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Chapter 32 Plant nutrition and transport
1) The uptake and transport of plant nutrients
a) Plants acquire their nutrients from soil and air
i)
Aristotle
(1) soil provided all substance for growth
ii) physician Jan Baptista van Helmont (17th century)
(1) experiment
(a)grew willow seedling in pot with 90kg of soil
(b) five years later
(i) tree = 78.6kg
(ii) soil only lost 0.06kg
(c)conclusion: water
iii) Stephen Hales (English Botanist)
(1) postulate it was mostly air
iv) 95% of dry weight is organic built mainly from CO2
v)
Figure 32.1A
vi) What happens to materials taken up from soil and air?
(1) sugar (photosynthesis)
(a)carbon and oxygen from CO2
(b) hydrogen from water
(2) sugars (two fates)
(a) used to make all other organic molecules
(biosynthesis)
(b) broken down to make ATP (cellular resp.)
(3) what about minerals from soil?
(a) lets look at N, P, Mg
(i) nitrogen
1. used in combination with glucose to make
hormones, ATP, coenzymes (chlorophyll),
nucleic acids, amino acids…
(ii) magnesium
part of chlorophyll
(iii)phosphorus
1. ATP, nucleic acids, phospholipids, etc…
b) The plasma membranes of root cells control solute uptake
i)
roots
(1) high surface area thanks to root hairs (extensions of
epidermal cells
(2) everything entering is in solution (just like respiratory
surface)
(3) need to move through epidermis  cortex  xylem
(4) Two routes to the xylem (Fig. 32.2B)
(a)intracellular - diffusion
(i) plasmodesmata – cytoplasm is continuous in roots
(ii) ion selection occurs at PM of root hairs
(epidermis)
(iii)endodermal cell discharges solution into xylem
(iv) only need to cross three membranes
(b) extracellular
(i) moves through porous cell walls
(ii) only need to cross two membranes
(iii)Casparian strip
1. stops solution from entering xylem via cell
walls (wall)
2. waxy belt through walls of endodermal cells
(iv) ion selection occurs at membrane of endodermal
cell
(c)In reality, solutions take a combination of these
routes passing many membranes along the way
(d) all routes are selective for ions however (no free
passes)
c) transpiration
i)
pulls water up xylem vessels
(1) xylems
(a)tracheid and vessel elements
1.
dead cells (only cell walls) lined up end to end
forming very thin, hollow tubes (through the cells
themselves) – holes in ends of cells
(2) xylem sap
(a)solution of inorganic nutrients flowing up from roots
(b) how does it flow upward against gravity? No heart or
skeletal muscles to help…
(i) root pressure
1. slight push from root cells actively pumping
ions into xylem, water follows by osmosis
2. can push it up a few meters
3. some giant trees like sequoia and redwoods
produce NO root pressure
(ii) transpiration (Fig. 32.3) (transpiration-cohesiontension mechanism)
1. xylem sap is pulled
2. loss of water from leaves and other aerial parts
through stomata by diffusion
3. Two properties of water make transpiration
possible
a. cohesion
i. sticking together of water molecules to
themselves (H-bonds)
ii. almost forms a continuous “string” from
roots to leaves
b. adhesion
i. sticking together of different molecules
ii. water to cellulose in the walls of xylem
4. water must break off top of string to leave the
leaf
a. thermal energy from sun pulls it out of leaf
down concentration gradient
b. as it is pulled out, it pulls on the waters next
to it and so on (cohesion) all the way to the
roots. Everyone moves up one.
