 times as
extensive as those of the spongy mesophyll, they carry on the bulk of photosynthesis in most dicot
leaves. In some leaves, the palisade layer is more than one cell thick.
●
Vascular bundles - Vascular bundles distributed throughout the leaf contain both primary xylem
(usually oriented toward the upper surface of the leaf) and primary phloem (usually oriented toward the
lower surface of the leaf) elements. Larger vascular bundles are often called veins. Visually, venation
appears to be netted (dicots) or parallel (monocots) in organization, but in every arrangement, vascular
systems are continuous and connect to the stem through the midrib. The midrib of the leaf acts as the
principal vascular conduit of the leaf and because it contains some secondary growth (lignification) the
midrib adds rigidity to the leaf (i.e., it acts as a "main support beam").
Note 1: lignin is a polyphenolic material that is deposited in secondary cell walls. It is extremely resistant to
degradation and is responsible for the durable characteristics of wood. If you witness the leaf decomposition
process you will note that the midribs remain long after the rest of the leaf blade has disintegrated.
Note 2: The midribs of simple leaves are continuous with the petiole or "leaf stem" which attaches to the stem
at the nodes. In compound leaves, each leaflet possesses a midrib which is attached to the petiole by a
petiolule, in pinnately compound leaves the leaflets are attached through the petiolule to a continuation of the
petiole called the rachis (See lab #1). Sorry -- natural variation sometimes makes naming structures somewhat
complicated.
A bundle sheath, a tightly organized group of parenchymatous cells surrounds each vascular bundle. This
tissue acts to prevent direct access of atmospheric gasses to the vascular system of the plant and insures that all
compounds entering the vascular system from the "outside" must pass through cellular membranes which
discriminate against potentially harmful substances. In most plants, these cells do not contribute much to the
photosynthetic capacity of the plant as they contain relatively few chlroplasts. However, there is an important
group of plants, C4 plants, in which the bundle sheath cells are extremely important for the photosynthetic
process (see below).
●
Guard cell function - As stated above, act as gate keepers, allowing gaseous interchange between the
interior of the leaf and the surrounding atmosphere only when it is to the plant's advantage (i.e., when
photosynthesis is likely to take place). Guard cells open and close stomates by virtue of their ability to
rapidly change shape in response to turgor pressure (the pushing force that the cytoplasm and cell
membrane exerts on the cell wall). At a given moment, the turgor pressure results from the relative
concentration of solutes (dissolved materials) in the cell and the effect they have on the importation of
water into the cell. Teliologically, nature tends to want to increase the distance between molecules of
dissolved solids so that their concentration (weight/volume) is at a minimum (one of the laws of
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HCS 200 Winter Quarter 2001
thermodynamics). In aqueous systems such as the cell cytoplasm, nature does this by adding water.
Remember, the plant cell membrane is permeable to water molecules; therefore, if the concentration of
solutes on the inside of the cell is greater than on the outside of the cell, water will move into the cell to
reduce solute concentration by a process called osmosis (see any biology text for a review of this
phenomenon). As the quantity of water inside the cell increases, the turgor pressure increases exerting
more force on the cell wall. Turgor pressure is maximized, of course, when the tensile strength (pulling
forces) in the cell wall will not allow it to be stretched further. Otherwise the cell would burst. Guard
cells that are fully turgid are open; those that are flaccid (the opposite of turgid) are closed (see a and b in
handout)
Guard cell turgor pressure is affected by several main controlling elements: light, CO2 concentration,
temperature and water concentration of the leaf (Fig. 4-11). In most plants, stomates are closed at night (see
exceptions below). When light strikes a leaf surface, photosynthesis commences causing the production of
ATP (energy), decreases in CO2 levels, and increases in the concentration of sugar and other solutes such as
the malate -2 ion.
Note: malic acid is a commom organic acid containing 4 carbon atoms; its anion is called malate. Because it
is associated with the Calvin (tricarboxyllic acid) cycle of respiration, it is ubiquitous and essential to all
higher forms of life.)
Then, the guard cell membranes open channels that permit the influx of K+1, and Cl-1 through active transport
(energy requiring) mechanisms from the surrounding cell wall and intercellular spaces. The influx of these
materials along with sugar and other cellular components increases the solute concentration and osmotic
potential of the guard cell. Water then enters the cell by osmosis and the turgor pressure increases, "inflating"
the guard cells and opening the stomates.
This whole process can be reversed by increases in ABA (which stands for abscisic acid, a plant hormone that
usually inhibits metabolic processes) concentrations. When water availability is limited (a function of soil
moisture levels, temperature and relative humidity), ABA concentrations in the leaf are increased by synthesis
in mesophyll cells or importation from the plant roots. Elevated ABA causes the efflux of solutes and water out
of the guard cell, thus, closing the stomates to protect against wilting or in extreme cases, leaf death.
●
Trichome function - Trichomes or hairs (collectively called pubescence)are specialized epidermal cells
that perform a number of important functions. Because they increase the surface area of the epidermis,
they aid the leaf in dissipating heat and retarding water loss. Some trichomes secrete oily resins which
also help maintain leaf water balance. Trichomes may also protect the plant by discouraging insect
predation or ovipositioning (egg laying) by a phenomenon known as antixenosis (xenos is Greek for
stranger or guest; antixenosis literally means repelling guests). In simple terms, the antixenotic response
occurs because insects simply do not prefer "hairy" leaf surfaces as they have to expend more energy to
eat or lay eggs there than on glabrous (non-hairy) leaf surfaces. Trichome (pubescence) density, color
and position varies, of course, with species and in the case of ornamental crops (e.g., african violets,
lambs ear) can be one of their most attractive features.
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C3 VS. C4 Leaf anatomy and function - Most crop plants capture CO2 by incorporating it into a 5-carbon
sugar phosphate (ribulose 1,5-bisphosphate) to form a transient 6-carbon compound that almost
immediately disintegrates to for 2 molecules of a 3-carbon compound (3-phosphoglycerate). The enzyme
that catalyzes this reaction is "nicknamed" rubisco (i.e., ribulose 1,5-bisphosphate
carboxylase/oxygenase).
C5 + CO2 C6 C3 + C3 catalyzed by rubisco
Thus these plants are called C3 plants.
The fate of the two C3 molecules is as follows: Five of the six carbon atoms are used to regenerate the initial C5
compound while the other carbon atom is used in the synthesis of a glucose (simple sugar) molecule.
C3 + C3 C5 + C used for glucose synthesis
This complex and cyclic process is called the Calvin Cycle (after the person who figured it out experimentally).
Obviously, since glucose contains six carbon atoms, the cycle must turn six times in order to fix the carbon
from six CO2 molecules to one molecule of glucose. As we will discuss in detail in Topic 4, this process takes
energy and requires the addition of electrons (i.e., it is accomplished via chemical reduction).
Although most of our crop plants are C3 plants, a number of notable exceptions have an additional scheme to
trap CO2. In these species CO2 can be added to a 3-carbon compound (PEP or phosphoenolpyruvate) to form a
4-carbon structure (oxaloacetate). The enzyme that catalyzes this reaction is called PEP carboxylase.
C3 + CO2 C4 catalyzed by PEP carboxylase
Thus, these plants are called C4 plants
The fate of the 4-carbon product is as follows: Three of the four carbon atoms are used to regenerate the initial
C3 compound while the other carbon atom is passed to the Calvin cycle to be used for glucose synthesis in a
process identical to that in C3 plants.
C4 C3 + CO2 passed to Calvin Cycle where it is used for glucose synthesis
This complex and cyclic process is called the Hatch-Slack Cycle (again after the researchers who figured it out
experimentally). Again this process requires energy and electrons.
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Although this additional system in C4 plants seems to add unnecessary metabolic complexity to the capture of
CO2, it has some real physiological advantages do to the nature of the two enzymes involved.
PEP carboxylase has a greater affinity for CO2 than rubisco does so photosynthesis rates are high in C4 plants
even when CO2 concentrations are limiting.
The operation of the Hatch-Slack pathway in C4 plants feeds carbon (CO2) directly to the Calvin Cycle keeping
it functioning at more optimum rates. If CO2 levels are low and oxygen levels are high, rubisco (Calvin Cycle
enzyme) adds oxygen to the C5 starting sugar which is the beginning of another cyclic phenomenon called
photorepiration. We will not discuss photorespiration in detail in this course except to say that it is a rather
futile cycle which uses oxygen and releases CO2 and seems to waste energy and accomplish nothing (i.e., its
positive attributes, if any, are not well understood). In C3 plants, photorespiration rates may be relatively high
under certain conditions so that as much as 50% of the carbon fixed by photosynthesis is reoxidized and
released as CO2. In C4 plants, rates of photorespiration are much lower because rubisco has a constant supply
of CO2.
C3 and C4 plants differ with respect to how photosynthetic rates are affected by light intensity. C4 plants
continue to respond to higher light levels long after C3 plants have reached their maximum photosynthetic rates
(see Topic 4 notes).
C4 plants exhibit greater water use efficiency than C3 plants (see below).
C4 metabolism has been observed in 19 plant families and may be active in thousands of plant species. C4
metabolism is perhaps most common in the Poaceae (grass family) which includes notable examples of crops
such as corn (maize), sorghum, sugarcane, millet and bermudagrass and of weeds such as crabgrass and
bermudagrass.
The leaves of C4 plants function a little differently than do those of C3 plants. Most notably, CO2 is originally
captured by the Hatch Slack pathway in the mesophyll cells and then is transferred to the Calvin Cycle which
operates primarily in the bundle sheath cells. The bundle sheath layer is more highly organized in C4 plants
(see Fig 26-26) and resembles a "wreath". Kranz is German for wreath and this arrangement in C4 plant leaves
is called Kranz anatomy. In C3 plants, the bundle sheath cells contain few chloroplasts and contribute little to
photosynthetic output. In C4 plants, the reverse is true, bundle sheath cells contain well developed chlorplasts
that are highly active (Calvin Cycle).
●
CAM plants - Crassulacean Acid Metabolism or CAM is a notable variation of C4 metabolism was
first witnessed in stonecrop an ornamental succulent in the Crassulaceae family. Thereafter, it was found
to be prevalent in a number of xerophytic (desert-dwelling) species, especially succulents and cacti. In
these species, stomates are opened at night when temperatures are relatively cool. At night, CO2 is fixed
via the Hatch Slack Cycle and stored as 4-carbon compounds in mesophyll cell vacuoles. During the
day, in the presence of light, these stored compounds are remobilized and further metabolized to sugar
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via the Calvin Cycle. Since the carbon supplied to the Calvin Cycle was previously fixed, stomates may
remain closed during the day, which drastically improves water use efficiency.
Metabolic Scheme
Water Use/CO2 Fixed (g/g)
C3
400-500
C4
250-300
CAM
50-100
Information from Raven et al., 1999
CAM metabolism is perhaps more widely spread than C4 metabolism since it is present in 23 families. The
pineapple is probably the quintessential example of CAM crop plant; prickly pears (a minor crop grown for
fruit or pads "leaves") also exhibit CAM metabolism. CAM metabolism is also active in many ornamental
species including wax plant, snake plant (mother-in-law tongues), bromiliads, cacti and euphorbs
Flowers
The diversity among species with respect to flower structure is as vast as the diversity among leaf types or any
other plant organ. As stated in Topic 2, differences in floral structure is one of the primary keys we use to
classify plants and to determine phylogenetic (evolutionary) relationships among species. As with other organs,
floral structure influences function and as such may influence how a crop is cultured. Last, but not least, floral
diversity among species provides us with a stunning array of ever-changing beauty (sorry - had to wax poetic).
Flowers are attached to an inflorescences or flower stalk by means of a pedicel, which in turn is attached to the
stem. As with leaves, the vascular system of a flower is continuous with that of the stem. Inflorescences can be
simple (bearing a single flower) or extremely complex (see Lab 1 for some details).
●
An Apple Flower - An apple flower (Fig. 2-39) is a good "typical" flower as it is simple and complete
(contains all possible flower parts). All flowering meristems are the result of transitions in previously
vegetative meristems. Thus, in some ways, flowers can be considered as modified leaves and stems.
Typically, flower parts are arranged in whorls. The outermost whorl contains the sepals (collectively the
calyx). Sepals are usually green in color but may also pigmented in a manner similar to that of the petals.
In many species, the sepals surround and protect the other floral parts during flower bud development,
but "peel back" or separate as the flower reaches anthesis (e.g., the transition from rose bud to open rose
flower). In others, portions of the calyx remain joined even after the flower opens (e.g., petunia,
carnation). The next outermost whorl is composed of the flower petals (collectively the corrolla).
Flower petal color extremely diverse among and within species and is mediated by at least three different
pigment classes (chlorophylls, carotenoids and anthocyanins). In many wind-pollinated grasses, flowers
are green in color and not usually conspicuous whereas in crops that require pollination by insects or
animals, flowers are often colored and showy (e.g., hummingbirds are attracted to red). In addition to
color, flower fragrance resulting from essential oils in the petals also attract certain pollinators; some of
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these fragrances are extremely strong (e.g., lilac, orange, viburnum); some are somewhat ethereal or
subtle (e.g., apple). Some fragrances are pleasing to humans whereas others are decidedly not (i.e., some
insects are attracted to smells reminiscent to us of rotten meat -- yuk!)
Note: nectars from nectaries or nectriferous glands also attract insects, bats and birds, as well as pigments in
other flower parts. For instance, the yellow carotenoid pigments in most anthers reflects ultraviolet light which
can be seen easily by bees.
In most simple flowers the petal number and sepal number are the same, but this is not always the case. The
inner most two whorls of the flower contain the sexual organs. The male sexual organ, the is called the stamen
(collectively the androecium). Stamens each have two parts, an anther, which is the site of male
gametogenesis, and a filament or stalk upon which the anther rests. The innermost whorl of the flower contains
the female sex organ, the pistil (collectively the gynoecium). Each pistil is composed of three parts, the stigma,
the style and the ovary. The stigma is composed of a relatively shallow tissue group and is the site of
pollination and pollen germination (i.e., its where the pollen grain lands and begins to grow). The style is a
columnar shape tissue group through which the germinated pollen grows, moving its male gametes toward the
female gamete located within an ovule of the ovary. The ovary is the site of female gametogenesis and
contains one or more ovules; each ovule contains an embryo sac (see below) containing the female gamete, the
egg. In some fruit, ovaries can be subdivided into chambers called carpels, each of which may be serviced by
its own stigma and style. In an apple flower, there are typically five stigmas and five styles. The apple ovary is
divided into five carpels, each with two ovules that when pollinated, for two seeds. In many fruits (ripened
ovaries), the ovary wall develops into the pericarp, which is sometimes edible (e.g., pea pods, outer surfaces
of the tomato fruit, the flesh of a peach) and sometimes not (the peel of an orange, the shell of a nut, and the
outer pit of a peach) The fleshy portion of the apple which we consume is actually derived from the
hypanthium, an accessory structure that surrounds the ovary, whereas the pericarp is located in the portion we
commonly call the core.
Flower structure often reflects a species' preferred pollination scheme. Flower structures which are open at the
time of pollination (chasmogamous; e.g., apple) invite cross pollination. Outcrossing species often bear flowers
that are wind pollinated. Cross-pollination can also be promoted if the pollen of an individual flower is shed
before the stigmatic surface is receptive (protandry) or if the reverse is true (protogeny). Species that are
monoecious, bearing unisexual flowers on the same plant (e.g., corn, pecan) dioecious species that bear male
and female flowers on different plants (e.g., hemp, fig, date) are outcrossing species. Species that produce
sticky pollen that adheres to the body of insect pollinators are also often cross pollinated. Outcrossing species
often develop showy fragrant and nectar-producing flowers in order to attract the appropriate pollinators.
In contrast, self pollinating species often produce flowers which are not conspicuous (e.g., wheat). Flowers
which are closed (cleistogamous; e.g., pea) when pollen is shed are obligatorily self pollinated. Flowers which
are pendulous (hang down) with stigmatic surfaces below the anthers (e.g., tomato, some peppers) are often
self pollinated, with gravity being the pollinating force.
Self or cross incompatibility are additional mechanisms that promote outcrossing or self-pollination,
respectively. Examples of species that employ incompatibility mechanisms can be found in almost every plant
family.
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Incompatibility refers to a chemically-based recognition system wherein proteins excreted by the pollen grain
interact with specialized receptor proteins in the cell walls of the stigma. Both stigma and pollen proteins are
coded for at a single locus - the S locus. The S locus of most species has many potential alleles. That is, most
species have the potential of producing a whole series of different S proteins; each stigma (2n) has the potential
to produce two protein types and each pollen grain (n) will produce only one type depending upon what alleles
are present within each genotype. The case of self incompatibility is illustrated below:
Pollen Grains
Stigma
S1
S2
S3
S1S2
In this situation, because the genotype of the stigma is S1S2, it will recognize any pollen grains that are either
S1 or S2 as being produced by itself. The recognition event will trigger the production of ribonucleases
(enzymes) in stigma cell which are released at the S1 or S2 pollen grain, destroying its RNA and effectively
neutralizing it. If a pollen grain from a different plant lands on the stigmatic surface, chances are its genotype
holds a different allele at the S locus (e.g., S3). If so, the stigmatic recognition system will not detect it and it
may germinate and grow unheeded, eventually effecting fertilization.
Note: In class, we discussed the topic of self and cross incompatibility under the section on fertilization and
embryo growth. I have moved it here, as this is where I had originally intended to present this material
●
Types of flowers - We have actually been discussing types of flowers since the beginning of this section,
but there are two additional sets of terms that you may encounter at some point in your career. They are:
Complete flowers vs. incomplete flowers. This pair of terms refers to whether or not all of the whorls of a
flower are present. A complete flower contains sepals, petals, an androecium and a gynoecium. An incomplete
flower lacks one or more of these whorls.
Perfect flowers vs. imperfect flowers. This pair of terms refers to whether or not both sexual whorls of a
flower are present. Perfect flowers contain both an androecium and a gynoecium. Imperfect flowers are
unisexual; when they contain only an androecium they are called staminate and when they contain only a
gynoecium they are called pistillate.
●
Flower bud initiation as controlled by daylength and other factors. Flower bud initiation is the point in
time when a vegetative meristem is transformed irreversibly to a flower meristem. For each species, the
timing of flower bud initiation is obviously quite important for the successful maturation of the fruit
under optimum environmental conditions and for survival of the seed. Flower bud initiation is definitely
under genetic control and as all other phenomenon we have studied, the exact timing varies
tremendously from species to species.
As well as being genetically controlled, flower bud initiation is often triggered in response to some
environmental que. One of the most studied of these ques is that of daylength. Some examples of daylength
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control of flower initiation are listed below.
LDP - long day plants, species in which flowering is triggered by long days (e.g. chrysanthemum)
SDP - short day plants, species in which flowering is triggered by short days (e.g. hibiscus)
IDP - intermediate day plants, species which flower at median daylengths (e.g. coleus)
DNP- day neutral plants, species in which flowering is unaffected by daylength.(e.g. cucumber)
In daylength sensitive plants, the length of darkness is actually the important factor! The controlling or
triggering mechanism is mediated by phytochrome, a ubiquitous plant pigment that controls a number of other
functions as well as flower initiation (e.g., the germination of some seeds, tuberization in potatoes, leaf
coloration in the fall, the onset of dormancy in temperate perennials just to name a few). A symplistic
explanation of how phytochrome controls plant function is illustrated in Figures 29-16 (legend in bottom right
corner) and 29-17 in your Topic 3a handout. Phytochrome exists in two forms, one which is sensitive to red
light (Pr) at 660 nanometers and one which is sensitive to far-red light (Pfr) at 730 nanometers.
Note: a nanometer equals 10-9 meters; a wavelength of 660 nm is 0.000000660 meters long.
In darkness, most of pigment will be in the Pr form. When light containing wavelengths of 660 nm strikes Pr, it
is almost instantaneously converted to the Pfr form. When light containing wavelengths of 730 nm strikes Pfr it
is reconverted back to the Pr form.
Note: For those of you who are biochemically inclined the actual chemical change that occurs in the molecule
when it is converted involves a shift in the double bond preceeding the 4th (right most) heterocyclic ring in Fig.
29-17 from the trans to the cis isomer. This, of course drastically changes the conformational shape of the
pigment and thus its function. For those of you who are not biochemically inclined, just remember that the
pigment's shape changes.
In sunlight, which contains both red and far-red light, an equilibrium between the forms will be established. In
the absence of light (i.e., darkness, night), pigment in the Pfr form will slowly be reconverted to Pr even though
no far-red light is present. This slow change is called dark reversion and it is the mechanism by which a plant
can measure daylength or nightlength. The longer the night, the greater the extent of dark reversion.
For most functions, the Pfr form of phytochrome is the one that elicits a biological response. LDPs require
phytochrome to be in the Pfr form for an extended period in order to initiate flowers. However, the converse is
true for SDPs, which rely on the absence of Pfr for an extended period before flowers are initiated (i.e., lots of
dark reversion during long nights).
Experimental evidence proved that the phytochrome-mediated floral response is dependent upon the dark
reversion process. For instance, when SDPs were grown under inductive conditions (i.e., long nights), they
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flowered as expected. But when the long nights were interrupted by a very brief burst of light (termed a night
break), flowering was inhibited. Presumably the night break phenomenon resulted from the instantaneous
conversion of Pr to Pfr during the light burst.
Flowering in response to daylength is obligatory in some species (i.e., they will not flower unless a critical
daylength is supplied) where as in others it is quantitative (i.e, flowering occurs more readily and abundantly
when the critical daylength is supplied, but some flowering may occur at other times as well).
The phytochrome-mediated response can be highly specific and highly sensitive. For example, rice is a SDP
that can be planted at various times throughout the year in the tropics. Before it will flower, two criterion must
be met. First, the plant must be physiologically "ripe to flower" which basically means that undergoes a basic
vegetative phase before it is ready to receive a floral stimulus. Second, the daylength must be less than some
minimum threshold (exact photoperiod is variety-dependent) before flowering will commence. If rice is planted
from Aug - Dec, there is sufficient time to meet the requirement for the basic vegetative phase while the
photoperiod is inductive (i.e., less than about 11.5 h Sept-April). Therefore, after the requirement for the basic
vegetative phase has been met, flower induction will occur. However, if the crop is planted in January, the
photoperiod will be too long by the time the basic vegetative phase is completed. Therefore, the plant will
remain vegetative (flowering will be inhibited) until the following Sept. when the next inductive photoperiod
occurs. Rice varieties can be extremely sensitive to the critical photoperiod. For the cultivar Siam 29 the
critical photoperiod is just under 12 h. When this cultivar was planted in Malacca Malaysia in Sept, the crop
cycle (planting to harvest) was 161 days; when it was planted in January, the crop cycle was 329 days. Since
Malacca is only 2 north of the equator, daylengths on June 21 and Dec 21 (the solstices) differ by only 14
minutes!
The expression of photoperiodicity can be highly influenced and in some cases overridden by temperature
effects. Most commercial strawberry cultivars are SDP that initiate floral buds in the fall. In temperate regions
like Ohio, these initiated buds remain dormant until the following spring, then flower. Flowering is relatively
synchronous, and in Ohio that means that only one inflorescence/year (one flowering cycle/year) is produced
from each plant. The fruit is ripe by June - hence the name Junebearing strawberries.
However, the flowering response to daylength can be overridden in strawberries if temperatures are cool
(especially night temps). California production illustrates how daylength and temperature interact to control
flowering in strawberries. In California, there are two coastal regions of production, one centered near Ventura
just north of Los Angeles, and the other located in Salinas, somewhat south of San Francisco.
Production of strawberries in Ventura occurs from Feb - June (i.e., floral induction occurs from Jan - May).
During this period, photoperiods are short enough and temperatures are cool enough that multiple cycles of
induction occur. That is, the strawberry plant undergoes several flowering cycles per year instead of just one
like they do in Ohio. As summer advances, both the LD photoperiod and the higher night temperatures shut
down floral induction so production ceases.
Production in Salinas occurs from late March - November. Early in the season, both SD photperiod and low
temperatures promote floral induction. However, unlike production in Southern California, plantings in Salinas
continue to flower throughout the summer when photoperiods are too long to promote flowering. Why does
this occur? It occurs because the ocean moderates summer temperatures so that nights stay cool, and it is the
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cool nights which override the the effects of an unfavorable photoperiod. Again several cycles of induction and
fruiting occur.
The net result of these two interactive factors allows California growers to typically harvest around 30
tons/acre/year of fruit whereas the rest of us do well to harvest around 5 tons/acre/year.
As stated above, phytochrome-mediated control of flowering is obviously an adaptive advantage in natural
communities which forces synchronous flowering at a time when the probability of fruit set, development and
maturation will be maximized.
Plants within species display phytochrome-mediated responses in relation to their specific area of adaptation.
For example, soybeans (a SDP) also respond to daylength in accordance with their region of development.
Some soybean varieties are so sensitive, that they flower at the correct time for maximum yield only when
cropped within a N-S band 40 miles N or S (i.e., 80 miles thick) from where they were developed. As with rice,
soybeans must undergo an obigatory vegetative phase before they will recognise the photoperiodic flowering
stimulus. However, if they have met this requirement, they will be induced to flower at some point after
mid-July as daylengths are getting shorter.
When `Lincoln' soybeans (adapted to Urbana, IL) were planted in mid-May at various locations from north to
south, the following harvest dates were observed.
Locations from N-S Harvest date
Madison Wisconsin Oct 2
Dwight, Illinois Sept 27
Urbana, Illinois Sept 17
Eldorado, Illinois Sept 8
Sikeston, Missouri Aug 30
Stoneville, Mississippi Aug 12
Obviously, as one goes south, the critical photoperiod is reached at an earlier date, so the crop matures earlier.
In northern locations, the obigatory vegetative phase is met long before the inductive photoperiod so that plants
waiting to be induced go through an additional vegetative phase. During this extra phase the plants are storing
carbohydrates and mineral nutrients which will be used later during fruit (sink) development. That is, this extra
vegetative phase maximizes yield.
However, northern-adapted soybeans are planted by southern farmers as a double-crop with winter wheat.
Although the yields are diminished, these farmers can make money by harvesting two crops off the same field
within a year. The nitrogen-fixing capacity of soybeans is also a plus in this cropping system.
● Flower bud development (FBD)
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Flower bud differentiation can be rapid, but it can also be a protracted process requiring several months to
complete. In addition, in temperate zone plants that initiate fall flowers, the process can be interrupted by
winter and completed in the following spring. In any event, the differentiation pattern is sequential - from
outside to inside - with calyx and petal whorl primordia being developed prior to stamens and pistils. Once all
structures are finally formed and fully developed, the reproductive cells are formed through the process of
meiosis. The entire process of FBD ends at anthesis. The following time line describes this sequence for apple
flowers.
Flower bud initiation June 15-30
Calyx primordia formed early July
Petal primordia formed early to mid July
Anther primordia formed mid to late July
Pistil primordia apparent mid August
Ovarian cavity formed late September
Meiosis in megaspore mother cells late September?
Rest subject to chilling requirement November - March
Meosis in pollen mother cells March
Ovule and pollen differentiation April
Anthesis May
Note that flower bud initiation for the next season is occurring simultaneously with fruit set of this year's crop.
Both floral initiation and fruit set depend upon critical levels of available energy (carbohydrates) and hormonal
balance. Overall plant vigor is extremely important. If the plant is weak or has set too many fruit,
carbohydrates available for floral initiation will be limiting and fewer flower buds will be formed for next year.
If the reduction in flower initiation is severe enough, the tree enters an "alternate bearing" cropping cycle in
which commercial crops are produced every other year. Alternate bearing is difficult to correct culturally once
it is initiated and it has disastrous economic consequences for the orchardist.
The length of the rest period is controlled by the plant's chilling requirement. The chilling requirement is
defined as "the cumulative number of hours below 7 C (45 F) needed to satisfy the rest requirement and
break dormancy in vegetative and floral buds." In some species, the chilling requirement for vegetative
buds is less than that for floral buds. In other species such as apple, the converse is true. Note that the
temperature range that satisfies chilling requirement is identical to that which satisfies vernalization and
stratification requirements. Chilling requirements are a selective advantage and confer adaptation to specific
growing regions. Many perennial crops fail in temperate growing regions not because they cannot withstand
midwinter cold, but because their low chilling requirement allows them to break dormancy during the first
warm spell of spring. Peaches and some other stone fruits are not well adapted to Ohio because they flower too
early; their flowers are damaged by late spring frosts. Conversely, oaks and maples never leaf out until May,
regardless of warm April temperatures.
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●
Gametogenesis
The formation of the male gametophyte (pollen grain) occurs in the anther. A typical anther consists of four
sporangia or pollen sacs. These sacs contain sporogenous cells or microsporocytes which will undergo
meiosis to form pollen grains during anther development and nutritive cells, collectively called the tapetum.
During the process of meiosis, each microsporocyte produces four microspores, each containing ½ of the
number of chromosomes present in the mother plant
Note: If you are fuzzy about the process of meiosis, I urge you to review it in any biology text. It is an extremely
important concept to understand.
Once meiosis is complete, each of the microspores undergoes mitosis to form two cells within each pollen
grain: a generative cell and the tube cell. The tube cell nucleus is responsible for controlling cellular activities
as the pollen germinates on the stigmatic surface and grows through the style toward the embryo sac. The
generative nucleus will mitotically divide a second time to form two sperm nuclei. In some species this is
completed prior to pollination whereas in others it occurs after pollen germination on the stigmatic surface.
Pollen grains are enclosed within very resistant outer and inner walls. The inner wall (the intine) is composed
of pectin and cellulose as are many primary cell walls, whereas the outer wall (the exine) contains a very
resilient material comprised of carotenoid polymers called sporopollenin. The outer walls of the pollen grain
are "sculpted" differently in each species in such a way that they can serve as a "fingerprint". Not only are these
patterns aesthetically pleasing and interesting scientifically, they also serve as a means to archaeologically
verify plant use by indigenous people and infer evolutionary relationships among plant species in the fossil
record. Pollen grains vary in size from 20 microns to 250 microns.
Note: a micron or µ is equal to 10-6 meters.
The formation of the female gametophyte occurs within the ovule. Each ovule contains a megasporocyte
which undergoes meiosis to form four cells which contain ½ the chromosome number of the mother plant. One
of these cells (usually the one distal to the micropyle or ovule pore) survives while the other three disintegrate.
The process from megasporocyte to surviving megaspore is termed megasporogenesis. Megagametogenesis
begins with the mitotic of the megaspore (three divisions in all) and the formation of eight genetically identical
nuclei. These nuclei are specifically arranged within the embryo sac so as to perform specific functions.
Membranes form around these nuclei to form 7 cells within the embryo sac. The three cells distal to the
micropyle are called the antipodals (function obscure) and the three cells near the micropyle form the egg
apparatus consisting of one egg and two synergids. Synergids are important in guiding the sperm nuclei to the
embryo sac. The egg, upon fertilization will become the embryo. The remaining two nuclei are positioned near
the center of the embryo sac. These nuclei will also be fertilized by one of the sperm nuclei to form the
endosperm.
The process of megasporogenesis and megagametogenesis shown here is typical but there are many variations
of these schemes among plant species.
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Fertilization and Embryo Growth
Pollination is perhaps the first step in the process of fertilization. When an anther and the pollen grains it
contains are mature, the anther wall dehisces (ruptures), releasing pollen into the environment. Pollen can be
transferred from the surface of a dehisced anther to the stigmatic surface of the same or different flowers by
wind, gravity, insects, or a variety of animals (birds, bats, etc.) including humans. The stigmatic surface in
many species is "sticky" because it is coated with a sugar-containing solution secreted by glandular cells within
the stigma. Climatic factors which either influence the activity of pollinators (e.g., bees don't fly when its cold
or rainy) or the status of the stigmatic surface (e.g., extreme wind, rain, heat, etc.) can affect the transfer of
pollen and its adherence to the stigma.
If the pollen is compatible with the stigma upon which it has landed, it will "germinate". A pollen tube then
begins to grow through the stigma and into stylar tissue, a process which is controlled by the tube nucleus. In
some species, the pollen tube grows through a channel within the stylar tissue whereas in others it penetrates
through the style's intercellular spaces (i.e., cell walls and middle lamella). In the latter case, tube growth is
directed to the embryo sac via specialized cells which form tissue transmitting strands. The two sperm nuclei
traverse the style through the pollen tube. In some species, the mitotic division of the generative cell forming
two identical sperm cells occurs in the style whereas in others, it precedes pollination. As the pollen tube nears
the embryo sac, one or both synergids located near the micropyle begin to disintegrate. Disintegration of
synergid cellular membranes release Ca+2 into surrounding tissue which acts as an attractant to the growing tip
of the pollen tube. As the tube enters the embryo sac through the micropyle, it releases the two sperm nuclei.
One of the sperm nuclei fertilizes the egg cell to become the zygote, whereas the other fuses with the two polar
to form the endosperm nucleus through a process called double fertilization.
Typically, the processes of pollination and double fertilization take about 24-48 hours.
Once double fertilization is complete, embryogenesis and seed development commences.
In many species, endosperm development precedes embryo development. In the early stages, the endosperms
of monocots and dicots develop similarly. First, the endosperm nucleus undergoes an extensive series of
mitotic divisions, resulting in a multinucleate "super cell". At this point the endosperm is without internal
structure. However, eventually, each nucleus in the super cell is surrounded by a cell membrane and wall and
the endosperm starts to develop into a recognizable tissue group. A some point endosperm development
patterns in monocots and dicots diverge. In monocots, the endosperm continues to enlarge and in the mature
monocot seed, it is the principle storage organ for starch and other potential nutrients that will be needed for
seed germination. The cotyledon in monocots aids in regulating seed metabolic processes and acts as a conduit
of energy from the endosperm to the developing new plant. On the other hand, in dicot species, the endosperm
is utilized during the development of the cotyledons so that in the mature dicot seed, little if any endosperm
tissue is left.
The zygote undergoes its first mitotic cycle, forming two distinct cells: the basal cell and the terminal cell. The
basal cell further divides transversely creating a stalk-like structure called a suspensor. The suspensor
suspends and anchors the developing embryonic plant in the ovule and connects it to the ovule integuments
(rudimentary wall-like structures) through an attachment of the basal cell. At the same time that the suspensor
is forming, the terminal cell undergoes several cycles of cell division in order to form a spherical mass that will
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eventually differentiate into primary meristematic regions or tissues (i.e., protoderm, ground meristem and
procambium). Notice that similar tissue groups are also formed from apical meristems of the root and shoot. As
the embryo develops root and shoot axes are formed, each with their apical meristems. Cotyledon tissue gains
prominence and in the case of dicot seeds, the cotyledons will form the bulk of the seed dry weight. The ovule
integuments develop into the seed coat.
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Fruit set and fruit development
A fruit is a ripened ovary or group of ovaries. The characteristics and morphology of fruits of course varies in
accordance with the flower and inflorescence structures which precede them. You will discuss this variation in
an upcoming lab, so I won't belabor the point in this lecture.
The early life of a fruit (i.e., shortly after ovule fertilization) is somewhat precarious. There are many physical
and physiological changes that must take place. If internal or external conditions are not adequate, the newly
formed fruit will abscise (drop off). Of course, sacrificing newly formed fruit in order to preserve maternal
plant health may be of selective advantage to perennial plants as there will be additional chances to produce
offspring in subsequent seasons. However, for annual plants, fruit drop is somewhat of a disaster biologically.
Fruit set occurs when this early critical period has passed; fruit which have set enjoy a good chance that they
will reach maturity.
Several factors affect fruit set including:
Successful embryo development - competent embryos which develop normally produce hormonal signals
(usually mediated by auxin or gibberellins) to ovary tissue, stimulating it to grow and enlarge. In some crops,
horticultural treatments using natural and synthetic growth hormones have been developed to enhance fruit set
or to limit fruit set.
Fruit set and subsequent development of seedless fruit (e.g., bananas, seedless grapes) results from the action of
the same hormones, but, in this case, they (the hormones) are produced in sufficient quantities by fruit tissues
themselves. The process of setting fruit without seed development is called parthenocarpy.
Carbohydrate reserves - See discussion above about alternate bearing
Competition - Many crops will drop fruit if fruit set is extremely high primarily due to competition for
nutrients and energy.
Other stress factors (e.g., heat, drought, low light intensity, cold weather etc.) can also result in fruit drop.
Fruit growth is characterized by abundant gas exchange and rapid and extensive increases in H2O and overall
dry weight. It is also controlled by hormonal balances. Early growth of all fruits results from cell division and
elongation enhanced by the presence of auxin and giberellins. The relative level of these compounds determine
the fruit's ultimate shape. Therefore, horticultural practices, such as treating 'Thompson Seedless' grapes with
giberellin to elongate the berries and the rachis (fruit stem), have been developed and routinely practiced.
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Fruits can be classified into two groups, depending primarily on hormonal changes (primarily ethylene, a
gasseous hormone) that occur as the fruit ripens. Climacteric fruits exhibit rapid rises in respiration and in
ethylene generation which signals the onset of the ripening process (i.e., softening, changes in color, sugar
content, etc.), whereas non-climacteric fruits seem to ripen without fluctuations in ethylene evolution. In
crops with fleshy fruits, whether or not a fruit is climacteric affects when it is harvested, and how it is stored
and how long its shelf life might be. Biotechnologically enhanced tomatoes (a climacteric fruit) have been
produced through the incorporation of an Arabidopsis gene coding for defective counterparts to enzymes that
responsible for ethylene production. The lack of ethylene production in these recombinants greatly increases
the shelf life of this product over that of common fresh market tomatoes. Conversely, fruits that are climacteric
can be ripened artificially by treating them with ethylene gas or products that evolve ethylene (e.g., banana,
tomato, etc.) Fruits such as apple are stored in chambers designed to limit respiration and ethylene evolution
(i.e., in controlled atmospheric storage chambers), which greatly prolongs their storage life.
As you might expect, environmental factors can affect fruit growth rate and there is considerable variation in
the time necessary for fruits to ripen (i.e., the obligatory time necessary to progress from pollination to fruit
maturation (harvest or abscission). However, there are really only two fruit growth patterns: sigmoid growth
curves and double sigmoid growth curves (Fig 9-2). In the double sigmoid pattern, fruit growth is retarded for a
period during mid-season to allow rapid growth and development of seeds. In fruits that follow a sigmoid
pattern (e.g., pecan), seed maturation often occurs in the latter stages of fruit development, or about the time
when fruit development slows (see Table 1 and its associated graph).
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #4A: Radiant Energy and Its Effect on Crop Growth - Part 1- Light.
References:
Text = Chapters 7 and 8
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science, growth, development
and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ. = Chapter 10
Janick, J., R.W. Schery, F.W. Woods and V.W. Ruttan. 1981. Plant science, an introduction to world crops.
H.W. Freeman and Co., San Fransico, CA. = Chapters 10 and 11.
Raven, P.H., R.F. Evert and S. E. Eichhorn. 1999. Biology of plants (6th Ed.). W.H. Freeman and Co., New
York, NY = Chapter 29.
Quotation:
"... Although a small amount of the energy to power civilization comes from the interior of the earth and more is
contributed by atomic fission, our most abundant source of energy is the sun"..... " The total incoming solar
energy reaching the outer edge of the earth's atmosphere averages 1.94 cal/cm-2/min-1, a value known as the
solar constant. We can put this in perspective by noting that in 1`0 days, the energy arriving at the periphery of
the earth's atmosphere is equal to the total known fossil fuel reserves." from Text.