(b)
adhesion counters downward pull of gravity
a. also holds water in xylem when transpiration
not occurring
d) guard cells control transpiration
i)
transpiration works for and against plants
(1) water loss
(a)transp. is greatest when sunny, warm, dry, windy
(i) increases evaporation
(b) average sized maple tree
(i) can lose 200L of water per hour during summer
(ii) not a problem if there is enough water in soil
(2) stomata
(a)each with a pair of guard cells (Fig. 32.4)
(i) change shape to control opening
(ii) open during day (photosynthesis) – need CO2
(iii)closed during night – save water
(iv) principle behind shape change
1. opening stomata
a. cells actively take up K+
b. water follows by osmosis
i. causes swelling and turgor
ii. cells bend away from each other due to
arrangement of cellulose fibers in wall
2. closing stomata
a. pump out K+
b. water follows
c. cells sag and close
(v) stimulation for stomata to take up K+
1. sunlight
2. low CO2
3. internal timing (biological clock)
(vi) close stomata
1. too much water loss during the day
a. CO2 uptake declines – crop yield decreases
with droughts
5.
balance b/w need to save water and need to
make sugar
e) phloem transports sugars
i)
phloem
(1) transport sugars made during photosynthesis
(2) sieve-tube members
(a)arranged end to end as tubes just like phloem, but
these are living
(b) sieve plates
(c)continuous cytoplasm
ii) phloem sap
(1) sugary solution moving through sieve-tube members
(2) hormones, inorganic ions, amino acids
(3) main solute = sucrose
(4) moves in ALL directions through plant
iii) sugar source
(1) location where sugar is being made or broken down
from starch
iv) sugar sink
(1) where sugar is consumed or stored
(2) growing roots, shoot tips, fruits, non-photosynthetic
stems, living tissue in tree trunks
v)
storage structure (tubers of potato plant, taproot of
beet, bulb of lily etc…)
(1) sugar sinks during summer (making sugar by photosyn)
(2) sugar sources during spring (for growth)
vi) so all phloem have a source and sink end
vii) What causes the flow from source to sink?
(1) as high as 1m/hr (way faster than diffusion – take 8
years for sap to move 1m by diffusion)
(2) pressure-flow mechanism (Fig. 32.5B)
(a)sugar enters phloem at sugar source by active
transport
(i) solute concentration rises in phloem
(b) water is drawn in by osmosis
(vii)
water pressure is high at source
(c)sugar is actively transported out of phloem at the sink
(d) water is drawn out by osmosis
(i) water pressure is low at sink
(e)hydrostatic pressure gradient
(i) high pressure at source and low pressure at sink
causes water to flow from source to sink
(ii) sugar moves with water
(f) xylem tubes carry water back by transpiration
(g) explains why it ALWAYS flows from source to sink
regardless of location
(3) Testing the pressure-flow mechanism
(a)difficult to test – disrupt structure and function of
phloem tubes
(b) used aphids
(i) feed by inserting stylet into phloem
(ii) honeydew released from anus
1. phloem sap minus some solutes that the aphid
absorbed for food
(iii)pressure in phloem sap force feeds aphid
(iv) swells to several times original size
(c)anesthetize while feeding ad sever from stylet
(i) stylet = miniature tap that drips for hours
(ii) closer stylet to sugar source, faster the sap drips
and greater the sugar concentration
f) Review
i)
water and inorganic ions enter from soil and
distributed by xylem via transpiration
ii) CO2 enters through stomata and converted to sugars
in leaves, distributed by pressure-flow mechanism of
phloem from source to sink.
2) Plant nutrition and the soil
a) plant health depends on a complete diet of essential inorganic
nutrients
(i)
i)
plants survive solely on inorganic substance unlike
their animal counterparts
ii) essential plant nutrient
(1) nutrient it must obtain to complete life cycle
(2) hydroponic culture
(a)used to determine essential nutrients
(b) roots of plant bathed in solution with known
minerals and concentrations
(c)air bubbled in to give roots O2 for cellular resp.