"It has been estimated that about 1.4 x 1014 kg of carbon from carbon dioxide in the air is converted to
carbohydrates each year by the green plants that live on the land and in the oceans, seas and lakes. A number of
this magnitude is beyond our comprehension. To put it another way, assume that the 1.4 X 1014 kg of carbon is
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converted entirely to an equivalent amount of coal, which would be 1.4 X 1011 MT. Assume further that a
standard-size railroad car holds 45.5 MT; then the carbon fixed annually by plants would yield enough coal to
fill 97 cars every second of every hour of every day all year long." from Hartmann et al.
Outline:
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What is radiant energy.
Radiant energy - radiant energy is that which is derived from the sun. It has a unique feature in that it behaves
as both a particle (photon - a descrete unit of radiation) and a wave. We typically measure the amount of
radiation striking or reaching a given area in photons but we measure the strength (energy level) of those
photons in terms of wavelengths. Short wavelengths are very powerful while longer wavelengths are less so. In
other words 100 photons of radiation at a 800 nm wavelength has only half the energy of 100 photons of
radiation at a 400 nm wave length. The entire range of wavelengths reaching the earth's outer atmosphere is
called the electromagnetic spectrum (see diagram in handout). Notice that visible light (wavelengths from about
400 - 700 nm) comprises only a small portion of this spectrum. Radiation at the highest energy level exhibits
extremely short wavelengths. The highly energetic radiation of X-rays and rays can cause serious damage to
biological systems because when they hit organic constituents in cells, "knock" electrons out of their orbits, thus
creating ions -- hence their alternate names,
cosmic or ionizing radiation. Although less dangerous, ultraviolet light is also somewhat ionizing and is the
chief culprit causing sunburn in humans and other animals. Prolonged exposure to high levels of ultraviolet
light is also linked to various types of skin cancer. Radiation at wavelengths longer than 700 are more or less
undetectable by the human eye, and those that are longer than 760 nm but less than about 10,000 nm are
responsible for what we commonly refer to as "heat". Electromagnetic radiation of extremely long wavelengths
(>100,000,000 nm) comprise radio waves. Wavelengths between heat waves an radio waves are considered to
be microwaves (useful for cooking and communication).
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Some definitions
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•radiation = the movement of energy without physical connection (e.g., light through space)
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•conduction = the movement of energy through one body to another (e.g., heat from electric stove
element to kettle bottom, light through a fiber-optic cable)
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•convection = the movement of energy by (air) currents (e.g., heat from gas furnace flame to upstairs
bedroom via forced air )
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•insolation = solar radiation striking the earth's surface
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•diffuse radiation = radiation that has been scattered or reflected by clouds or atmospheric particles. The
amount of diffuse radiation is dependent on cloudiness, latitude, season, time of day and elevation
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•radiant flux density = number of photons/cm2 surface area
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•foot candle (lumen) = quantity (intensity) of light which falls on a 1 ft2 surface area generated from a
candle that is 1 foot away.
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•lux = lumens/m2 1 lumen = 10.76 lux
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•Einstein = energy in one mole of photons.
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•microEinstein = energy in 1 µmole of photons.
At noon on a summer's day, insolation is roughly equal to about 108 K lux, 10,000 ft. candles or 1800
microEinsteins.
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What happens to the radiant energy arriving at the earth?
About 20% of the light arriving at the earth's outer atmosphere penetrates directly to the earth's surface, about
25% of it is scattered in the atmosphere and reaches earth as diffuse radiation, about 20% of it is absorbed by
atmospheric particles and about 35% of it is deflected back out into space without ever entering the atmosphere.
Other than the fact that we receive enough radiation to make life as we know it possible, there are are several
important consequences to this scheme that bear some discussion
First, light scattering in the atmosphere is a function of particulate matter such as dust, smoke and water
droplets. These large particles scatter all visible wavelengths of light in equal proportion so that on a cloudy or
hazy day, the sky appears to be white and on a smoggy or dusty day it appears to be brown, the color of the
polutants. However, smaller particles such as atmospheric gases scatter shorter visible wavelengths while
allowing longer ones to pass through without incident. That's why on a clear and dry day the sky appears to be
blue.
Radiant energy that is absorbed by the atmosphere before it reaches earth's surface is comprised mainly of
wavelengths that are shorter than those in the visible spectrum. Ozone and oxygen are primarily responsible for
absorbing much of the ultraviolet rays which might otherwise have drastic consequences (mutations, cancer,
etc.) on terrestrial life forms. The depletion of the ozone layer by chlorofluorocarbons (freon), methyl bromide
(a fumigant) and other such compounds became a matter of concern to scientists and then to enviromentalists
and now to the public at large. There is still great controversy as to how large of a problem we have created for
ourselves and whether or not the steps we have taken to correct it will be effective.
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Of the direct and/or diffuse radiant energy that reaches the earth, about half is in the visible range. This fact has
undoubtedly affected the evolution of both plants and animals. Those animals that had "sight" (i.e., keenly
sensed radiant enery in the 400-700 nm range), or those plants who were strong photosynthesizers had a
selective advantage over those that didn't. The relatively unrestricted penetration of visible light is called the
window effect. (Note: glass is impervious to ultraviolet light --- that's why you will never tan while working in
a greenhouse.
Once reaching the earth, only about 2-3% of solar radiation is utilized directly by plants in the process of
photosynthesis. Much of the light striking the earth's surface is radiated back into the atmosphere as heat (longer
infrared wavelengths). This longer wavelength radiation has far less of a chance of passing out of the
atmosphere than light has of entering it. Atmopheric gases such as CO2 from combustion of carbon-based fuels
such as wood, coal and gasoline impede the escape of heat further. The lack of energy balance (i.e., more light
energy enters than heat energy escapes) results in the "greenhouse effect" which is now a common concern
among scientists, politicians, enviromentalists and the public at large. The increased retention of heat in the
atmosphere due to man's activities within the 20th Century may be radically warming our atmosphere (i.e.,
global warming), again with some potentially nasty side effects such as the melting of the polar ice caps,
massive flooding, increased frequency and intensity of violent storms, catastrophic climatic changes, just to
name a few. As with the depletion of the ozone issue, the extent of global warming and its consequences are
still a matter of controversy that fosters continued debate worldwide.
Of the 2-3% of radiant energy utilized by plants for the photosynthetic process, much of it absorbed by
plankton, algae, etc. in aquatic communities (Table 11-1). Cultivated land accounts for only about 5% of net
production and only about 0.4% of world biomass accumulaion yearly.
Some climatic and geographic factors that affect the path of radiant energy in the atmosphere (besides those
mentioned above) include lattitude, season, time of day (all which affect the angle of incidence (obliqueness)
with respect to the earth's surface and thus, the amount of atmosphere traveled through prior to terrestrial
contact. Elevation affects the intensity of sunlight (greater at higher elevations) and the ability to lose heat (also
greater at higher elevations). Therefore, although higher elevations receive more of the sun's radiation, they
loose heat easily and are often cold.
4. Plant growth and development as affected by light
A. Photosynthesis
The primary light harvesting molecules (pigments) of plants are of course, chlorophyll a and chlorophyll b. If
you are interested in the exact chemical structure of these molecules, you will easily be able to find them in
many of the plant science texts. However, for our discussion, remember that they each are composed of a
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tetrapyrrole or porphyrin ring ( a complex ring structure containing four N atoms exposed to the ring's center. A
Mg+2 ion is coordinately held by these four N atoms comprising the "business end" of this pigment. Electrons
within the ring can be excited (i.e., bumped to a higher orbit) after intercepting a photon of light. When they
return to ground state, three possible events may occur:
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the energy can be converted to a combination of heat and fluorescence (light of longer wavelengths)
the energy, but not the excited electrons can be passed directly to another chlorophyll molecule, exciting
its electrons, in turn while its own return to ground state. This is called resonance energy transfer.
the energy and excited electrons can be passed on to enzymes responsible for ATP and NADH synthesis
characteristic of the light reactions of photosynthesis. This leaves an "electron hole" in the chlorophyll
molecule that must be filled with the addition of electrons through oxidation of a suitable substrate (see
discussion below for clarification).
Note: Various carotenoids serve as additional or "accessory" pigments involved in the light harvesting
process. Radiant energy captured by these pigments must be transferred to chlorophyll prior to its use in the
photosynthetic process
Chlorophylls absorb light only in two regions of the visible spectrum (Figure 7-8) in the blue-violet range (i.e.,
420-460 nm) and in the red range (630-660 nm). THESE WAVELENGTHS COMPRISE THE
PHOTOSYNTHETICALLY ACTIVE RADIATION or PAR. Notice that chlorophyll absorbs very little
light of wavelengths from 500 to 540 nm (i.e., the green region), but reflect it instead. This is why we "see"
plants as being green. It's also interesting to note that carotenoid (accessory) pigments, do absorb some light in
the green region. Although light harvesting by carotenoids may offer a selective advantage, the primary
function of carotenoids is as antioxidants protecting the chlorophyll molecule from light induced oxidative
degradation.
The effect of PAR can be demonstrated by examining the rate of photosynthesis and the rate of growth (height)
of plants grown at a series of monochromatic wavelengths (Figs. 7-6 and 7-7). In these examples, bean plants
photosynthesized most actively and grew tallest when grown using light of red and blue wavelengths.
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Photosynthesis Basics
The basic reaction of photosynthesis is 6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6O2.
Glucose
The carbon in CO2 is "fixed" within the glucose molecule via a series of enzymatic reactions within the
chloroplast known as the Calvin cycle. As we studied earlier, CO2 is first combined with a C5 compound called
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ribulose bis phosphate to form an unstable C6 intermediate (catalyzed by the enzyme rubisco). The C6
intermediate is almost immediately cleaved to for two C3 compounds. Several reactions later, one carbon atom
is used in the formation of glucose while the other five are rearranged to reform ribulose bis phosphate,
completing the cycle. Therefore it takes six turns of the cycle and six CO2 molecules to form one molecule of
glucose.
In order for the Calvin Cycle to work, the carbon atoms of CO2 (oxidation state +4) must be reduced (have
electrons added to them) in order to be transformed into the carbon atoms of glucose (oxidation state average of
0). To reduce molecules in a biological system almost always requires two things --- Energy in the form of
ATP and a source of electrons carried by NADPH. To build a glucose molecule via the Calvin Cycle, it
requires the energy stored in 18 ATP molecules and 24 electrons delivered by 12 NADPH molecules.
Ultimately light energy (24 photons) will be used to garner both of these required "building" materials via the
energy transduction or light reactions of photosynthesis catalyzed by membrane bound enzymes within the
chloroplast. The source of the needed electrons is water. In the light reactions, 12 H2O molecules are broken
down to their constituents 6O2 and 24H+. Notice in this reaction that each oxygen atom went from an oxidation
state of -2 in water to an oxidation state of 0 in O2. Hence the oxygen of water was oxidized (electrons were
taken away).
Diagrams 7-13 and 7-14 illustrate how all of this works. The process starts with the harvesting of light energy
by Photosystem II. Photon energy may be captured by any chlorophyll molecule. Electrons within the porphyrin
ring are first excited and then return to their ground state passing the captured energy to another chlorophyll
molecule in the process (see above). Eventually, the energy is transferred to a chlorophyll a molecule in a
specialized reaction center. This also results in the excitation of the electrons in the chlorphyll molecule of the
reaction center. However instead of returning to ground state. The energy along with the excited electrons are
passed to an electron acceptor which is part of an electron transport chain of pigment molecules. The excited
electrons with the energy they hold are passed down this chain from pigment to pigment in a series of energy
favorable reactions forming an ATP molecule in the process. Finally the electrons and the residual energy they
hold are passed to a chlorophyll a molecule at a reaction center of Photosystem I. Again an excitation event
occurs and the excited electrons are passed along to another electron acceptor in yet another membrane bound
electron transport chain. The net product of energy and electron movement down this chain is the formation of
an NADPH molecule.
This process leaves the entire system two electrons short (i.e., the reaction center chlorphyll a molecule in
Photosystem II is missing two of its electrons. They are replaced by the cleavage of water and the oxidation of
the oxygen molecules within as described above.
One additional option worth mentioning here is that ATP may also be formed in an alternate Photosystem I
scheme by a process called photophosphorylation.
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The Effect of Light Intensity on Photosynthetic Rate
The effect of light intensity on net photosynthesis is depicted in Figure 7-8. The light compensation point is the
amount of light necessary to stimulate a level of photosynthesis in plants that is equal to their level of
respiration. A typical light compensation point might be reached at about 2% full sunlight or about 40
microEinsteins. Notice that net photosynthesis is zero at that point. The Light saturation point occurs at the light
intensity level which saturates the photosynthetic mechanisms. I.e., for a given set of conditions (CO2,
temperature, water availability etc.), the photosynthetic process is operating at a maximum rate - light intensity
is no longer the limiting factor
Values for light compensation and light saturation points vary tremendously among plant species, but is mainly
dependent on dark reaction mechanisms (i.e., C3 , C4 , and CAM metabolism). A typical C3 plant will reach its
light saturation point at around 800 microEinsteins (less than ½ of full sunlight on a cloudless day). A typical
C4 plant exhibits a much higher light saturation point than the typical C3 plant -- as much as 2X higher. Thus,
C4 plants have a definite advantage in environments that are usually cloudless because they can more efficiently
use the solar radiation provided to them.
Factors which affect light intensity (the number of photons) reaching leaf surfaces, affect photosynthetic rates
including:
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time of day
season of the year
lattitude
elevation
the level of atmospheric pollution
cloud cover
atmospheric moisture
Additional factors which may affect photosynthetic rates include
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temperature
nutritional status
water status
genetics (species and cultivar differences)
leaf age
Atmospheric CO2 concentration
Ecosystems often exhibit characteristic photosynthetic maxima in relation to some of the factors listed above.
See Table 12-3.
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Light saturation points also vary among shade tolerant (100 microEinsteins) and shade intolerant species
(500-800 microEinsteins) (Figure 12-4). Berry and his coworkers conducted an interesting experiment wherein
they collected three plants from their native habitat: Tiderstromia oblongata, a C4 plant from Death Valley,
Atriplex hastata, a C3 plant native to the Pacific Coast, and Alocasia macrorrhiza, a rainforest floor species
native to Queensland. (Note: The arrows on each data line indicate the typical light levels of the species' native
habitat). They then subjected these plants to various levels of light intensity and measured photosynthetic
output.
Alocassia behaved like a typical shade tolerant plant because:
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It outperformed all others at very low light intensities,
It exhibited a very low light compensation point, and
It became saturated at very low light intensities so that increased PAR had no effect on photosynthetic
rate.
The morphological and physiological adaptations shade tolerant plants have undergone to survive on the forest
floor typically render these plants in capable of taking advantage of full sunlight. They survive, but grow
slowly.
Tidestromia behaved like a typical C4 plant because:
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It outperformed all others at very high light intensities
It exhibited a light compensation point similar to sun tolerant C3 and C4 plants, and
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It did not reach a light saturation point at full sunlight or beyond
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These species are best capable of taking advantage of their typically sunny environments. Growth rates are
usually very rapid.
Atriplex behaved like a typical C3 plant because:
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It's performance at very high and very low light intensities was intermediate between sun-loving C4 and
shade tolerant plants
It exhibited a light compensation point similar to sun tolerant C3 and C4 plants, and
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It reached it's light saturation point at light intensities much less than full sunlight (typically one-fourth to
one-half full sunlight).
These plants typically do not capitalize on all of the available sun light they receive. Their typical growth rates
are much faster than shade-tolerant species, but not nearly so rapid as C4 plants. Remember, however, there is
great diversity among C3 plants with respect to light saturation. Peanut and sunflower are examples of C3 plants
whose light saturation points are near full sunlight.
Can sun-loving and shade-loving plants be "re-conditioned" to survive in the alternate environment?
Those of you who will go on to take H&CS 310 will likely have a whole section on acclimatization of indoor
foliage plants to low levels of light. Sun-grown plants vary tremendously with respect to their tolerance of being
placed in the shade (genetic limits) and of course, leaf morphology and physiology at maturation is influenced
by the light levels present during leaf development (see below). However, some degree of acclimatization is
possible.
Representative plants of two clones of Solidago virgaurea were collected, one from a sunny location and the
other from a shaded area of the forest floor. Representatives from the sunny location and representatives from
the shady location were cultured under both high and low light intensities and later measured for their response
to various irradiance levels (Figure 12-6 a,b).
Representatives from the sun clone exhibited the typical C3 pattern when grown at high light intensities (their
natural condition). When grown at low light intensities the representatives of the sun-adapted clone tended to
behave somewhat as a shade tolerant species in that both their light saturation and their light compensation
points were reduced by exposure to low light culture. Thus, they had become somewhat adapted to the low light
environment. However, light saturation points and photosynthetic maximums of the sun-adapted clone growing
in the shade (Fig. 12-6 a) were lower than for shade-adapted clone growing in the shade (Fig. 12-6 b). In other
words, although some acclimatization had taken place in the sun-clone during its growth at low light, changes in
its physiological status did not render it as efficient at low light intensities as was its shade-adapted counterpart
which was both physiologically and genetically suited to perform well in this environment.
Representatives from the shade clone exhibited a typical pattern for shade tolerant species when grown at low
light intensities. However, unlike their sun-clone counterparts, they were entirely unable to capitalize on the
additional radiant energy when cultured at high light intensities. Successful acclimatization of a shade plant to
full sun is much less common than the reverse.
B. Photoperiodism
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Light
Photoperiodism, or crop development as a function of daylength (night length) is mediated by the pigment
phytochrome (see previous lectures for details). Processes we have studied (or will study) which are
photoperiodic include floral initiation, dormancy, cold hardiness, potato tuberization and membrane
permeability.
C. Phototropism
Phototropism, or the orientation of plants toward light, can be transient or permanent.
Examples of transient orientation (alternate names: solar tracking, heliotropism) include: sunflower heads that
change orientation throughout the day to follow the sun; leaves that change orientation throughout the day so
that their upper surface is always perpendicular to the sun angle; and leaves that orient themselves to avoid
direct sunlight during periods of drought.
Transient orientation results from osmotic changes in specialized cells near the base of the moving leaf or
flower called pulvini. Because cell osmotic potentials (i.e., resulting from the relative concentration of solutes
in and outside the cell) can be regulated by hormones and other membrane or cellular components (enzymes,
etc.), changes in cell turgor and thus changes in orientation of the structure in question can be relatively rapid.
The permanent orientation of plants to light (e.g., shaded plants that "reach" for sunlight) occurs via a different
mechanism. Well over 100 years ago, Darwin noticed that if oat seedlings were grown with a light source at one
side rather than overhead, the coleoptile of the seedling would bend in the direction of the light (Figure 35-2).
He also discovered that this would not happen if the seedlings apical meristem was damaged or removed. He
and his son conducted a series of experiments to demonstrate the necessity of an operating meristem for
phototropic response using decapitation and various types of transparent and opaque caps. They concluded that
the meristem controlled the process, but the actual bending occurred somewhere below the meristem. They also
concluded that the meristem must be sending some sort of signal to the cells below directing them to grow at a
differential rate in order to bend the coleoptile. A little later, Boysen-Jensen confirmed that a signal was indeed
being translocated from the meristem to the rest of the coleoptile using permeable and non-permeable blocks.
Later F.W. Went experimented with this phenomenon further and in the process discovered the plant hormone,
auxin (indoleaceticacid) (Figure 35-3).
Auxin is a growth promoting substance (usually) which is integral to cell elongation. It is produced in
meristems, such as the apical meristem of an oat seedling. Auxin normally is translocated downward evenly
throughout the coleoptile cylinder so that expanding cells are exposed to similar levels of this promoting
substance. However, auxin is easily degraded by light. If one side of the cylinder is illuminated and the other
shaded, auxin levels will be greater on the shaded side and thus cell elongation will be greater. THIS is the basis
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Light
for permanent phototropism.
D. Photomorphogenesis
Hypocotyl elongation
Hypocotyl elongation in several species is controlled by phytochrome in the Pr form. Therefore, longer nights,
or treatment with far-red light simulates their growth whereas long days and night breaks would inhibit it.
Seed germination
Seed germination of some species (e.g., lettuce) is controlled by phytochrome in the Pfr form. Therefore,
continuous light or treatment with red light simulates germination.
Leaf thickness
Relative to their position in the canopy, leaves on a given plant may either develop in full sunlight or in various
levels of shade. The light intensity available to a leaf as it develops often affects its morphology and its function
when fully developed.
Shade leaves are usually larger than, but thinner than sun leaves. Shade leaves typically exhibit:
● a poorly developed palisade layer,
● large intercellular spaces and loose organization of the spongy mesophyll
● very thin (if any) cuticle layers
Shade leaves often have higher chlorophyll contents (by weight) than sun leaves but less rubisco and other
photosynthetic enzymes (both light and dark reaction enzymes). In other words, these leaves have invested
more effort in producing pigments for harvesting light than for fixing carbon. This makes some intuitive sense
in that the limiting factor for photosynthesis in these leaves will be light availability. These leaves are set up to
be able to fix some carbon under low light intensities with minimum expenditures of energy to maintain the
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Light
Calvin cycle and electron transport systems at levels appropriate for the amount of sunlight likely to be
received. In other words, why develop a massive CO2 fixing capability if it is unlikely ever to operate at full
capacity due to limited light.
Sun leaves typically exhibit:
●
●
●
●
a well developed palisade layer which may be more than one cell thick,
tight organization among cells of the palisade and/or spongy mesophyll
relatively small intercellular spaces within the spongy mesophyll
very thick cuticle layers
Since sunlight is not a limiting factor in their development, these leaves are seemingly set up to maximize
carbon fixation rates.
Leaves developed in partial shade have characteristics between these two extremes
All of this notwithstanding, shade and sun leaves on a given plant may show a five fold difference in
photosynthetic capacity.
5. Methods to control light
●
Field orientation
Plant row east-west to minimize shading within the row.
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Plant spacing
In field, landscape and greenhouse situations, the amount of carbon fixed per unit land area will be proportional
to the amount of the land area covered by leaves. For this reason greenhouse and container-grown materials are
densely planted as seedlings and then repotted several times as their size increases during their production. This
practice not only maximizes photosynthetic capacity/unit area, but also minimizes costs and maximizes profit.
Under field and landscape conditions the amount of area covered by leaves is dependent on plant spacing. The
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Light
literature is replete with studies concerning the effects of plant spacing on yield and or product quality, so no
example will be given here. If you want specific information about a species, it is probably available through
many sources. For each species and in some cases cultivars, there is an optimum spacing.
The optimum spacing for a variety does not necessarily optimize the photosynthetic capacity of individual
plants. Rather, it is the spacing at which the photosynthetic capacity of individuals is balanced against
other limiting factors and production is maximized over the entire area.
If plants are grown at closer than optimum spacings, interplant competition for water, nutrients and sunlight will
have a deterimental effect. Often this is a factor of within row spacing. In apples, a 2m X 10 m spacing will
have a greater detrimental effect than a 4m X 5m spacing, even though these schemes both result in a plant
density of 500 trees/HA. If plants are grown at wider than optimum spacings, individual photosynthetic rates
may increase, but at the expense of overall production. In general, it is the balance of limiting effects that is of
importance.
In general, plant spacing in field and landscape production is fixed at the time of planting. Therefore, until
maximum canopy area is achieved, sunlight energy is being wasted. However, under certain conditions, (eg.
high value crops) it may be advantageous to overplant the area originally, with the intent of removing
"temporary plants" as permanent plants become mature.
●
Leaf area index and leaf orientation
Leaf surface area is both a vertical and horizontal phenomenon. Leaf area index (leaf surface area (one
side)/unit land area is affected by both. The vertical aspects of LAI affect the photosynthetic capacity through
the effects of shading.
In general, about 70 percent of the light which strikes the leaf surface is absorbed. Therefore PAR rapidly
decreases as light penetrates the leaf canopy. Shading can have a dramatic effect on ps rates of lower leaves. In
general LAIs between 4-8 are optimal for most crop species. If the canopy is too dense (i.e., the LAI is too high)
there will be many leaves which do not photosynthesize enough to counteract their respiratory activities. Under
these cases overall yield will be reduced. Notice also that the optimum LAI is somewhat dependent on the
average light intensity (see above right). For a given species, the optimum LAI will be less under environmental
conditions where cloudy weather is the rule.
Leaf orientation also effects the optimum LAI. The shading effects of upper leaves is far greater in crops with
planophile leaves than in those with erectophile leaves. This phenomenon is most evident in C4 plants because
their light saturation point is high.
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Light
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Supplemental lighting - see additional handout distributed in class.
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic #4A: Radiant Energy and Its Effect on Crop Growth - Part 2- Heat.
References:
Text = Chapters 7 and 8
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science, growth, development
and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ. = Chapter 10
Janick, J., R.W. Schery, F.W. Woods and V.W. Ruttan. 1981. Plant science, an introduction to world crops.
H.W. Freeman and Co., San Fransico, CA. = Chapters 10 and 11.
Raven, P.H., R.F. Evert and S. E. Eichhorn. 1999. Biology of plants (6th Ed.). W.H. Freeman and Co., New
York, NY = Chapter 29
Quotations:
"All matter is composed of atoms that are in a state of vibration that depends on their relative heat. The
temperature of a substance is a measure of the relative speed with which its atoms are vibrating. If they are
vibrating fast, the temperature is high. Theoretically, at absolute zero (0°K, -273°C), all vibration ceases and
atoms at that temperature are absolutely still" Janick et al., 1981.
"The ability of plant life to adapt to changing temperatures within the life range ...... is remarkable. The critical
range varies widely from species to species. Banana, sweet potato, cucurbits and many tropical plants may be
seriously injured by exposure, however brief, to temperatures below 4°C. A properly acclimated apple tree on
the other hand, seldom suffers injury at -35°C" - Text
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Outline:
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Heat vs. temperature; temperature measurement
heat is a radiant energy form. It will move by conduction or convection from a warm body to a cold body
temperature is a qualitative measure of heat intensity but does not measure energy quantity directly (Note: see
heat of fusion or heat of vaporization below, where energy is added or given off during a phase change, but
temperature stays the same.)
Temperature can be measured using various instruments. Some of the common instruments include:
●
•thermometers - thermometers are based on the fact that materials (most notably liquids and gases)
expand when they are heated and contract when they are cooled. Common thermometers are constructed
with a reservoir filled with either mercury or alcohol which is attached to a calibrated capillary column.
As the temperature increases, the liquid in the reservoir expands and fills the capillary proportionally.
●
•thermocouples - thermocouples are based on the fact that the electrical conductivity of metals is
influenced by temperature. They are composed of two wires of different metals (usually iron and
constantan) that are fused (arc welded) at their tip. As the temperature changes, the difference in relative
conductivity of the two metals change. Conductivity differences are then measured electronically.
●
•thermisters - thermisters are also based on the fact that the electrical conductivity of a metal is influenced
by temperature. However, most thermisters are constructed of a single alloy which is extremely sensitive
to changes in temperature. Changes in the electrical conductivity in proportion to temperature fluctuation
are measured electronically. The advantage of thermisters is that they can be constructed to be very small,
so they are great for measuring temperatures of very small or delicate items (e.g., the temperature of a
bee, or the temperature of a seedling root).
●
•infrared radiometers - infrared radiometers measure directly, the amount of infrared radiation being
given off by a body (i.e., the heat escaping from the body). They are very useful for estimating the
temperatures of large bodies (e.g., the temperature of a corn field).
●
Some definitions
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•specific heat - the amount of energy required to change 1 g of a given substance by 1°C.
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See specific heats of various substances in Table 8-1. Notice that the specific heat of water is 1.00 and that it is
relatively high in comparison to other materials. Another way of putting this is that water is slow to heat up and
slow to cool down.
●
•heat of fusion - the amount of energy required to change 1 g of solid to 1 g of liquid at the melting point
●
•heat of vaporization - the amount of energy required to change 1 g of a liquid to 1 g of vapor at the
boiling point
See heats of fusion and vaporization of various substances in Table 8-2. Notice again that the heat of fusion and
the heat of vaporization of water are relatively high.
Figure 8-1 illustrates nicely the concepts of specific heat, heat of fusion and heat of vaporization. Starting at a
temperature of -100°C, energy is applied to a 1 kg block of ice over time at a constant rate (i.e., 100 Kcals/min).
As the specific heat of ice is 0.50 cal./g/°C (equivalent to Kcal./Kg/°C), it takes just ½ minute to raise the
temperature of the block of ice from -100°C to 0°C. As additional energy is applied, the block of ice begins to
melt. Since the heat of fusion of water is 80 cal/g (equivalent to 80 Kcals/Kg) it takes 0.8 minutes for the
melting process to be completed. Since the energy being added during this time went into the melting
process, temperature didn't change!!! Once the melting process was complete, the liquid water began to
increase in temperature in response to the added energy in proportion to the specific heat of water, 1 cal./g/°C
(equivalent to 1 Kcal/Kg/°C.) Therefore, in one minute, the temperature of the water went from 0°C to 100°C).
Then it was time for another phase change. The transition from liquid to steam took 5.4 minutes to complete as
the heat of vaporization of water is 540 cal/g (equivalent to 540 Kcal/Kg). Thereafter the temperature of the
steam rose in proportion to the energy supplied and its specific heat.
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•calorie - a calorie is the amount of energy needed to raise the temperature of 1 g of water by 1° C.
Note: dietary calories (those we count as we eat a hot fudge brownie with ice cream, whipped cream and
chopped nuts on top) are actually kilocalories (Kcals) = 1000 calories.
●
•BTU - British Thermal Units (residential and commercial building industry measures heat using these
units -- the output of your furnace will be expressed in BTUs). One BTU is the amount of energy needed
to raise 1 lb. of water by 1°F. 1 BTU = 253 cals.
●
Factors influencing temperature
•Latitude and season
●
Table 8-3 summarized latitude's effect on temperature for the Northern Hemisphere. No surprises here -however, please note that the temperature difference at the two solstices is almost nothing at the equator, but
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gets larger as one moves toward the north pole. For those who don't believe this, I suggest moving from Central
Ohio to Central Michigan --- not a great distance north but the winters are much more brutal.
Differences in temperature with respect to latitude actually result from latitude's effect on sun angle and day
length. Sun angle affects the relative intensity of radiation reaching the surface and it is 5.8 X greater at the
equator than at the north pole. Likewise, day length flucutations are extreme at the poles and almost
non-existent at the equator.
The effect of latitude on biological communities is illustrated in the diagram to the right.
●
•Elevation
A general rule of thumb is that for every 100 m rise in elevation temperature decreases 0.6° C. Consider two
cities that more or less are at the same latitude. Belan Brazil (19 m in elevation) has a mean temperature of
28°C whereas Quito Ecuador (3000 m in elevation) has a mean temperature of 13°C. The effects of elevation
and latitude mimic each other (Figure10-2). Latitude and elevation may act in concert. A typical snow line in
tropical regions is 4500 m where as in temperate regions they are only 3000 m.
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•Aspect or slope exposure
If you ever take a trip through mountains the effect of exposure (whether or not the slope faces N, E, S or W) on
temperature will be made obvious by the differences in vegetation. In the west, it is not uncommon at all to see
desert scrub on the south-facing slope while the adjacent north-facing slope supports juniper or pine. Crops also
are affected by slope exposure. In general southern or western-exposed sites are warmer than eastern or
northern-exposed sites.
●
•Time of day
A substantial portion of the radiant energy that reaches earth's surface is converted to heat. Diurnal fluctuation
in temperature is obviously a function of the varying amounts of incoming insolation at different times of the
day, including the night when insolation is absent (See Figure 8-6). However, diurnal fluctuations in
temperature typically lag behind the curve of radiant energy gain and loss throughout the day. Just after sunrise,
the ambient temperature and the relative energy (light and heat) gained from the sun are at a minimum. As the
angle of the sun becomes more direct as the day advances to noon, incoming energy increases. Surfaces (such as
leaves) exposed to sunlight absorb heat and become hotter than the surrounding air. Eventually they begin to
radiate that heat into the surrounding air. However, as it takes some time for this to occur so that the maximum
temperature occurs in mid afternoon, several hours after the insolation peak at noon. After that point, the
surrounding air also begins to cool, but is far more buffered than surfaces. After dark, surfaces are actually
cooler that ambient air temperature. The same sort of phenomenon controls the relationship between daylength
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and seasonal temperatures (i.e., summer and winter start on their respective solstices because the earth
temperature fluctuation lags behind changes in the duration or intensity of insolation.
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•Temperature inversions
Normally, the atmosphere on a cool night is layered such that the warmest air is nearest the earth's suface
(Figure 11-13 A). However, if the nights are long, skies are clear, the air is dry and there is no wind, cold air can
"drain" down-slope, setting up an inversion layer where the temperature gradient is reversed from normal. This
phenomenon is extremely important to understand for both frost avoidance (i.e., plant orchards on the sides of
hills) and frost control (i.e., using the inversion layer air to alleviate freezing conditions via wind machines).
●
•Large bodies of water
Large bodies of water have a moderating effect on temperature and adjacent land masses often enjoy a milder
climate than they otherwise might have. Figure 8-8 compares the seasonal fluctuation of monthly means in two
cities - St. Louis and San Francisco. Both cities are approximately situated at the same latitude and both have a
yearly mean temperature of 13°C. However, as San Francisco is a coastal city, their winters are warmer and
their summers are cooler than those of St. Louis. St. Louis temps range from -1°C to 26°C whereas San
Francisco's only ranged from 8°C to18 °C.
This phenomenon results from the fact that water heats up and cools down much slower than does air. The same
situation is responsible for "lake effect" snow as cold air crosses a "still warm" Lake Erie, picking up moisture
as it goes and then dropping it as it crosses land. The lake effect is also why it is possible to grow European
wine grapes all around Lake Erie and why there is an extensive fruit growing region in Michigan near the
eastern shore of Lake Michigan.
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Plant growth and development as affected by temperature
●
•Cardinal temperatures -
One system of modeling crop growth involves the determination of cardinal temperatures. If one measures
growth rate as a function of increasing heat, the first cardinal temperature one would reach is that of the
cardinal minimum, or the lowest temperature at which growth will occur. Presumably increasing heat would
accelerate growth in some sort of predictable way. Growth is accelerated primarily because the added heat
increases enzymatic activity. The Q10 is a relative measure of enzymatic activity.
Q10 = the increase or decrease in the rate of enzymatic activity in response to raising the temperature 10 °C.
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When temperatures are moderate Q10s are typically about 2 but as temperatures reach extremes these values are
much less. Presumably one could increase temperature and experience increased growth rate up to the point
when any additional increases in temperature would result in decreases in both Q10s and growth rate. This
temperature would constitute the cardinal optimum. Finally, if one continued to increase heat past the cardinal
optimum, Q10s and growth rates would continue to drop until the plant ceased to grow thus reaching its
cardinal maximum temperature.
Cardinal temperatures will be specific to cultivar, to stage of development being monitored , to plant organ of
interest, and growing systems or conditions.
●
•Degree days (growing degree days, heat units)
The calculation of degree days is yet another way to model crop growth and to predict maturity dates
In simple terms, a degree day is calculated as follows:
GDD = mean daily temperature - a crop-specific constant
The crop-specific constant is a base temperature for each crop which is experimentally determined.
Degree days are monitored daily and accumulated over time, presumably giving an estimate of how many
additional degree days will be needed to reach maturity. If the mean daily temperature is below the crop specific
constant, the GDD count for that day is defined as "0". In this modeling process, it is presumed that the
relationship between growth and heat is linear, even though that probably is not true.
Corn has a crop-specific constant of 50°F. If the mean daily temperature on a given day was 80°F then the corn
crop would have received 30 degree days for that day. If the crop needed 2400 degree days to maturity, then one
would need 80 days worth of 30 degree days in order to harvest.
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•Onset of dormancy
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Temperate woody perennials often require a brief period(s) of cold weather in conjuction with receiving critical
daylengths in order to "harden off" or to prepare for dormancy. Several light freezes prior to entering dormancy
actually increases winter hardiness.
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•Cold requirements (i.e., vernalization, stratification, chilling requirement - see previous notes and
handouts)
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•Heat stress
In latitudes from 20° to 40° N, midseason radiation = 400-500 cal/cm2
Since less than 5% of that insolation is used for photosynthesis, the rest is essentially converted to heat.
Remember that only 1 cal of heat is necessary to raise 1 g of water by 1°C. Since a 1 cm3 leaf volume contains
less than 1 g of water, heat build up is possible. In mild cases, heat stress can cause midday wilt. It can also
cause dehydration, denaturation of enzymes and metabolic imbalances in photosynthesis, respiration and
photorespiration.
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•Cold stress
Chilling injury - read second quotation on front page
Freezing injury - results from the formation of ice crystals (water expands when it freezes).
Intracellular events cause cell rupture and death; intercellular ice crystals cause tissue damage (tearing etc.)
which may disrupt the vascular system of flowers or new leaves during spring frosts.
No time to discuss it, but read about ice nucleation sometime - its kind of a neat phenomenon.
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•Frost tolerance
There are two types of frost tolerance to consider - tolerance to severe midwinter cold and tolerance or
avoidance to spring frosts. Species and cultivars differ with respect to both types of tolerance. Of the two,
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spring frosts have probably caused more economic losses over time than winter kills have (my opinion).
Read more if you have time.
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic 4B: Soil and Water and Their Effect on Crop Growth
References:
Text = Chapters 7 and 8
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science - growth,
development and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ. = Chapter 10.
Janick, J., R.W. Schery, F.W. Woods and V.W. Ruttan. 1981. Plant science, an introduction to world
crops. H.W. Freeman Col., San Francisco. = Chapters 10 and 11.
Raven, P.H., R.F. Evert, and S. E. Eichhorn. 1999. Biology of plants (6th Ed.). W.H. Freeman and Co.,
New York = Chapter 29.
Quotations:
"To many people, soil is merely dirt. From a plant's perspective, howver, soil is crucial for survival
because it provides support, water and a variety of elements essential for growth". (Raven et al., 1999)
"The importance of water for crop production cannot be overemphasized. Within a given temperature
zone, the availability of water is the most important factor in determining which plants can grow and
what their level of productivity will be". Text.
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1. What is soil and how is it formed?
"Soil is the unconsolidated, weathered, biochemically-modified portion of the earth's surface which is
composed of organic matter, minerals, air, water and organisms".
"Soil is the portion of the earth's crust that has formed through physical, chemical and biotic forces, in
which the roots of plants grow".
Parent rocks (igneous, sedimentary or metamorphic) are weathered to form soil parent material which is
then further decomposed until it differentiates into distinct layers or horizons - see below.
The factors involved in soil genesis include climatic weathering agents (i.e., temperature extremes, water
movement, ice, wind, and other physical forces), chemical weathering processes (i.e., hydrolysis,
hydration, carbonation, oxidation etc. - see lecture handout for details), and biologic factors (plants,
insects, worms, bacteria)
●
Soil profiles
Soil profiles are composed of horizons
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•The O horizon, the top most layer is typically less than 1" thick. It is composed primarily of
organic matter that is just beginning to decompose. - a litter layer or peat layer.
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•The A horizon is top soil (0-25" in depth) containing highly decomposed organic matter and
highly weathered mineral elements. Top soil in Ohio typically contains 2-5% organic matter most
of which is humus. Humus is highly decayed, colloidal organic matter that is chemically stable.
Humus improves soil structure.
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•The B horizon is subsoil (25-36" in depth) containing less organic matter, but still showing
significant weathering of mineral elements.
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•The C horizon is substratum (greater than 36" in depth) which may or may not include some
parent material (rocks). The C horizon has no clay or organic material.
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•Parent Rock lies below the substratum.
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Soil texture
Soil texture refers to the size of mineral particles in the soil.
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●
●
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•Gravel is > 2.0 mm = particles are visible to the eye
•Sand is 0.05 to 2.0 mm = particles are visible to the eye
•Silt is 0.002 to 0.05 = particles are visible with light microscopes
•Clay is <0.002 mm = particles are visible by electron microscopy
The texture triangle is a generally accepted scheme for classifying soils with respect to their relative
concentrations of sand, silt and clay. For most crops, loam soils (those which contain approximately 20%
clay, 40% sand and 40% silt) are considered optimal.