(d) omit a particular element and watch growth
(e)compare experimental to control groups
(i) most common symptoms
1. stunted growth
2. discolored leaves
(3) results (mostly done on house and crop plants)
(a)17 elements essential to ALL plants
(b) few other elements essential to specific plants
(4) The 17 essential
(a)macronutrients
(i) 9 of the 17
(ii) require large (macro) amounts
(iii)C,N,O,H,S,P – obvious – macromolecules
1. 98% of plants dry weight
(iv) Ca, K, Mg
1. 1.5%
2. Calcium (Ca++)
a. formation of cell walls
b. combines with proteins to form a “glue” that
holds plant cells together in tissues
c. maintain structure of cell membranes
d. regulate selective permeability
3. Potassium (K+)
a. cofactor of several enzymes
b. main solute for osmotic regulation
i. opening and closing of stomata
Magnesium (Mg++)
a. component of chlorophyll
b. cofactor of several enzymes
(b) micronutrients
(i) Fe, Cl, Cu, Mn. Zn, Mo, B, Ni
1. components or cofactors of enzymes
a. Fe – found in proteins of ETC – accepts and
donates electrons
2. catalysts – used over and over, and recycled
3. only need very small quantities
a. 1 Mo for every 16,000,000 hydrogens
b. deficiency will kill plant though
(5) quality of soil determines quality of your own nutrition
(Figure 32.6B) –
(a) nitrogen deficient crops will have lower food value
b) You can diagnose some nutrient deficiencies in your own
plants
i)
nitrogen shortage
(1) single most common problem
(2) soil usually has nitrogen, but not as NO3- (nitrate ions)
or NH4+ (ammonium ions).
(a)stunted growth and yellow-green leaves
(b) older leaves usually show signs first
ii) phosphorus deficiency
(1) second most common ailment
(2) plenty of phosphorus in soil (rock), but not as water
soluble HPO42- or H2PO4-.
(a)growth rate reduced
(b) new growth spindly and brittle
(c)purplish color in some plants
iii) potassium deficiency
(1) most obvious in older leaves
(a)turn yellow
(b) develop dead, brownish at edges or in spots
(c)stunting of growth
4.
c) soil contains rock particles, humus, organisms, water and
crucial solutes
i)
fertile soil
(1) adequate water and dissolved nutrients
(2) provides conditions that enable plant roots to absorb the
substances it needs
ii) Soil horizons (layers of soil) – Fig. 32.8A
(1) topsoil – A horizon – top 20 cm in figure
(a)subject to extensive weathering
(b) fertile topsoil
(i) rock particles of various size including sand and
clay
1. extensive surface area to retain water and
nutrients while allowing oxygen to diffuse in
(ii) humus
1. decomposing organic material
2. important source of nutrients
3. retains water while keeping soil porous for
good aeration of roots
(iii)living organisms
1. bacteria, fungi, protozoa, earthworms, round
worms, burrowing insects
a. loosen and aerate soil
b. contribute organic material as they live and
die
c. bacteria and fungi
i. breaks down organic to usable inorganic
(c)plant roots extend into B horizon
(2) B horizon
(a)much fewer organisms, less organic material
(b) less subject to weathering
(c)clay and nutrients dissolved in water drain down and
get stuck here
(3) C horizon
(a)partly broken down rock
iii) Figure 32.8B – root hairs in soil
(1) root hairs in contact with soil water
(2) O2 diffuses into soil from air spaces
(3) cation exchange
(a)mechanism by which root hairs take up cations
(positively charged ions)
(i) Ca++, Mg++, K+
1. adhere to neg. charged surface of clay
2. keeps them from draining away
(b) root hair release H+ (protons)
(i) displace other cations and root hairs absorb free
cations
(ii) this is also why acid rain is not good for soil
(4) negatively charged particles (anions)
(a)NO3-, H2PO4-., etc…
(b) don’t stick to clay (obviously) and drain out easily
(i) into rivers and lakes…eutrophication!