Soil texture influences many of the soil properties that are important to crop growth. Study Table 8-2,
which is self-explanatory.
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Soil structure
Soil structure refers to how the soil particles are aggregated into secondary units. These patterns of
aggregation also affect soil performance.
For our purposes here, the effect of both soil texture and structure on pore space is of most importance as
it affects both water and air movement through the soil. Pore spaces typically comprise 40-60% of the
soil volume and are either filled with water, air or a mixture of both. In general, light sandy soils,
although the have large pore spaces, the have less overall pore space than heavier soils with greater clay
content. Therefore, the water holding capacity of clay soils is greater than that of sandy soils. Conversely,
the speed at which water percolates through sandy soils is greater than that of clay (See Figures 8-19 and
8-20).
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Poor soil management often results in soil compaction which severely decreases pore space, limits the
amount of gas exchange and also impedes water movement through soils. In some instances, it physically
inhibits plant growth.
Conversely, in many horticultural situations, soil and additives such as peat, perlite, vermiculite, etc. are
mixed together for use as growing medium (potting soil). Aside from their effects on nutrition, these
additives also greatly affect soil porosity (Table 6-1). Soilless mixes (primarily peat mixtures) are also
very popular as growing media.
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Soil water - soil water potential
As we begin to discuss water movement in the plant, we need to mention that it is primarily caused by
tensile (pulling or sucking) forces rather than compression (pushing or squishing) forces. By definition,
tensile forces are expressed as negative numbers. Remember that lower numbers (those that are more
negative) indicate greater forces. A number of units are used to report these forces
1 bar is roughly equal to 1 atmosphere of pressure (actually, 0.987 atmospheres)
1 Mpa (a megapascal) = 10 bars = 9.87 atmospheres.
At this point in the lecture refer to the handout supplement entiled "A Short Discourse on Plant-Water
Relations". Additional diagrams 8-26, 8-25, and 30-7 simply augment the information provided in the
handout supplement.
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Functions of water in plants (quoted from text)
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•It is a necessary constituent of all living plant cells and tissues.
•It serves as a biochemical medium and solvent as nutrients from the soil and some organic
compounds move in solution from their site of uptake, production or storage, to sites of utilization.
•It is a chemical reactant or product in many metabolic processes, including photosynthesis,
although relatively little water is actually consumed or produced in metabolism.
•Without cell turgor resulting from water movement into cells, young cells would not expand.
•Functioning of stomata and normal plant turgidity depend directly upon adequate amounts of
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water
•It is a coolant and temperature buffer (remember, water has a relatively high specific heat. The
same phenomenon is responsible for the fact that sandy soils - with less pore space and moisture
holding capacity - warm up faster in the spring and are ready to plant early).
The transpiration stream
Review all pertinent material in the supplementary handout first.
When water is not limiting, the transpiration stream is controlled by the vapor pressure deficit (i.e., the
potential for water to vaporize into the atmosphere) at the leaf surface. Notice in the diagram below, that
the water potential of the atmosphere at 22°C and 50% relative humidity is around -100 MPa. Therefore,
it exerts a far greater pull on water than any other component of the continuum. The vapor pressure
deficit is -96 to -99 MPa. in this case
There is only one factor in the transpiration story that we have not mentioned yet. That is the
phenomenon of positive root pressure, a pushing force fueled by osmotic potential. When nutrients are
actively loaded into the xylem in the root, it creates a localized concentration of solutes greater than that
in the cortex or in the rest of the xylem system. Since the xylem sap in the roots have a greater
concentration of solutes than the rest of the system, there is an osmotic potential for these materials to
move out of the root zone to the aerial regions of the plant. This force is not nearly so large as that of the
typical vapor pressure deficit, but it is large enough in some species to cause xylem sap to exude from the
cut surface of a decapitated plant.
Transpiration rates are affected by irradiance, temperature, wind speed, soil moisture, relative humidity
and other plant factors.
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Water use efficiency
Each crop and perhaps each cultivar has specific water requirements (Table 31-1). However, the amount
of water used is not necessarily useful information unless it is related to growth rate or yield. For
instance, a field of prickly pear cacti (Opuntia spp.) may use very little water over a season, but it also
adds biomass rather slowly when compared to corn, for instance. Transpiration ratio (one measure of
water use efficiency) expresses water use in terms of dry matter yield (Table 9-2). Notice that corn,
which was designated as a high water user in Table 31-1 has a relatively efficient transpiration ratio (i.e.,
it develops a fair amount of dry matter for the water it consumes.
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No matter what the WUE a specific crop has, it can be grown most efficiently if all other factors of
production are in balance so that dry matter can be accumulated at optimum rates.
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Agronomic/horticultural methods to conserve water.
The availability of water for plant use is obviously not evenly distributed in all agricultural areas. In areas
where moisture is limiting, the following methods of conservation are often practiced.
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•Use of arid-adapted (drought tolerant) crops - sorghum
•Fallowing - cropping a field every other year allowing moisture to accumulate in off years.
CAREFUL- the dust bowl !!!!!!!!
•No-till systems - crop residue reduces run off, evaporation and increases water infiltration
•Mulches - can moderate soil temperature, control weeds, lower disease pressure, keep crops
cleaner AND conserve water ---- but, they can be expensive. Organic vs. synthetic mulches.
•Use of any cropping system that reduces run-off and controls erosion (e.g., terracing). This
promotes percolation of water into the soil
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic 4C: Nutrients and Their Effect on Crop Growth
References:
Text = Chapters 10 and 11
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky and W.J. Flocker. 1988. Plant science, growth,
development and utilization of cultivated plants. Prentice Hall, Inc., Englewood Cliffs, NJ. = Chapter 9
Raven, P.H., R.F. Evert and S. E. Eichhorn. 1999. Biology of plants (6th Ed.). W.H. Freeman and Co.,
New York, NY = Chapter 30.
Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil fertility and fertilizers (5th Ed.).
MacMillan Publ. Co., New, York, NY. = Chapters 1-6.
Quotation:
"Plant nutrition involves the uptake from the environment of all the raw materials required for essential
biological and chemical processes, the distribution of these materials within the plant, and their
utilization in metabolism and growth." Raven et al., 1999.
Outline:
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Essential elements
Soil fertility has been studied for at least 5000 years - a rich history of discovery
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•Greeks knew the value of adding materials such as animal waste (manure), green manure (crop
residues) and certain inorganic salts such as saltpeter or nitre (KNO3) marl (dolomitic limestone)
wood ash and other substances to soils in order to improve their fertility. They were also aware
that crop husbandry using such practices as tillage and crop rotation also improve plant growth.
Many of these practices were described fully by Theophrastus, a historian and philosopher.
The experimental approach to plant nutrition began during the Rennaisance. Philosophers of the time
were interested in identifying the "essence" of vegetation, of which they considered water, soil, saltpeter
and air, likely candidates.
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•Around 1600, Jan Baptiste Von Helmont conducted an experiment as follows: He planted a 5 lb.
willow sapling in 200 lbs of oven-dried soil held in a big tub. He then covered the tub so that
nothing could be added or taken away from the soil. He watered it solely with rainwater for a
period of 5 years. At the end of this period, he disassembled his apparatus and reweighed the soil
and tree. The tree now weighed 167 lbs and the soil weighed about 199 lbs and 14 oz (i.e., it had
lost only about 2 oz. in 5 years). From these results, he concluded water to be the principle of
vegetation. Of course he was not familiar with the importance of CO2 fixation by photosynthesis
as it had yet to be discovered for about 200 years.
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•Around 1760, Francis Home experimented with several different salts and discovered that all had
an effect on crop growth.
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•By 1840, oxygen, carbon dioxide had been discovered and the process of photosynthesis was
understood at a rudimentary level. About that time, Justis Von Liebig published a treatise on crop
husbandry in which he maintained the following; nitrogen could be garnered for the plant from soil
or air, and that minerals were absorbed by roots from the soil solution. He further stated his law of
the minimum--
"... By the deficiency or absence of one necessary constituent, all the others being present, the soil is
rendered barren for all those crops to the life of which that one constituent is indispensable"
In other words, crop growth requires several nutrients to be present at optimum levels to achieve
optimum growth and crop growth under limiting conditions will only proceed to the extent that the least
available element will allow.
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•In 1939, Arnon and Stout published a paper in which they describe the concept of essential plant
elements. For an element to be essential it must fit these three criteria:
a) a given plant must not be able to complete its life cycle without it.
b) the function of the element must not be replaceable by another element.
c) the function of the element must be directly involved in plant structure (e.g., cell membranes) or
metabolism (e.g., an enzyme component or it must be required for a distinct metabolic step such as an
enzyme reaction).
Some definitions and concepts
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•nutrition = supply and absorption of chemical compounds (ions) needed for growth and
metabolism
•nutrients = chemical compounds (ions) needed
•metabolism = mechanism by which nutrients are converted to cellular material (energy and
structure)
•nutrition and metabolism are related
Table 30-1 lists seventeen essential elements in the relative order of their occurrence in plants. Those
elements that are required in lesser amounts are called micronutrients whereas those required in greater
amounts are called macronutrients. Notice that some essential elements are taken up by plants as cations,
others as anions, others as both cations and anions (e.g., nitrogen) and still others as neutral molecules.
Note that elements not in this list may be essential for certain species (e.g. rice requires silicon for proper
growth and legumes require cobalt for symbiotic nitrogen fixation.
Table 30-2 lists some of the functions of essential elements as well as their deficiency symptoms.
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Elements can be classified by function as follows;
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•elements involved primarily in structure or in major constituents = C, H, O, N, S, Ca
•elements involved with energy transfer = P
•elements involved in oxidation/reduction reactions = Fe, Cu, Zn, Mo
•elements with diverse functions (e.g., K as an osmoregulator or Mn as an enzyme cofactor)
Hidden hunger and the proactive approach to fertility management.
Figure 11-7 charts the relative growth and production of a crop as a function of nutrient availability. In
theory, an individual diagram such as this generic one might be created for each essential element using
data derived through experimentation with a given crop. When the concentration (availability) of an
essential element is low or very low, it is said to be in its deficiency range. Deficiencies will lead to poor
or sub-optimal performance and visible symptoms such as the interveinal chlorosis of leaves typical of
Fe deficiency and others. When the nutrient is readily available (i.e., not limiting) to the plant, it is said to
be in its optimum range. The range of concentrations than can be considered optimum varies
tremendously among species and among elements. For instance, nitrogen can be available over a wide
range of concentrations and still be considered optimal, but certainly some species are "happy" over a
wide range of N concentrations while others are more sensitive to variability in N concentration.
Likewise, N typically has a prolonged optimum range whereas certain micronutrients such as copper,
have a very narrow optimum range (i.e., just a few parts per million) between deficiency and toxicity
(i.e., decreased plant function or even plant death at high concentrations). Some elements may not result
in toxic reactions even when their concentrations are very high. However, however these high levels may
not result in better plant performance (i.e., plant growth has been "maxed out" by another limiting factor
such as water availability or photosynthetic capacity). When elements are available at levels above where
they result in increased plant performance, they are said to be in their luxury consumption range.
In terms of managing fertility, it might be important to know whether or not a nutrient was available in
its luxury consumption range so that the expenditure of additional funds and effort to apply fertilizer
containing the element might be avoided. However, a more critical situation for fertility management is
that of hidden hunger (i.e., the nutrient is available at near optimum range, but it is still deficient).
Hidden hunger results in decreased growth and performance but usually does not result in visual
symptoms. Yields will be perhaps adequate, but not optimum under these conditions.
In order to combat hidden hunger, it is necessary to take a proactive approach to fertility management.
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Because visual symptoms are absent or hard to recognize, the only way to detect nutrient concentrations
in the hidden hunger range is to submit periodic samples of the soil and of the crop foliage for nutrient
analysis by a competent public or private laboratory. Soil tests are usually conducted prior to planting a
crop and periodically thereafter. Tissue samples should be tested periodically as well, because
availability in the soil or medium does not insure that the plant will be able to assimilate it. For instance
the soil could contain adequate levels of Fe, but if the pH is near neutrality, plants may not be able to
take it up. Recommendations for soil and tissue testing are crop specific. Consult State Cooperative
Extension information or information from individual labs about the correct timing and protocols for
these tests. Most crops won't require more than one soil and one tissue test per year for adequate fertility
management. However, some high value crops and greenhouse grown crops are monitored much more
closely (perhaps even continuously with the aid of computer-controlled nutrient delivery systems).
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The soil as the source for mineral elements
Mineral elements ultimately available for plant growth are found in two soil components
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•The soil solution = dissolved (ionized) salts in matric water. The soil solution may also contain
water soluble organic substances from biological sources (e.g., sugars from a near by root).
•Soil colloids = amorphous solids which are dispersed in the soil solution. The two types of
colloids that are important to plant nutrition are clay and highly decomposed organic matter
particles called humus. For a visual image of what colloid suspensions (dispersions) are like,
consider catsup -- catsup has particulate tomato matter that is dispersed or suspended within a
liquid medium. Repelling forces keep particulate material from "settling out" (see discussion
below) without the addition of force such as centrifugation.
Properties of soil colloids
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•They are small - usually about 2 microns in diameter
•Like other small spherical objects, they exhibit a high surface to volume ratio
•They have a net charge (dependent on the colloid type, but mostly negatively charged)
Negative charges on clay colloids result from isomorphous substitution of elements within the clay
crystalline lattice (e.g., Al+3 ions are replaced by Mg+2 ions) or from fracturing of clay colloids resulting
in localized cation deficiencies within the crystalline lattice. Note: some clays that are highly weathered
(oxidized) actually exhibit a positive charge. The negative charge on organic matter particles results from
ionization of organic acid groups associated with humic acids
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•They are dispersable in aqueous media. In order to maintain a colloidal dispersion, it is necessary
for colloids to repel one another. Otherwise they would coalecse into larger units and fall out of
the suspension. As most soil colloids are negatively charged, they naturally repel each other. The
polar nature of water molecules helps keep them apart as well. The partially positive end of each
surrounding water molecule associate loosely with the colloid (i.e., they are drawn electrically by
the negative charge) whereas the partially negative oxygen end of the water molecule extends out
into solution. Thus the oxygen atoms of water surround the colloids with an additional negative
charge.
Cation Exchange Capacity (CEC)
Clay and organic colloids act as a "bank account" for cationic nutrients. Just as water molecules can be
loosely associated with negatively charged soil colloids, it is electrically favorable for cationic nutrients
to be loosely held as well (Figure 2.2). However, these cations are not irreversibly bound and free ions in
the soil solution can be "exchanged" for those that are bound. Consider the two K+ ions in the diagram.
As these cations in free solution move toward the colloidal surface, the colloid's pull on them increases
(Figure 2.1) and when they are sufficiently close, it is possible for them to "dislodge" a Ca++ ion which
had associated with the colloid previously. Following this exchange, the K+ ions are now loosely bound
and the Ca ++ ion is now in free solution.
"Cation exchange is important because the exchangeable ions (those held on the exchange complex)
are (1) available to plants, supplementing the small quantity in solution, and are (2) retained in soils and
not lost with leaching water."
Rules governing the cation exchange process
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•the outgoing charge must match the incoming charge (e.g., it takes two K+ ions to dislodge one
Ca++ ion
•cations of higher charge are more tighly held and are harder to replace
•among cations of the same charge, those with smaller hydrated sizes are held more tightly (see
Table 3-2)
•even though the hydrated size of H+ is large, it is generally tightly held.
Cation Exchange Capacity (CEC) = the number of moles of exchangeable charge held by a Kg of soil.
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Note this is measured as charges not as atoms.
If a Kg of soil had a CEC = 1, it could potentially hold a mole (6.02 X 1023) of potassium ions as they
each would only require one of the colloid's charges. As the gram molecular weight of K is 39, it means
that this Kg of soil could retain 39 grams of K.
However, this same Kg of soil could only hold ½ mole of Ca ++ ions because each ion would take up two
negative charge sites on the colloid. Thus, since the gram molecular weight of Ca is 40, it means that this
Kg of soil could retain 20 g of Ca.
Lastly, for the same reasons, only 27 g of Al+3 could be held on this Kg. of soil even though aluminum
weighs 81 g/mole.
Other factors that affect soil fertility
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•pH
pH = log 1/[H+]. The pH scale is based upon the fact that pure H2O at 25°C dissociates into OH-1
(hydroxyl ion) and H3O+ (hydronium ion) at an equilibrium frequency of 10-7. Since pure water is
assumed to be neither acidic or basic, plugging this into the equation defines pH 7 as being neutral.
Effect of pH on cation exchange site occupancy
pH primarily affects soil fertility by affecting the ease at which cations can be released from cation
exchange sites on colloids, and thus, their relative concentration in the soil solution (see figures 1-3 on
the following page of your handout).
Since the beginning of the 20th Century, soil scientists have been studying the effect of pH on the
availability of nutrients in various mineral soils (soils that typically contain 2% or less decomposed
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organic matter). Although nutrient behavior is somewhat different in each soil type, the body of work
taken as a whole has resulted in the classic diagram shown in Fig. 1 (bottom right). Notice that as the soil
becomes more acid, the availability of N, P, K, S, Ca, Mg, and Mo drops off. Conversely, in basic soils,
N, P, Fe, Mn, B, Cu and Zn are less available. Therefore, the best "window of nutrient availability" in
these soils is at pH 6.5-6.8.
Similarly, pH effects on nutrient availability in organic field soils (muck soils containing @ 30% organic
matter) have been studied. The average findings of these studies resulted in the diagram depicted in Fig.
2 (top right) In general the pattern is similar to that for mineral soils. However, differences do occur
because the functioning of cation exchange in organic matter is different than in silicate clays. The
relative mix of cations occupying the cation exchange sites of clays is affected by the number of
hydrogen ions in solution, but the the CEC (number of cation exchange sites) will not change.
Conversely in organic soils, as the negative charge of organic material is dependent upon the ionization
of organic acid moieties, both the relative mix and the CEC will be altered by pH. As pH gets lower (i.e.,
hydrogen ion concentration gets higher), these organic acid moieties will not be ionized and will
therefore cease to be a cation exchange site until the acidity of the soil is neutralized. Therefore, the best
"window of availability" for this soil type is from pH 5.5-5.8.
At some point during your careers, you will likely use soil-less mixes (Pro Mix, Metro Mix and others)
for greenhouse culture. As the name implies, these products do not contain soil; rather, they are
essentially products made of nutrient fortified peat moss supplemented with horticultural texurizing
agents such as vermiculite, perlite, sand and composted bark. Aside from the slight nutrient-holding
capacity of vermiculite, the peat moss and composted barks in these mixes serve as the primary CE sites.
For reasons stated above, the window of availability for peat-based soil-less mixes is very different from
that of mineral field soils
John Peterson, an ex-OSU professor of floriculture, designed a study to determine the best window of
availability in Metro Mix. He adjusted the pH of this material pots to range from pH 4.3 to 7.8 using
FeSO4 or Ca(OH)2, added fertilizers, then after an incubation period, he examined the leachates from
pots at various pHs to determine the availiability of nutrients. The results of his study were summarized
in Fig 3; details can be obtained by reading the article passed out in class. Data on Mg and Mn were
particularly interesting.
Acidity (pH)
4.3
4.8
5.1
Mg (ppm)
164
324
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Mn (ppm)
4.1
2.7
0.8
5.6
135
0.3
Essentially, he found the best window of availability to be from pH 5.2-5.5.
Effect of pH on base saturation
The base saturation of a given soil is a relative measure of its fertility which is determined primarily by
clay type and amount and by soil pH. The term base saturation is defined as the % of CE sites held by
Ca, Mg and K. When the pH of the soil is decreased through the continual supplementation of crops with
ammoniacal fertilizers, acid rain (if it exists) or other means, the increased level of H+ ions cause
exchange events to occur where H is adsorbed onto the CE site while a basic cation is released into the
soil solution. Once in the soil solution, these basic cations, if not immediately taken up by plants, are
subject to leaching.