d) soil conservation is essential to human life
i)
we must conserve soil fertility
ii) may take centuries for soil to become fertile
iii) Three critical aspects of soil conservation
(1) proper irrigation
(a) can turn desert into garden
(b) huge drain on water resources
(c) flooding fields tends to make soil salty
(i) water comes in with salts, evaporates quickly, but
salts stay, more water, more salts, more
evaporation…
(ii) plants cannot tolerate salty soil
(d) solution
(i) drip irrigation
1. use perforated pipes that drip slowly
2. conserves water and reduces water loss by
evaporation (less water, less salt) and drainage
(2) prevention of erosion
(a)blowing
or washing away of soil
(b) 1000’s of acres lost per year in US
(c)plowed soil is especially vulnerable
(d) plow to turn over weeds and crop stubble, they
decompose and add nutrients
(e)solutions
(i) minimal tillage farming
1. do not plow every year
2. use herbicides to kill weeds
a. now your putting chemicals into soil…fix
one problem with another
(ii) plant trees along field edge
1. prevent wind erosion
(iii)hills
1. contour tillage
a. plant in rows that go around hill rather than
up and down
b. slows runoff
(3) fertilization
(a)most fertilizers are commercially produced
containing minerals that are mined or prepared by
industrial processes (pollution, etc…)
(b) quick release, not retained in soil
(c)excess gets into water supply, rivers, etc…
(d) organic fertilizers
(i) manure, fishmeal, compost (decaying plant
matter)
(ii) biological origin
(iii)organic material must be broken down by
saprobes (bacteria and fungi) to inorganic
1. slowly released, retained in soil as insoluble
organic material
(e)problem: if we stopped using inorganic fertilizers,
famine would likely occur!
e) organic farmers avoid the use of commercial chemicals
i)
relies on principles of ecology rather then synthetic
chemicals or pesticides
(1) use compost or manure to fertilize
(2) use positive insects to combat negative ones
(3) some synthetic chemicals are used
(a) thought not to harm environment…
(4) bottom line: restore as much to the soil as you take out
of it
(5) 12,000 farmers in US are now organic
(6) benefit
(a)fewer dangerous chemicals in soil, air, water
(b) less exposure to workers, consumers and wildlife
(c)picked when ripe and sold locally (no preservatives)
(7) problem
(a)crop yields are lower
(b) more expensive to grow and buy
(c)organic label is no guarantee of organic
f) Fungi help most plants absorb nutrients from soil
i)
roots need large surface areas
(1) root hairs help, but are they enough?
(2) most (almost ALL) plants team up with fungi
(3) mycorrhiza (mykes = fungus, rhiza = root (Greek))
(a) beneficial association of plant roots and fungus (Fig.
32.11)
(b) great in nutrient poor soil
(c)provide enormous surface area
(i) absorbs water and inorganic ions (especially
phosphate) more rapidly than roots alone
(ii) some water and ions transferred to plant
(iii)fungus may secrete acid to increase solubility of
certain minerals
(iv) convert minerals to more readily usable form by
plant
(d) plant gives nourishment from photosynthesis
(e)fungus may protect plant from pathogens
(f) citrus
trees need less fertilizer in presence of
mycorrhizae
(g) this association is observed back to the beginning of
plants and may have been necessary for them to
move on land
g) plant kingdom includes parasites and carnivores
i)
some plants can get nutrients from other plants or
animals
ii) Figure 32.12A
(1) dodder – parasitic plant (yellow-orange threads)
(a)can’t photosynthesize
(b) obtains organic molecules from other plants
(i) use specialized roots to tap into vascular tissue
iii) Figure 32.12B
(1) mistletoe parasitizing an oak tree
(2) 1000 species of mistletoe
(a)photosynthetic
(b) supplements diet by siphoning sap from vascular
tissue of host
(3) doodler and mistletoe – may kill host by blocking too
much light or taking too much food