Other effect of pH on soil fertility.
pH also effects the rate of irreversible binding to clays and the rate of release of toxic ions (e.g., Al-3 at
low pHs -- see table on left margin of lecture handout). pH also affects the activity of soil
microorganisms (see supplemental handout on nitrogen fertility)
REMEMBER, the optimum pH for growth and performance is crop-specific (Figure 16-2 located on
page of your class handout which follows pH availability diagrams)
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•Fertilizer regimes and leaching
When a fertilizer such as NH4NO3 is added to the soil and subsequently enters the soil solution (after
dissolution), the soil solution concentration of NH4+ will rise and by their shear abundance, will begin to
replace ions currently held at cation exchange sites. One ion likely to be displaced by this process is K+.
If the soil were overfertilized with the nitrogen fertilizer, much of the potassium would enter the soil
solution and be subject (at risk) for leaching (percolating down through the soil beyond the reach of plant
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roots).
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•Soil biology
The area just surrounding the root is called the root rhizosphere. In this area, soil bacteria, beneficial
fungi, roots and other organisms secrete compounds (e.g. phytic acid) which affect (either positively or
negatively) the plant's ability to absorb nutrients. Our time will not permit much exploration of this
fascinating subject.
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Movement of nutrients from the soil solution to the root zone by contact exchange, mass flow, and
diffusion (see supplementary handout).
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Ion absorption by the root.
Root structure and development were thoroughly handled in Section 3, so only a brief "refresher"
treatment of the subject will be offered here. Remember that the developing root has an apical meristem
that is protected by a root cap. The apical meristem gives rise to developing tissue groups which
eventually differentiate and begin to function in the process of nutrient uptake. As you might expect,
most nutrient uptake is accomplished by young roots rather than those with secondary growth. However,
nutrient uptake does not typically begin in root tissues until the vascular system is completely
differentiated (Figures 4-2 and 4-3). Wiebe did an experiment where he fed radioactive 32P to various
regions of young developing barley roots. He noticed that radioactive material would accumulate even
right near the meristematic region, but it would not translocate out of the area until xylem and phloem
member were completely differentiated.
As we discussed earlier as well, the soil solution more or less has free access to the apoplastic regions of
the root cortex (i.e. apoplastic regions = cell walls and intercellular spaces; the apoplast stops at the cell
membrane). Following the fate of an individual nutrient molecule (e.g., K+) which has entered the
apoplast, one might see -- a) the K+ remains in the apoplastic region of the root cortex and perhaps
becomes loosely associated with (attracted to) a negatively charged cell wall component; b) the K+
eventually migrates out of the apoplast and back into the soil solution at large; c) the K+ is taken up
through a root hair, epidermal or cortical cell and is moved through plasmodesmata (intercellular
connections) toward the vascular system at the center of the root --- in order to do this, it must pass
through an endodermal cell prior to its entry into the vascular system (i.e., remember the function of the
Casparian strip); and d) it might get taken up directly into a endodermal cell.
THE TAKE HOME MESSAGE is that in order for nutrients to be absorbed into the plant, they must
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cross the semi-permeable membrane of a living cell. This will have its consequences - see below
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Active and passive assimilation of nutrients.
The fact that nutrients must pass through at least one living endodermis cell prior to xylem loading offers
the possibility of both active and passive uptake (i.e., active uptake can only occur in living cells.
Hoagland (a very famous plant nutritionist) demonstrated active movement of nutrients accross cell
membranes using two different species of algae: Nitella, a fresh water alga and Valonia, a salt water alga
(Table 2.1). Notice that in both algae the concentration of K+ is far greater in the cell sap than it is in the
surrounding water indicating movement against a concentration gradient (a dead give-away for active
transport). Notice also that Valonia appears to be actively excluding Na+ from its cell sap, as the
concentration in sea water is 5X the concentration inside the cell. If only passive systems were at work,
these concentrations would be nearly equal. From this table we can infer that for at least some nutrients,
uptake is selective and that it occurs against a concentration gradient.
In another experiment depicting active transport of K+, plants were placed in a nutrient solutions that
were optimal for uptake or were sub-optimal due to lack of aeration, low temperatures or denatured
(non-functional) enzymes (i.e., metabolism in general and specifically ATP metabolism was restricted.
Under optimal conditions, there was an immediate rise in the K+ concentration representing free
movement into the apoplastic regions of the epidermis and cortex of the root. Thereafter, uptake occurred
at a constant rate (i.e., enzymes responsible for active uptake were working at capacity). At the time
indicated by the arrow, the plant was removed from nutrient solution and placed in distilled water.
Thereafter, there was an immediate loss of K+ as that which was still in the apoplastic regions diffused
out into the distilled water medium. Under sub-optimal conditions, movement into the apoplast was again
evident. However, there was no uptake of K+ into the plant and when it was returned to the distilled
water, the efflux (outflow) of nutrient from the apoplast to the surrounding medium was equal to that
which had entered the apoplast at the beginning of the experiment. In another trial, these authors also
showed that Ca++ was a necessary component of this system. From these trials we can determine that
active uptake is irreversible and that it requires energy, enzymatic activity and functional
membranes in order for it to occur.
Actually both active and passive uptake of nutrients occurs in plants. The conditions necessary for
passive uptake or diffusion of cations are actually set up through active processes (see below). In general,
passive uptake of cations will occur under two conditions: a) there is a favorable chemical gradient where
concentrations of cations are greater outside the cell than inside the cell, or b) there is a favorable
electrical gradient, i.e., a negative charge on the inside of the cell, which causes movement of positive
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charges from outside to inside the cell. In actuality, condition "a" is often not met -- the concentration of
cations inside the cell is almost always greater inside the cell than it is on the outside of the cell. In this
case, if cations are going to continue to move into the cell, the cell's electrical gradient (negative charge)
must be great enough to offset the unfavorable chemical gradient.
How does the cell maintain its negative charge? -- through the active excretion of H+ ions via a "proton
pump". Proton pumps utilize the enzyme ATPase in order to cleave the terminal high energy phosphate
bond of ATP to form ADP and both H+ and OH-1 ions. The H+ ions are secreted during the the formation
of ADP whereas the OH-1 ions are secreted using a separate carrier protein that imports divalent anions
such as SO4-2. Therefore the net result of each ATP expenditure is the increase of one negative charge on
the inside of the cell. The inside of cortical cells usually maintain a negative charge of -70 to -150
millivolts, which is enough to insure the passive uptake of cations.
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Movement of ions in the xylem and the phloem.
●
•Ion transport in the xylem is primarily a function of mass flow. That is ions are more or less
carried along with the movement of water during the process of transpiration (see water notes).
Therefore, nutrients in the xylem move primarily from the root to the leaves at a rate that is
determined by the "tug of war" between the water potential of the soil and the vapor pressure
deficit at the leaf surface. Additional factors that effect ion movement in the xylem include:
possible transient association of cations with organic acid ions which are structurally part of the
cell wall; the size and the age of the plant (primary vs. secondary growth); and the time of day.
●
•Ion transport in the phloem is bidirectional from a source to a sink and its rate is determined
primarily by sink strength. The phloem is rich in most nutrients except perhaps, calcium (Table
3.8). Inputs into this phloem ion pool come from two primary sources - roots and senescent leaves
- although some materials flow directly from the xylem to the phloem through parenchymatous
vascular rays in some species. Recipients of material from the phloem ion pool (sinks) include
shoot apices, developing leaves and developing fruit. Cations often move in the phloem as salts of
organic acids such as malate (Figure 3.10)
●
•Although the phloem sap is rich in many nutrients, nutrients are differentially mobile (i.e., it is
easier to move some than others). Table 3.9 separates highly mobile nutrients such as K+ from
those that are intermediately moveable such as Zn++, and from those that are immobile such as
Ca++. In particular, the immobility of calcium in the phloem causes severe calcium deficiency
problems in fruit and other sinks under stress conditions (i.e., too cold, too hot, too dry, etc.).
Moreover, movement of calcium into sinks from the xylem is hampered by the unidirectional
movement from roots to leaves (i.e., fruit don't transpire as much as leaves do). Perhaps the most
famous of these nutritional deficiency diseases is tomato blossom end rot - similar diseases occur
in many fruits and vegetable crops, but unfortunately, they all have different names.
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●
•Not only are elements differentially moveable in the phloem, they are also differentially added to
the phloem ion pool by senescent leaves. Some nutrients are remobilized easily in senescent leaves
whereas others that have become parts of cell structures or are covalently bonded in organic
molecules such as enzymes, are not readily remobilizable. Notice that remobilization of calcium, a
major constituent of cell walls and cell membranes, is almost nil.
●
•Differential mobility and moreover, differential remobilization have important consequences
concerning nutrient deficiency diagnosis from visual symptoms (Table 6-5). Nutrients that are high
mobile and remobilizable will become deficient in old leaves first as they sacrifice their nutrient
content to supply developing sinks. Nutrients that are not mobile or remobilizable can not supply
developing sinks. Therefore in these cases, nutrient deficiencies will show first in developing
leaves.
●
•Soil biology
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Horticulture and Crop Science 200
Winter Quarter 2001
Lecture Topic 4C (Continued: Nutrients and Their Effect on Crop Growth
References:
Text = Chapters 10 and 11
Finck, A. 1982. Fertilizers and fertilization. Verlag Chemie., Deerfield Beach, FL.
Follet, R.H., L.S. Murphy and R.L. Donahue. 1981. Fertilizers and soil amendments. Prentice-Hall, Inc.,
Englewood Cliffs, NJ.
Jones, U.S. 1982. Fertilizers and soil fertility. Reston Publ. Co., Reston, VA.
Marschner, H. 1986. Mineral nutrition of higher plants. Academic Press, NY.
Mengel, K., and E.A. Kirkby. 1987. Principles of plant nutrition. Int'l. Potash Inst., Berne, Switzerland.
Plaster, E.J. 1985. Soil science and management. Delmar Publ. Inc., Albany, NY.
Tisdale. S.L., and W.L. Nelson. 1975. Soil fertility and fertilizers. MacMillan Publ. Co., New York, NY.
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*Handouts Have Individual Reference Lists
Quotation:
"When the soil elements essential for efficient plant nutrition and economic production are low in
availability, or are not in balance, chemical fertilizers and soil amendments are required....... The efficient
use of fertilizers and lime supplements the nutrient-supplying capacity of the soil minerals and soil
organic matter and decreases specific toxicities to achieve optimum agronomic and economic plant
nutrition and production (Follet et al., 1981)
Outline:
●
Nutrient Cycles and Their Relevance to Plant Nutrition - Please refer to handout on nitrogen cycle
as an example.
As you study the nitrogen cycle in the handout, pay attention to the factors that affect nitrogen
availability to plants. Understand how NH4+ is converted to NO3- in the soil.
●
Fertilizer programs
A. Program objectives
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●
Supply necessary nutrients
Nutrient requirments are crop-specific and in some cases, cultivar specific. Know whether the
crop/variety is a heavy user of nutrients.
Available nutrient levels are dependent on soil interactions. For instance, for nutrients subject to leaching
such as nitrogen, experts may recommend much higher levels of supplemental N fertilizers for crops
grown in sandy soils than for crops grown in heavier soils.
Nutrient recommendations such as those found in Extension bulletins are general. Each cropping system
is unique. The only way to ensure that your crop is adequately nurished, is to adopt a PROACTIVE
APPROACH to nutritional management. Follow guidelines for periodic soil and tissue testing, then
provide supplements as suggested in test reports.
●
Balance nutrient supply and the cropping cycle
Crops do not use inputs at a constant rate throughout the season. (I.e., some developmental activities
require more input than others).
For example, if one were to optimize the available N for wheat throughout its cropping cycle, one would
add total N for the season at the following times:
- 20% N applied at planting to stimulate early growth and tillering. Overall growth is important to
promote flowering and eventual high grain yield. Likewise, yield is directly related to the number of seed
heads (more tillers = more seed heads).
- 60% N applied during the period of stem elongation. This application adds to the development and
weight of the shoot. Stem diameter and strength positively related to its resistance to lodging (falling
down under the weight of the seed head). In addition, N during stem elongation increases stored
carbohydrates for use later by the developing inflorescence. (I.e., N at this time increases the # of
seeds/head and the wt./seed).
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- 20% N applied at flagleaf stage (just prior to flowering). This application stimulates grain filling
(wt./seed) and may increase overall protein content within the seed.
B. Types of fertilizers and their methods of application
See self-explanatory diagram from p. 2 of notes.
Fertilizer formulations often express the level of N, P, and K as a series of numbers - for example,
5-10-5 is straight forward - it indicates that this fertilizer is 5% nitrogen by
weight. The second two designations, however, are not so easy to interpret directly. The 5-10-5 refers
5-10-5. The first number,
to the percentage of phosphorus as expressed as phosphorus pentoxide (P2O5, the anhydrous form of
5 refers to the percentage of potassium as expressed by potash (K2O, the
phosphoric acid) and the 5-10simplest oxide of potassium).
From a molecular weight standpoint: N = 14; O = 16; P = 31; K = 39
P2O5 weighs 31 + 31 + 16 + 16 + 16 + 16 + 16 = 142
The phosphorus in P2O5 weighs 62
Therefore the percentage of phosphorus in P2O5 = 62/142 = 0.436
K2O weighs 39 + 39 + 16 = 94
The potassium in K2O weighs 78
Therefore the percentage of potassium in K2O = 78/94 = 0.830
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Conversion formulas are as follows -
% P2O5 (on bag) X 0.436 = % P
% P X 2.29 = % P2O5
% K2O (on bag) X 0.83 = % K
% K X 1.2 = % K2O
●
High pressure liquids (Gasses)
Materials such as anhydrous ammonia (a commonly used N fertilizer) are in a liquid state when
pressurized in a delivery tank, but vaporize (i.e., change to the gas state) when released into the soil.
NH3 is toxic to plants (especially to seeds) so these materials are typically applied prior to
Because it is somewhat dangerous (i.e., its toxic to us too, its basicity causes irritation to eyes, lungs
etc.), it is sometimes applied by a custom applicator. However, many Ohio farmers have the equipment
and expertise to do it themselves. You must be licensed or certified to do this.
The equipment used consists of a nurse tank, an applicator or injector which releases the liquid below the
soil surface, a means of filling the applicator from the nurse tank and a vehicle to pull the apparatus.
NH3 is applied by injecting it directly into the soil. Once in the soil, it rapidly undergoes this reaction
NH3 + H2O ------- NH4OH (ammonium hydroxide)
Therefore the pH of the soil rises drammatically to @ pH 9. Later by the action of nitrifying bacteria
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NO3- is formed and the pH returns to near its pre-treated level.
The major advantage of this method of application is that the N can be very evenly distributed if applied
correctly.
The major disadvantage is that the material is subject to volatilization (i.e., escape into the atmosphere).
Volatilization is mimimized by good application technique and proper soil conditions (good tilth, no
clods and a soil water potential near field capacity).
●
Fluids
●
Spray applications
Spray applications of liquid formulations are typically applied with "boom-type" sprayers. Liquid
fertilizers are dissolved or dispersed in water and held in a pressurized tank equipped with an agitator.
The material is delivered to the soil surface from the tank through a nozzle.
Application efficiency is affected by the following: nozzle type, nozzle number, nozzle height, tractor
speed and speed consistency (pressure changes can occur when speed fluctuates), wind, soil/field
conditions ("lumpiness") and accuracy of swath markings
Spray applications of liquid formulations have the advantage over dry fertilizers in that they can be more
evenly applied to the soil. Applications may also be made in conjuction with other materials, but
CHECK TO MAKE SURE THAT THE MATERIALS COMBINED IN THE SAME TANK ARE
COMPATABLE!!!!!!!!!
The disadvantage of spray applications is that the water in which they are dissolved drastically increases
the weight of material applied and thus transportation costs.
●
Fertigation
Fertigation (i.e., applying fertilizer through an irrigation system) can be accomplished using a number of
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delivery systems, but under field conditions, most typically using overhead irrigation or trickle irrigation.
In the plains states, approximately 60-70% of the overhead sprinkler systems supply N and other
nutrients to growning crops.
Fertigation through trickle systems may even be more common.
The use of fertigation in greenhouse crops is also extremely common. Greenhouse fertigation systems
use fertilizer injectors and are often automatic and under computer control.
The major advantage of a fertigation system is that the producer can provide nutrients at optimum levels
at all times. In other words, by applying nutrients repeatedly in small doses, allows the producer to
"micro-manage" nutrients delivered to the plant and to increase or decrease dosages rather rapidly in
response to environment or crop cycle. For instance strawberry growers in California and Florida apply
N through the trickle system during vegetative periods and when berries are forming, but incrementally
reduce N levels as the berries ripen to prevent oversoftening. Because only small amounts of fertilizers
are applied with each application, chance for fertilizer burn, nutrient costs, and leaching are all
minimized. Fertigation systems can be used in hydroponic culture or with inert media (like rockwool)
which eliminates soil or media variability.
The major disadvantages of a fertigation system is that they are expensive (high capital outlay for
injectors, filters, etc. and potentially increased maintenance costs). If water is not pure enough (e.g., pond
water used) or if the wrong fertilizers are combined, precipitates can form, clogging lines and sprinkler
heads/emmitters. Clogs can also be caused by soil particles, slimes or bacteria. In the case of trickle
fertigation, salts can build up in the wetting zone causing localized changes in pH and soil osmotic
potential. Perhaps most importantly, because nutrient levels are maintained at optimum levels through
frequent application of fertilizers, fertilizer reserves are low or non-existent, which, in turn, makes
management of the system critical and management mistakes very costly.
●
Foliar feeding
Applying fertilizers to above ground portions of plants is an old technique. In 1789, Forsythe in England
rubbed a mixture of manure, wood ashes, lime and urine onto the bark of young trees and noted that it
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stimulated their growth. Later, Griss (1844) published a report describing the benefits of foliar
application of Fe sprays to relieve Fe chlorosis in tree species.
The advantages of foliar feeding include the following: Material can be applied locally to the organ in
need and conversely the fertilizer will only affect the plant in a localized area. There are no soil
interactions which may bind added nutrients making them unavailable. Plant response to foliar feeding is
rapid.
The disadvantages of foliar feeding are as follows: Only small quantities are absorbed through the leaf
cuticle. Therefore, the positive effects of the applied materials are short-lived. Frequent application can
increase overall costs of operation.
Foliar feeding is typically used only for high value crops. Specific uses include:
Supplying micronutrients (B, Cu, Fe, Mn, Zn) to orchard crops
Supplying micronutrients when soil interaction is a problem (e.g., micronutrient availability at high pH
Supplying nutrients at critical times (e.g., spraying CaCl2 on apples just after petal fall to control cork
spot (a fruit development disorder) or on ripening fruit to prevent bitter pit (a storage disorder).
Applying urea to a broad range of crops -- urea applied to the surface of the leaf is converted NH3 and
CO2 by the enzyme urease NH3 is taken up in gas form and then immediately detoxified in the leaf
Foliar feeding efficacy is dependent upon
Uniform deposition on leaves. Surfactants or wetting agents are often used to improve dispersion of the
aqueous mixture on the leaf cuticle (which repels water naturally).
Deposition on the underside of the leaf. Spraying the undersides of leaves with foliar-applied nutrients
allows for greater penetration and uptake as cuticle layers are not as thick and stomates are more
prevalent (usually)
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Leaf age - young leaves absorb more (as much as 2X) nutrients than older ones.
Environmental factors --- High light levels increase cuticle thickness and cut down on absorption.
Temperature and humidity should be moderate -- apply during the morning hours.
Species difference -- banana, coffee, apple and pear can be foliar fed. Stone fruits do not respond well
Variety difference -- there is a five fold difference among apple cultivars in the efficacy of Ca treatments
for bitter pit.
●
Solid fertilizers
●
Methods of application
Broadcasting - the spreading of fertilizer evenly over the soil surface. Commonly a broadcast
application is followed by incorporation through disking or similar tillage treatments. This is perhaps the
most common fertilizer application technique. It is accomplished using commonly available equipment
and can be accomplished using various levels of technology (i.e., consider a homeowner spreading
fertilizer by hand). The techinque is also applicable for liquids and dispersibles. For annually seeded
crops, fertilizer is often broadcast prior to planting. When fertilizer is applied after the crop has emerged,
the process is called topdressing. Broadcast methods are also used with perennial crops.
The major advantages of the broadcast method are as follows: Application can be made rapidly.
Broadcasting is less labor and technology intensive than some other methods, Different fertilizers can be
blended and then applied without precipitation concerns of liquid applications.
The major disadvantages of the broadcast method are as follows: The efficacy of treatment is dependent
on the uniform application of material. Fertilizers must be dissolved in the soil solution before they are
availability for plants. If incorporation is not even, fertilizer burn may result.
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Banding - Banding fertilizer is the localized placement of fertilizer beside or near plant roots. In annual
crops banding applications are often implemented as beds are seeded. Banding is commonly used to
apply P and sometimes K and micronutrients. Phosphorus moves very little in the soil so placement of P
applications in the vicinity of the root zone assures that it will be "mined" by the developing plant roots.
If fertilizer is banded after the crop is emerged, the process is called side dressing; if the fertilizer is
placed directly in the seed bed, the process is called pop-up. Pop up treatments typically contain N, P
and K nutrients at low application rates. Because of the danger of salt injury, only low salt index
fertilizers and no NH3 generating fertilizers are used.
The advantages of banding include the following: localized placement of fertilizer prevents losses due to
leaching or fixation onto clay surfaces (irreversible binding). Less fertilizer is used. Fewer weed
problems develop as fertilizer is not generally spread throughout the field.
The disadvantage of banding is that concentrated applications can cause fertilizer burn.
Banding is used effectively in cold soils, wet soils, soils of low or high pH, soils that are low in soil P
and soils with high concentrations of Al or Fe
● Types of dry fertilizers
Pulverized fertilizers - made from crushing fertilizers into a powder (i.e., powdered rocks); They are
dusty and tend to absorb moisture which results in caking (e.g., rock phosphate)
Granules - treated to obtain large evenly sized grains which resist moisture absorption. Granules usually
spread easily but still contain fines which results in dust (e.g., ammonium sulfate)
Prills - smooth, rounded, uniform and relatively dust free pellets. Prills are coated with diatomaceous
earth (a silicate compound made from diatoms or plankton) to prevent caking and to facilitate pouring,
flowing and spreading. (e.g., urea)
Fritts - shattered glass containing approximately 3-6% fused mineral nutrients (Cu and other trace
elements). A type of slow release fertilizer.
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Chelated fertilizers - chelated fertilizers are composed of multivalent cations that are stabilized in the soil
solution through association with organic molecules (ligands). Chelate is derived from the greek word
chela which means claw. Chelation helps to offset the effects of media pH on nutrient binding and allows
nutrients to remain soluble when, if unprotected, they would adhere to soil colloids very tightly.
Iron and zinc are two micronutrients that are relatively unavailable at neutral pHs that are often
formuated as chelated fertilizers. Their association with EDTA (ethylene diamine tetraacetic acid) and
EDDHA (ethylene diaminedi-o-hydroxyphenylacetic acid), respective are shown in the lecture outline
along with information about Fe availability in soils and characteristics of additional chelating agents.
Plants produce and exude their own chelating agents such as citrate and alpha keto gluconate which help
cycle nutrients from soil colloids.
Slow release fertilizers. Fertilizers formulated to release nutrients slowly and evenly over time (i.e., they
have lasting residual effects). Because they release nutrients slowly, they have a low burn potential.
Typically they are very expensive.
There are several types of slow release fertilizers: 1) fertilizers that are released through biological
degradation of their coatings (e.g., UF) 2) fertilizers that are released as their coatings weather (e.g.,
IBDU and sulfur-coated urea) 3) fertilizers that are covered with a semi-permeable or impermeable
plastic coating (e.g., osmocote) 4) organic fertilizers.
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Horticulture and Crop Science 200
Winter Quarter 2001
Additional Material 1: Plant cells, cellular components and selected tissues
We will not discuss the following information in class, but you should be familiar with it! Most of this
material you should have covered in high school or university biology classes. You can find
diagrams/illustrations of the plant cell, cell types, etc. in just about any botany, biology or plant science
texts. I will be glad to discuss it individually with you if you need help.
The plant cell
The cell is the basic unit of living matter; the plant cell is the basic structural and functional
(physiological) unit of the organism. All plant cells are similar in the early stages of their development,
but as they mature, they can become highly specialized -- leading to the tremendous diversity we see
(and perhaps take for granted) in the plant kingdom. Individual plants then are composed of highly
differentiated cells, each adding to the structure and function of their organ or tissue group. The study of
how these cells are differentiated and then organized in tissue groups or organs is called the study of
anatomy.
A typical vascular plant contains hundreds of billions of cells. A typical apple leaf alone is composed of
50 M cells. Plant cells also vary tremendously in size and shape (e.g., comparison of meristematic
parenchyma cells with xylem vessels or tracheids). Plant cells are differentiated from animal cells
because they have cell walls.
General features of eukaryotic cells
The eukaryotic cells of higher plants have defined nuclei. Eukaryotic cells are compartmentalized;
individual compartments are called organelles (e.g., mitochondria, chloroplast, vacuoles) and these
entities are bound by membranes. The membranes typically exhibit biochemical functionality (i.e., they
contain membrane-bound enzymes which perform critical metabolic reactions).
Cell components (refer to any diagram of the plant cell in any text)
cell wall - cell walls provide protection, support and shape for each cell. Primary cell walls are composed
primarily of cellulose, an indigestible polymer of glucose (as opposed to starch, which is a digestible
polymer of glucose), hemicelluloses, polymers of xylose and other sugars) and pectins (polygalacturonic
acid). Primary cell walls are formed during the early stages of a cell's growth and therefore, must be
expandable (i.e., must be able to incorporate newly synthesized cell wall constituents during cell
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expansion). Secondary cell walls are synthesized inside the primary cell walls only after growth ceases;
they are not elastic so they add overall strength and protection to the cell . Secondary cell walls also
contain cellulose and other complex sugar-derived molecules, but in addition, they contain polyphenolic
materials called lignins. Lignins are incredibly durable compounds; they render the cell wall rigid.
protoplast - the protoplast is the "living" portion of the cell - it includes the plasma membrane, the
cytoplasm and all organelles, etc. found inside the cell.
plasmalemma (cell membrane) - cell membranes are semipermeable guardians of what enters or leaves
the cell. They are composed primarily of phospholipids, and proteins in a somewhat complex structural
arrangement. See Raven et al., or any other text for a discussion of the Danielli-Davidson model of the
cell membrane and how it functions. Its fascinating!!
cytoplasm - Cytoplasm is the medium in which all cell solutes (e.g., sugars and proteins) are dissolved
and all cell particles (organelles, membranes etc.) are suspended. It is primarily composed of water, but
because of all the material dissolved in it, it has a viscosity similar to raw egg (i.e., sort of slimy). The
cytoplasm is the site of many important metabolic reactions supporting cell/plant life. Organelle
movement (which can be seen by microscopy) with this body is called cytoplasmic streaming.
vacuole - Vacuoles can be considered to be the cell's storage cupboard or perhaps its garbage dump, as it
contains primarily materials that are no longer needed, or in some cases, are detrimental to cell function.
The solution in the vacuole is often referred to as cell sap. Vacuoles, through their regulation of specific
ions, also regulate the pH of the cytoplasm (a very important function). The vacuoles of newly-formed
cells are rather small and as the cell ages, these bodies coalesce to eventually form one body that fills up
80-90% of a mature cell's volume. The membrane surrounding the vacuole is called the tonoplast.
nucleus- If vacuoles are the garbage dump, then the nucleus could be considered as the cell's brain. It is a
structure which is bounded by a double membrane called the nuclear envelope. The liquid (gel-like)
medium in the nucleus, called the nucleoplasm, is composed of chromatin (DNA, RNA and proteins).
Obviously, chromatin contains the genetic code which directs all cell activity. During replication, DNA
is transcribed and the message is "copied" per se in the form of messenger RNA. More info below and in
later lectures.
nucleolus- The nucleus also houses one or more nucleoli, spherical bodies also composed of DNA, RNA
and proteins, which are the sites of ribosome subunit synthesis.
endoplasmic reticulum- a long membranous network extending throughout much of the cytoplasm. The
ER is a direct extension of the nuclear envelope and, therefore, is also a double membrane. It is very
important as a "highway" (sort of) for the transport of materials around the cell. It is extremely important
as the site of translation, or the reading of the genetic code (as messenger RNA) to form specific proteins
(enzymes). Enzymes serve as templates enabling the cell to synthesize or degrade all biochemical entities
as needed for cell growth and function. When ER is functioning as a site for enzyme synthesis, it is
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associated with spherical particles called ribosomes (mostly ribosomal RNA and protein). Because these
particles can be seen by microscope, ER associated with ribosomes is termed "rough ER" and ER without
ribosomes is "smooth ER".
ribosomes- Ribosomes are particles responsible for the reading of messenger RNA and for the translation
of this message into functional enzymes. In other words, they're sort of like "work benches" where amino
acids (the building blocks of enzymes) are assembled into chains as dictated by the blueprints provided
by messenger RNA. The amino acid pattern in the chain ultimately determines its three dimensional
structure which allows the enzyme to function.
plastids - Plant cells may contain a number of double membrane-bound organelles called plastids. They
vary in size, shape, structure and function. The typical plastids are chloroplasts (responsible for the
process of photosynthesis), chromoplasts (storage of pigments) and leucoplasts (storage of starch)
chloroplast- (refer to diagram 3.1 at bottom of Page 2 of the original handout). The chloroplast is the site
of photosynthesis. It can be estimated that in a single leaf cell could contain from 20 - 100 chloroplasts
and that a cubic mm of leaf palisade could have 500K chloroplasts. Chloroplasts contain chlorophyll, the
green pigment responsible for light energy harvest, but also contain carotenoid pigments as well. It is the
latter pigments that give leaves their typical fall color. Stroma, the liquid matrix of the chloroplast,
contains the enzymes necessary for fixing CO2 , eventually forming glucose. Within the stroma, is an
elaborate system of membranes, collectively called lamellae. These lamellae contain bound enzymes
responsible for many of the biochemical reactions of photosynthesis. Concentrated bundles or stacks of
lamellae are called grana. Grana lamellae specifically hold the membrane-bound enzymes responsible for
the capture of light energy.
mitochondrion- Mitochondria are also composed of a semigel stroma rich in enzymes. They also contain
membrane invaginations called cristae, which serve to drastically increase overall surface area in the
organelle. Mitochondria are the site of respiration, the chemical process which breaks down sugar and
other fuels to form ATP and NADH. ATP is a molecule which stores and eventually transfers chemical
bonding energy during enzymatic reactions whereas NADH has a similar function, but involving
reducing equivalents (electrons).
NOTE: Most likely because they both evolved long ago from bacteria, chloroplasts and mitochondria
both contain some DNA of their own. This DNA is not involved in the process of meiosis and is
therefore only inherited maternally.
golgi apparatus (dictyosomes) - a network of highly polarized membranes which are involved in
secretion (to vacuoles or outside of the cell). They also function in providing materials for both cell wall
and cell membrane synthesis.
Types of cells
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parenchyma - Parenchyma cells are thin-walled cells. Primary cell walls are the most prominent in these
cells, but occasionally, secondary cell walls are formed. They are usually polyhedral (many-sided) in
shape and have large central vacuoles. They are found in abundance in younger tissues such as
developing leaves or fruit; their thin-walled nature allows for rapid enlargement. However, they are also
commonly found throughout the plant in pith, vascular, cortical and epidermal tissue systems. They are
highly metabolic, performing a variety of functions throughout the plant, including photosynthesis.
collenchyma - Collenchyma cells are specialized parenchyma cells that function as support during the
primary growth phase of plants. For instance, the "strings" found in celery stalks are composed of
collenchyma cells. These cells are typically "living" when they function in the plant.
sclerenchyma - Sclerenchyma cells are highly specialized cells which function primarily without
protoplasts (i.e., after they are dead) as cells which provide support, protection or a means to conduct
water and nutrients . They have thick, heavily lignified secondary cell walls. Two subtypes of
sclerenchyma are fibers and sclerids. Fiber cells are highly elongated cells with tapering ends, usually
they are arranged within the plant body in an overlapping manner which allows a plant to hold a stem
erect or to manage a heavy seed-head. Sclerids are more polyhedral in shape and are usually used for
protection. Seed coats, nut shells, pits, etc., contain sclerids. The stone cells found in pears (the cells in
the fruit which give it a slightly gritty texture) are also sclerids.
Xylem and Phloem (see any plant science text for illustrations)
Xylem tissue is a complex tissue that essentially has the job of conducting water and dissolved minerals
to all regions of the plant. A typical xylem tissue has four types of cells: tracheids, vessel elements, fibers
and parenchyma.
Tracheids and vessel elements are sclerenchymatic cells function only after the protoplast is dead. They
are similar in function as they are the cells that conduct water & nutrients. Tracheids are long and
tapered; water is conducted from one tracheid to another by passing through "pit pairs" or matching holes
in adjacent cell walls. Vessel elements are similar except that they are somewhat shorter and they are
arranged end to end in one long continuous column. As these cells die, their end walls disintegrate,
leaving little to impede the flow of liquid through the vessel. Xylem fibers are very thick walled
sclerenchymatic cells that function as support. They are perhaps, most like tracheids. They also lack a
functioning protoplast at maturity but have much fewer "pit pairs". Parenchyma cells in xylem are either
arranged in vertical files or scattered. They are of course, living, and act as food storage sites and lateral
transport between xylem and phloem.
Phloem tissue is also complex; its primary function is to transport food (sugars) and other metabolites
throughout the plant-- essentially moving it from leaves to other organs (stems, roots, fruit, etc.). Phloem
has essentially four types of cells: sieve cells, companion cells, fibers and parenchyma.
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Sieve cells are elongated thin-walled, large-vacuoled, parenchymatous cells with protoplasts that are
pressed against the inner surface of the cell wall. Although they are living at maturity, the protoplasts
contain no nuclei. In dicots, the sieve cells are positioned vertically to form sieve tubes; each sieve cell in
a sieve tube is considered to be a sieve element or a sieve tube member. At each end of a sieve element is
a porous sieve plate. During the working life of a sieve tube, these plates allow for the mixing of
cytoplasm from one sieve member to another, greatly facilitating the transport of materials throughout
the plant. As this tube ages and eventually ceases to function, these sieve plates often get "plugged up"
by deposits of callose, a carbohydrate polymer. Companion cells, found primarily in angiosperms, are
living and do contain nuclei. Their function is to regulate the metabolism of the enucleated sieve cells.
Fiber and parenchyma cells in phloem have the same function that they do in xylem.
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I. History of Agriculture
A. Agriculture developed through the domestication of :
● Plants
● Animals
B. 10,000 years ago (Near East)
● wheat and barley
C. plentiful wild fruits and nuts precluded domestication of
these crops until later 8,000; 9,000 years ago in W. Hemisphere (Mexico and Peru)
● Squash, chili peppers, maize, and avocado
● Later beans, cotton, fruits
II. Agriculture and Social Change
A. End of Nomadic Lifestyle
In the book "Crops and Man" Jack Harlan describes the era before Agriculture as the "Golden Age". He
studied existing hunting and gathering communities (Australia, southern Africa). Women and children
did the gathering. Men past puberty did the hunting. Hunting is a high-risk, low return activity.
Gathering is a low-risk, high-return activity. Hunting was more for sport than necessity. The meat
acquired through hunting was not necessary for nutrition but it did make the diet more interesting.
Everyone had more leisure time compared to the modern world. Hunters and gatherers have survived in
areas where agriculture has been unable to penetrate.
B. Increase in population
C. Concentration of populations in urban areas surrounded by rural areas (rural/urban interface)
D. Fostered trade
E. Fostered sedentary way of life
F. Diets narrowed
G. Led to work specialization
H. Creation and accumulation of wealth
I. Social distinctions increased
J. Loss of appreciation for rural inhabitants
K. Manufacturing launched
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L. Back now to rethinking past 100 years.
The past 100 years have been the era of "scientific agriculture". Productivity increased dramatically,
especially for major food crops like wheat, rice, and corn. Up until 1970 we relied on the use of external
inputs like inorganic fertilizers and pesticides. We are now concerned about the place and impact of
agricultural/horticultural activities on the world around us. Most of our newer research looks at methods
of maintaining productivity with reduced inputs and more accurate use of external inputs with the goal
being more favorable effects of crop production and utilization on the environment.
Thought questions:
Why do you think agriculture (agrarian) societies evolved from hunting gathering societies in spite of
such negatives as a narrower diet and less leisure time?
Why do you think that agriculturists came to be looked upon as lower class citizens as agrarian societies
evolved into manufacturing societies? In other why words, why does it suck to be you and me since
everyone is this room has some interest in agriculture (even if it is only to pass this class to fulfill a
GEC requirement)? Why do students with majors outside this college look at you funny when you have
to talk about your major/interests in classes across the river?
III. What is a crop????
A. Plants grown and utilized by humans for economic gain
B. Food, fiber, forage, forestry, ornamental, and recreational uses
C. Definitions:
● Agronomy " agros" and "nomos"; the science of crop production and soil management
● Horticulture ("hortus") the science of intense cultivation of plants
● Forestry &ntilde; foris &ntilde; the science of forest management and wood production
IV. History of Crop Science
A. Earliest crop scientists were observers and selectors
● observed better growth of plants near water and waste heaps
● selected certain species for yield , usefulness, harvest predictability
B. Followed by scientists studying ways to improve production
● Tillage (managing of soil in fields, landscape, athletic facilities for drainage, root and seedling
growth, greenhouse; nursery soilless growing mixes)
● Planting (mechanization/automation; including seed drills, field transplanters for vegetable crops,
greenhouse and nursery seeders and transplanters)
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●
●
●
●
●
Cultivation (managing of growing crops; weed control in field crops, mowers, aerators, etc for
landscapes and athletic facilities, energy intense greenhouse environmental control)
Irrigation (fields (agronomic, horticulture, intensely managed forests, landscapes, athletic
facilities, greenhouses, and nurseries)
Harvest (combines, corn pickers, fruit and vegetable harvesting machines, mobile benches or
conveyance systems in greenhouses and nurseries).
Storage (post-harvest handling); grain elevators, refrigerated storage/transport systems, fast
transport systems, modified/controlled atmosphere storage, chemical preservatives
Processing traditional uses of grains, legumes, fruits, vegetables and floral crops.
C. Today's scientists study integrated approaches to crop and environmental management
● Crop diversity - germplasm (gene) preservation, traditional breeding, biotechnology and gene
transfer
● Low-input/impact crop production - reducing chemical dependency [Bt crops, Round-up Ready (
crops, integrated pest/crop management (IPM/ICM) systems], more energy efficient
environmental manipulation of greenhouses, low-till/no-till
● Environmental stewardship - reducing NO3- and pesticide run-off (fields, greenhouses/nurseries,
athletic facilities), top-soil conservation, wildlife habitat on farms and golf courses, wetland
preservations
● Value-added; ethanol, specially packaged foods, double-use floral crops, specialty wood products
from formerly waste products, Flvr-Savr tomatoes, Lunaria (money plant) common garden
ornamental now being cultivated and harvested for neuronic acid used in treatment of M.S. and
premature birth problems,
V. Crops
A. Traditional
● Food
● Fiber (including wood products)
● Medicine
B. New
Ornamental
● landscape floral
● recreational
Industrial; mainly replacements for petroleum products
Lost medicine; ethanobotany
Land preservation/restoration
● prairie
● forest
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●
●
●
wetlands
wildlife habitat
problem; large ecosystems difficult to maintain in small (often middle of crop lands) areas
VI. Crops defined
A. Grains
●
Almost all are cereal grains in the Grass Family (Poaceae)
Wheat (Triticum spp., T. aestivum most common species grown)
1) most widely cultivated
2) primary human food grain in U.S.