iv) carnivorous plants
(1) grow in bogs where soil highly acidic
(a)acid causes organic material to decay slowly
(b) little inorganic nitrogen available to plants
(2) sundew and venus flytrap
(a)get nitrogen by digesting flies
(b) sundew
(i) modified leaves to form club-shaped hairs
(ii) secretes sticky sugar substance
1. attracts insects and traps them
2. hairs bend and leaf cups around prey
3. hairs then secrete digestive enzymes
(c)venus flytrap
(i) insects touch sensory hairs twice in a row
(ii) trap
closes
(iii)digestive enzymes secreted
h) most plants depend on bacteria to supply nitrogen
i)
plants cannot use N2 (atmospheric nitrogen)
ii) nitrogen fixation (review of nitrogen cycle)
(1) nitrogen-fixing bacteria
iii) ammonifying bacteria
(1) decomposes organic matter generating ammonium
iv) ammonium tends to remain stuck to clay (ammonium
is positively charged, clay is negatively charged)
(1) plants can’t get it so well
v)
nitrifying bacteria
(1) ammonium to nitrate (negatively charged)
vi) plants convert nitrate back to ammonium to
incorporate into amino acids (biosynthesis)
(1) amino acids used to make proteins and other nitrogencontaining organic molecules
i) Legumes and certain other plants house nitrogen-fixing
bacteria
i)
legumes
(1) have own built in source of nitrogen-fixing bacteria in
their root nodules (mutualism)
(2) plants with pods around seeds
(a)peas, beans, clovers, alfalfa, peanuts, etc…
(3) root nodules
(a)swellings on roots with plant cells that contain
nitrogen-fixing bacteria
(i) they are actually INSIDE the plant cell
(ii) could this be another endosymbiotic event in the
making?...
(iii)convert N2 directly to NH4+
(iv) excess leaks into soil making soil more fertile
1. farmers will rotate crops, one year of legumes
and one year of non-legumes (legumes to
increase nitrogen content of soil)
(4) Rhizobium
(a)genus
of most N-fixing bacteria in nodules
(5) actinomycetes
(a)other N-fixing bacteria found in roots of
nonleguminous plants like alders
(6) plants give carbohydrates and other organic compounds
to bacteria
j) A major goal of agricultural research is to improve the
protein content of crops
i)
most people are predominantly vegetarian
ii) depend on plants for protein
(1) many plants have low protein content
(2) proteins present deficient in essential amino acids
iii) scientist aim to improve quality and quantity of
protein in crops
(1) corn, wheat and rice “super varieties”
(a)require more nitrogen
(b) usually supplied by inorganic fertilizers
(i) cost a lot to produce
(ii) countries most in need are poor ones
(2) improve output of rhizobium bacteria of legumes
(a)negative feedback mechanism regulates rate of NH4+
formation
(b) when NH4+ reaches a certain level, bacteria turn the
nitrogen-fixing genes off (an operon)
(c)mutants that keep making nitrogen compounds have
been isolated
(i) make soil more fertile, keep adding nitrogen to
make excess proteins in crop.
k) genetic engineering is increasing crop yields
i)
gene gun
(1) shoot foreign genes into a plant cell
(2) fires plastic bullets with tiny metal pellets coated with
DNA
(3) bullet stays in gun, pellets enter cell
(4) DNA integrates into cells DNA
(5) cell will now make the protein coded by the inserted
gene
ii) many new crops have been made this way and by
using bacterial plasmids (Module 12.18)
(1) ex. cotton plants that resist viruses,
(2) potato plants making own insecticide
(3) tomato plants producing slow spoil fruit
(4) what if we can get them to synthesize medicine?
(5) more nutritious food?
(6) grains with all the essential amino acids?
iii) make N2 fixing bacteria that are more efficient
iv) put genes for nitrogen fixation into nonleguminous
plants
v)
problems
(1) what if these super genes get loose?
(a)crop plants hybridize with relatives causing “super
weeds”
(b) plant itself gets into wild and outgrows native
species
(c)proteins cause allergies?
(d) what to do?...