3) winter vs. spring wheat
a) winter wheat planted in the fall, germinates and overwinters as seedling, matures and produces seeds
in the summer, needs cold (vernalization) to become reproductive
b) spring wheat planted in spring, germinates in spring, produces seed in summer. No vernalization to
become reproductive
c) winter wheat grown where winters are mild enough to allow survival of seedlings (central U.S., west
of Mississippi, (Eastern, Southeastern, and Pacific Northwest U.S
d) Uses
bread, Spring wheat -- pastries, cookies, cakes
pasta made from T. durum grown mainly in ND
Rice (Oryza sativa)
1) Chief food of nearly world's population
2) Types:
a) paddy. flooded during it&iacute;s growing season
b) upland. can grow without standing water
3) 7-8% protein (amino acids - CarbonHydrogenOxygen +Nitrogen, Sulfur),
75-80% carbohydrates (CHO only)
4) Wild rice is different species (Zizania aquatica) and is native to N. America. Minnesota, S. Canada
primary production areas.
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Corn (Zea mays)
1) Corn occupies acreage of wheat, worldwide production is equal to wheat
2) Leading US grain crop
3) 90% used for livestock feed
a. grain
b. silage
5) Human consumption: cereals, sweeteners, corn meal, starch, alcohols and spirits, popcorn, sweet
corn
6) Industrial uses: adhesives, plastics, laundry starch, gasohol, paints
Sorghum and millet
1) Diverse group used for cereals, syrups, forage, livestock feed, birdseed
2) Short growing season
3) Drought tolerant
4) Warm temperatures (Nebraska and Texas primarily)
B. Pulse crops
1. Legumes with edible pods
2. Grown on less than 10% of non-forage cropland but extremely valuable as source of high quality
protein and oil
Soybean (Glycine max)
a. 2/3 world's production in US and Brazil
b. less than 10% consumed by humans
c. most processed for oil and high protein mean
d. lack sulfur containing amino acids
Peanuts (Arachis hypogea)
a. direct food source for oil and protein, contain all essential amino acids
C. Oil, sugar, fiber, and pleasure crops
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1. Oil: sunflower, safflower, cottonseed, olive, coconut, canola, others
2. Sugar: sugar cane, sugarbeet, maple sap
3. Fiber: cotton, flax, jute, hemp (a.k.a. Cannabis sativa), kenaf
4. Pleasure: tobacco , coffee, tea
D. Forages : plants that produce vegetative matter that is fed to animals either in a fresh or
preserved state
1. Grazed pasture or range
2. Harvested or gathered
● Hay - preserved in dry form (15-20% H2O)
● Silage preserved moist (40-55% moisture) under limited O2 (fermented)<BR>
3. World wide more land in forage than all other crop land combined
4. The value of forages is to support animals for human diet (milk and meat products) or pleasure
(horses and other foraging pets).
E. Vegetables and fruits
1. Plant parts needing little or no processing to be consumed by humans (FL, CA, TX, AZ, Southeast
States, NJ (Garden State), NY, OH, MI),
a. Vegetables: non sweet e.g. tomato, potato, squash, greens (lettuce, spinach), green beans, peas, corn
b. Fruits sweet e.g. apples, pears, cherries, grapes, peaches, figs
2. Note: botanically speaking, a fruit is a fertilized and fully grown ovary. Tomatoes are technically a
fruit, as are beans, peas, etc. but we call them vegetables in everyday language (common vernacular).
F. Ornamental or recreational crops
1. Floral crops plants grown for the aesthetic appeal of their flowers and sold mainly for indoor use, e.g.
chrysanthemums, poinsettias, Easter lilies, 100's of others (FL, TX, CA, OH, MI)
2. Foliage crops plants grown for the aesthetic appeal of their leaves and sold mainly for indoor use, e.g.
pothos, ficus (same family as figs), spathyphyllum, many others (FL, TX, CA)
3. Landscape plants grown for aesthetic appeal of either flowers or foliage and intended for use as
plantings outdoors in the landscape, e.g. ornamental trees and shrubs, annual bedding plants (100's of
species, perennial herbaceous plants
3. Turf: grasses grown and maintained for golf courses, athletic fields and landscapes.
Types of grasses used in turf applications: Bentgrass, Bermudagrass, Kentucky Bluegrass, Buffalo
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Grass, Carpetgrass, Centipede Grass, Ryegrass, St. Augustinegrass, Tall Fescue, Zoysia
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Introduction to Experimental Design
Introduction to Experimental Design and
Data Analysis
How to study crop production
Studying improvement of crop production is not a simple task. Crops are rarely grown in isolated,
perfectly controlled environments. Rather, they are cultivated in the systems shaped by weather,
geological history of earth, human activities, and by other surrounding organisms. All these factors
affect crop development and yields. Therefore it is important to take them into consideration when
setting up the experiments and interpreting the experimental results.
The main objective of the lab portion of this course is to introduce you to the principles of
experimental design, collection of useful information (data), interpretation and analysis of experimental
data. Once you are comfortable with these techniques, you will be able to set up your own experiments
to test whether different treatments can increase crop yields.
This interactive website is designed to help you get acquainted with the experimental process.
Experiments on each website contain brief summaries of known information on the subject we are
about to investigate. Based on this information you can decide which unexplored phenomenon you and
your teammates would like to study in your experiment.
To help you prepare for more productive lab work, we have included pre-lab assignments. Please,
take your time to complete these assignments. It is always a good idea to prepare a flow chart of an
experiment that you are about to begin. In your flow chart, identify the hypothesis you want to test, the
independent and dependent variables and the control treatments.
Principles of Experimental Design
Before you start any experiment, identify the prediction (hypothesis) to be tested. Suppose, you have
decided to test whether a new fertilizer "WonderGro" really increases corn yields by 25% as the
manufacturer claims. The prediction you are about to test is:
Under field conditions, application of the WonderGro fertilizer at the rate of 500
kilograms/hectare (446 lb/acre) increases grain yields of corn by 25%.
This statement anticipates the outcome a fertilizer application might have on crop yield. This statement
is called a hypothesis. A hypothesis should:
● describe the conditions for conducting the experiment (field conditions in our example);
● identify the dependent variable (corn yield in our example);
● identify the independent variable (fertilizer application);
● define what data will indicate a relationship between the variables (yield);
● predict what effect the independent variable will have on the dependent variable (25% yield
increase).
What are the variables in this experiment? The variable to be tested is the dependent variable. In
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other words, a dependent variable is the result observed in response to the independent variable. Grain
yield of corn depends on fertilizer application, therefore, in this example grain yield is a dependent
variable. Fertilizer application is an independent variable.
Are there any other variables in this experiment? Can history of the field (crop rotation, previous
herbicide application, field disease history, form of tillage, etc.) affect the dependent variable?
Unfortunately, yes. The variables that obscure the relationship between the independent and dependent
variables are known as confounding variables.
To reduce the effect of the confounding variables on the dependent variable, three important
experimental design principles are used: control, randomization and replication.
A control treatment elicits either no response or a highly predictable response, which serves as a
standard to compare with the results from other treatments in the experiment. To test the above
hypothesis one can set up the following treatments:
Control: no fertilizer applied
Treatment: Apply 500 kg/ha of the fertilizer.
Application of no fertilizer should result in a certain yield that could serve as a standard for
comparison with the yield of the Treatment. Even with the proper controls, confounding variables can
still greatly affect your experiment. Randomization and replication are the two techniques that further
minimize the obscuring effect of the confounding variables. Simply stated, randomization is using
chance to assign individual plants, seeds, etc., to the treatments, and assigning the treatments to
locations within the experimental area. Repeating the treatment at different locations or over time is
called replication. Often, 3-4 randomized replications are sufficient to generate reliable results from the
treatments. Although better in theory, ten replications may be too expensive and impractical for a
scientist to manage. An experiment that exceeds a scientist's resources (including labor, space and time)
is unlikely to succeed.
How to analyze data experimental data?
Upon completion of each experiment you will collect very valuable information. Now your goal is to
analyze this information critically, interpret, evaluate and present the data in order to decide whether the
data support or do not support the hypothesis you intended to test in the experiment.
Suppose you have completed the experiment described above. You have harvested from four
randomized plots:
Control: 40, 40, 40 and 40 bu/plot
Treatment:30, 50, 20 and 60 bu/plot
Both samples have the same mean value (40 bushels/plot), yet the yield in the Control is much less
variable than the yield from the Treatment. Listing only the average value omits an important
σ
component of the data - the variance. The variance 2 gives an indication of how variable your data
(note that the word "data" is plural) are from one observation to the next. You can calculate the variance
using the following formula:
where, N is the number of observations made (four in our example)
Xi is the value of the i-th observation, and
X is the mean value of all the observations made in the sample.
The standard deviation (SD) is the square root of the variance. Most
calculators and computer programs (Excel, Lotus 1,2,3) will calculate both
variance and SD for you.
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In our example,
Control:
SD = 0
σ2 = ((40-40)2+(40-40)2+(40-40)2 +(40-40)2)/3 =0 (no variance)
σ
Treatment: 2=((30-40)2+(50-40)2+(20-40)2+(60-40)2)/3=333 (high variance)
SD = 18.25
In general, less variable data are more reliable. Higher values of SD often indicate that the effect of the
confounding variables was very significant, often even more significant than the effect of the
independent variable on the dependent variable.
What conclusions about the effect of the WonderGro application (independent variable) on the yield
(dependent variable) will you make based on the given information?
Concept Check
You can earn extra-points (up to 2 points per correct answer, counted toward your final lab grade) when
you answer the following questions correctly. To receive credit, please E-mail your answers to your
instructor within a week from this lab.
1. Suppose you wanted to investigate a manufacturer's claim that WonderGro performs better than a
cheaper leading fertilizer brand. What controls would you set up in this case? What would dependent
and independent variables be in this experiment?
2. What factors might confound the results of an experiment conducted in a greenhouse?
3. Why are highly variable data less reliable? How can one decrease this variability?
All materials on this website are for personal use only. Pictures, text or files cannot be legally
reproduced or duplicated in any form. For commercial or instructional use of this website or materials
from it, please contact Dr. P. McMahon or Max Teplitski.
©Copyright by M.Teplitski and P.McMahon, 2000
For more information, email us at [email protected], [email protected].
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Crop ID Lab
It is hard to underestimate the importance of knowing how to identify plants. Each one of us can think
of a few reasons why we need to be able to ID both crops and wild plant. It is easy to identify plants
using special plant ID guides which require knowledge of basic identifying features of the plants. To
identify a plant correctly, one has to describe form, shape, color, odor and arrangement of a plant's
leaves, flowers, fruits, stems and roots. Let's go over the most common nomenclature used to describe
and identify plants.
What to look for to identify plants?
Leaves
Leaf shape, size, color, odor, venation, pubescence -- all help to identify a plant
correctly. Most leaves consist of a blade (lamina) (1) with veins (2) in it. Leaves are
connected to the node by a short stalk, called "petiole" (3). You have noticed that the
ivy leaf (left) looks glossy because it is covered by a layer of cutin, a waxy substance
that protects leaves. Shapes of the leaf blades, presence of petioles and other features
differ between species. We will discuss these differences in a moment. Microscopic
structure and functions of leaf tissues, as well as leaf modifications will be discussed in
the upcoming labs.
Leaf shapes can greatly help in identifying plants. The two figures on the right illustrate
15 most common leaf shapes: 1 - lobed leaf of canola, 2 - heart-shaped leaf of tall
morning glory, 3 - lanceolate leaf of lady's-thumb, 4 - linear leaf of tall fescue, 5 fan-shaped leaf of Ginkgo biloba, 6 - arrowhead leaf of honeyvine milkweed, 7spade-shaped leaf of buckwheat, 8 - star-shaped leaf of Japanese maple, 9 - pentagonal
leaf of ivy, 10 - obovate leaf of crab apple, 11 - oblong leaf of sugar beet, 12 - elliptic
leaf of tobacco, 13 - spoon-shaped leaf of Portulaca, 14 - ovate leaf of pigweed, 15 rhombic leaf of lamb's- quarter. Here is an easy way to distinguish between ovate,
obovate, elliptic and oblong leaf shapes: ovate leaves are slightly wider at the base (near
the petiole), obovate leaves are somewhat wider at the top half of the leaf, elliptic leaves
are the widest in the middle of the leaf, while oblong leaves are equally wide on the top,
bottom and the middle of the leaf.
Leaves we have discussed above consist of only one leaf blade, - they are simple leaves.
On the right, there are three common "compound" leaves. Compound leaves consist of
several leaf blades, or leaflets. For example, A is a trifoliate leaf of clover, B - palmate
leaf of Ohio buckeye, C - pinnate leaf of crown vetch.
In pinnately compound leaves (rose, vetches and garden pea), leaflets occur along an extension of a
petiole called a rachis. If all leaflets arise from a common point (buckeye, Virginia creeper), it is a
palmate leaf. To distinguish between these two terms, it might help to remember that pinnae means
"feather" and palmae is Latin for "palm" . Trifoliate leaves (strawberry, clovers, beans) haved three
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leaflets arising from a common point on a petiole.
Leaves vary in their venation, i.e. the pattern of leaf veins. Dicots usually have either
pinnate or palmate venation (left). In a pinnately veined leaf of sugar beet (1) there is
a main vein (the midrib), with secondary veins branching from it. Palmately veined leaf
of cotton has several veins which arise from the base of the blade. Corn leaf (3), like
almost all monocots, has parallel veins. Leaf of an ancient plant ginkgo (4) has
dichotomous leaf venation, with each vein branching in two smaller veins.
Pubescence (tiny hairs on different plant parts) can help to
distinguish between seedlings of alfalfa and clover, thyme and
mother-of-thyme, which otherwise are very much alike. On the left
are magnified leaves of mother-of-thyme, on the far left are
pubescent leaves of thyme.
Leaves are arranged in a specific manner on a plant. On a stem, leaves grow from nodes
(4). Nodes are spaced along stems, with internodes (5), spaces between nodes. There are
four main leaf arrangements: alternate, opposite, whorled and a rossette. Leaves of tomato
(on the right) are arranged alternately, with one leaf at each node.
In plants with oppositely-arranged laves (left), the two leaves at the
node are opposite from each other. Members of the Mint Family (sage,
basil, thyme, catnip, etc) characteristically have opposite leaves. Other
examples of oppositely arranged leaves include first, unifoliate leaves of
soybean, leaves of lilac, etc.
When there are three or more leaves arising from a node, leaves are
arranged in a whorl (right).
Leaves of strawberry (left) are arranged in a rosette. Rosette is a structure in which
leaves are arranged in a tight spiral on a short stem (dandelion, strawberry, etc). Members
of Mustard Family (cabbage, radish, wild mustard, etc), some members of Aster Family
(lettuce, daisies, etc), and Carrot Family (celery, anise, etc) spend part of their life cycle
as rosettes, and then "bolt" , i.e. produce long stems with flowers. Bolting is controlled by
a number of environmental conditions (daylength, temperature, light quality, etc).
Grasses have other morphological features that aide in plant identification. Below, is a diagram of a
grass.
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Grass stems are called culms, they are usually hollow. Grass leaves (4)
are simple, linear with parallel veins. Leaves can be either folded at
growing point (D) or rolled (E). Grass leaves don't have petioles.
Lower part of a grass leaf that wraps around the culm is called a
"sheath. The place where sheath meets the blade is known as "collar".
At the collar, a grass leaf might have a ligule (1) which can be
membranous (B), or hairlike (C), in some grasses ligules are absent
(A). Some grass leaves have claw-like projections, known as auricles
(2). If auricles are present, they are either short, long, or clasping. On
the right is a micrograph of a barley collar. Barley has a membranous
ligule (1B) and clasping auricles (2).
Grasses can have three types of modified stems, shown on the grass diagram above.
● tillers (5), secondary stems that arise from nodes at the base of the main stem, and are almost
vertical in position;
● stolons (6), horizontal aboveground stems that can produce new plants from their nodes.
Bermudagrass (as well as white clover and strawberries) propagate themselves with stolons;
● rhizomes (7), horizontal underground stems that can produce new stems from their nodes.
Smooth bromegrass, Kentucky bluegrass and johnsongrass use rhizomes to propagate and store
food.
Grass flowers are arranged in compound inflorescences (3 on a grass diagram above). A
"unit" of grass inflorescence is called a "spikelet". On the left is a spikelet of tall fescue.
Click on the image to learn more about spikelet anatomy.
Spikelets are usually arranged in spikes (wheat, rye, etc) or panicles (rice, fescue, oat,
Kentucky bluegrass, etc).
Stems
Shape of a stem in cross section, color, modifications and pubescence can help in plant ID. Shape of
a stem in cross section may be round, flat, square or triangular. Most plants have round stems.
Members of the Mint family (coleus, peppermint, basil) have square stems. Sedges have distinguishing
triangular stems. Click here to review stem morphology.
Roots
Most monocots have fibrous root system. Most dicots have a tap root system. Roots of legume may
also have root nodules, which are sites of nitrogen fixation. We will talk more about root and stem
anatomy and functions in the upcoming labs.
Flowers and inflorescences
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As you recall from Plant Biology classes, no two plant species have identical flowers.
The number of flower parts is one of the major identifying characteristics of a plant
family. Click here to review flower anatomy.
Sweet potato (left) and some other plants has single flower. Most plants have their small
flowers in inflorescences.Why do you think it is advantageous to a plant to have its small
flowers arranged in an inflorescence?
Often it is hard to decide whether one is dealing with a single flower or an
inflorescence. Roses, and peonies are single flowers. Sunflowers, daisies and
chrysanthemums are actually inflorescences. The outer flowers of these inflorescences
are called "ray florets". The centers of these inflorescences are comprised of many
small short disk flowers.
Flowering structures of dogwood (right) and Poinsettia contain enlarged and brightly
colored leaf structures called bracts. Actual flowers are small yellow. They are
arranged in an inflorescence surrounded by bracts.
Below are examples of some common inflorescences.
umbell of apple
compound umbell
of carrot
head of daisy
corymb
spike of wheat
male flowers of
birch in a catkin
inflorescence
panicle of brome
raceme
spadix of
Anturrhium
Fruits and Seeds
The number of cotyledons in a seed will immediately steer you in a right direction when identifying
plants. As you remember, dicotyledonous plants have two cotyledons, while monocotyledonous plants
have only one. Seed size, color, shape, and presence of various decorations (hooks, wings, etc.) are also
very helpful in plant identification. Grass fruits have many other characteristics that will help identify
grass grains.
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Crop ID Lab
Lab Activities
1. Choose 5 plants (crops or weeds) your group would like to describe.
2. Label five blank sheets of paper with the common names of the crop or weed you decided to
describe.
3. On the labeled sheet of paper, describe identifying features of the plant as thoroughly as possible.
Use the following guide in your description:
a) Leaves: shape, simple/compound, venation, color, odor, arrangement, pubescence, petiole (present or
absent). Describe shape of leaflets in compound leaves. In grasses describe ligules and auricles;
b) Flowers: single or inflorescence, type of the inflorescence, color, odor. Count the number of flower
parts;
c) Stem: shape, color, modifications (if present), pubescence;
4. Select a pot with two grass species (one of the grasses is a grain crop, the other is a weed common in
this area). Using the grass ID guide, decide which grass species are growing in the pot.
After class, turn in your work.
Materials on this website are for personal use only. Text or files cannot be legally reproduced or
duplicated in any form. For commercial or instructional use of this website or materials from it, please
contact Dr. P. McMahon or Max Teplitski.
©Copyright by M.Teplitski and P.McMahon, 1999
For more information, email us at [email protected], [email protected].
For additional information about the crops, click on their highlighted names
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http://www.hcs.ohio-state.edu/hcs200/images/stems&roots/root.jpg
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