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CHAPTER 39
LECTURE
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Plant Nutrition and Soils
Chapter 39
Soil
• Highly weathered outer layer of the Earth’s
crust
– Mixture of sand, rocks, clay, silt, humus,
mineral, and organic matter
• The Earth’s crust includes about 92
naturally occurring elements
– Most are found in the form of inorganic
compounds called minerals
• Also full of microorganisms
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• Most roots are found in topsoil
– Mixture of mineral particles of varying sizes,
living organisms, and humus
– Characterized by their relative amounts of
sand, silt, and clay
– Soil composition determines the degree of
water and nutrient binding to soil particles
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• Only minerals dissolved in water in spaces
among soil particles are available for
uptake by roots
• Membrane potential maintained by the
root, as well as the water potential
difference inside and outside the root,
affects root transport
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• About half of the soil volume is occupied by
pores
– May be filled with air or water
• Some of this water is unavailable because it
drains immediately due to gravity
• However, water that is held in small pores is
readily available to plants
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Soil Loss
• If topsoil is lost, soil’s water-holding capacity and
nutrient content are adversely affected
• Drought and poor farming led to wind erosion of
farmland in the 1930s
• The southwestern Great Plains of the U.S.
became known as the “Dust Bowl”
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• Measures to prevent erosion
– Intercropping – mixing of crops in field
– Conservation tillage – minimal or even no-till
approaches to farming
• Measures to prevent fertilizer runoff
– Site-specific farming
– Integrated nutrient management
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Acidic and Saline
Soils
• Acidic soils release minerals, such as
aluminum, that are toxic to plants
• Saline soils alter water potential, leading to
a loss of water and turgor in plants
• Draining marshland in southern Iraq
resulted in a salty desert
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Plant Nutrients
• Photosynthesis is major source of plant
nutrition fixation
• Also need
– Macronutrients – used in relatively large
amounts
• Nine = C, O, H, N, K, Ca, Mg, P, and S
– Micronutrients – used in minute amounts
• Seven = Cl, Fe, Mn, Zn, B, Cu, and Mo
– Deficiency of any one can have severe effects
on plant growth
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Healthy wheat plant
Chlorine-deficient plant
Copper-deficient plant
Zinc-deficient plant
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• Grow plants in hydroponic culture to
assess nutritional requirements
– Plant seedling is first grown in a complete
nutrient solution
– Seedling is then transplanted to a solution
lacking one suspected essential nutrient
– Growth of the seedling is monitored for
presence of abnormal symptoms
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• Hydroponic cultures – plant roots are
suspended in aerated water containing
nutrients
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• Food security, avoiding starvation, is a
global issue
• Food fortification is an active research
area
– Focuses on ways to increase a plant’s uptake
and storage of minerals
– Using genetically-modified plants
• Secrete citrate to solubilize phosphate
• Genes for plasma membrane transporters
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Special Nutritional Strategies
• Plants need ammonia (NH3) or nitrate
(NO3–) to build proteins
– However, they lack the biochemical pathways
necessary to convert N2 to NH3
• Symbiotic relationships have evolved
between plants and nitrogen-fixing
bacteria
– Legumes form nodules that house the
bacterium Rhizobium
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• Rhizobium bacteria require oxygen and
carbohydrates to support their energetically
expensive lifestyle as nitrogen fixers
– Plant host supplies both
• Nodule formation requires extensive signaling
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• Symbiotic associations with mycorrhizal
fungi are found in about 90% of vascular
plants
– Substantially expand the surface area
available for nutrient uptake
– Enhance uptake of phosphorus and
micronutrients
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• Carnivorous Plants
– Often grow in acidic soils that lack nitrogen
– Trap and digest small animals, primarily
insects, to extract additional nutrients
– Have modified leaves adapted for luring and
trapping prey
– Prey is digested with enzymes secreted from
specialized glands
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• Pitcher plants
– Have pitchershaped leaves
with cavity filled
with digestive fluid
• Venus flytrap
– When hairs are
touched, the two
halves of the leaf
snap together
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• Sundews
– Glandular
trichomes
secrete both
sticky mucilage
and digestive
enzymes
• Waterwheel
– Uses trigger
hairs and snaps
to capture and
digest small
aquatic animals
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• The snap-trap mechanism was acquired by a
common ancestor of the Venus flytrap and the
aquatic waterwheel
• Pitcher plants are not related to this clade
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• Parasitic Plants
– May be photosynthetic or non-photosynthetic
– At least 3,000 types of plants
– Dodder (nonphotosynthetic)
• Wraps around its host
• Relies on host for its nutritional needs
– Indian pipe (nonphotosynthetic)
• Hooks into host trees through mycorrhizae
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Carbon–Nitrogen Balance
• The Intergovernmental Panel on Climate
Change (IPCC) has concluded that CO2 is
maybe at its highest level in 20 million
years
– Correlates with increases in many human
activities, including the burning of fossil fuels
• Increased CO2 levels may alter C–N ratio
in a plant
– Important for the health of the plant and the
herbivore
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• Calvin cycle fixes CO2 into sugar
– Ribulose 1,5-bisphosphate
carboxylase/oxygenase (rubisco) catalyzes
the first step
– Can bind CO2 or O2
• If CO2 binds, a 3-C sugar is made, that can be
used to make glucose and sucrose
• If O2 binds, photorespiration occurs
– Neither nutrient nor energy storage
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• C3 photosynthesis occurs in mesophyll
cells
• C4 photosynthesis uses an extra pathway
to shuttle carbon deep within the leaf
– This reduces photorespiration by limiting the
Calvin cycle to cells surrounding the vascular
tissue where O2 levels are low
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• In C3 plants, as CO2 increases, the Calvin
cycle becomes more efficient
– Should lead to increased photosynthesis and
plant growth
– However, the plants have less nitrogen and
minerals per unit mass – ratio of carbon to
nitrogen increases
– Results in lower nutritional value for
herbivores
• More plant matter must be eaten to obtain same
amount of nutrients
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• Free Air CO2 Enrichment (FACE) studies
– Rings of towers that release CO2 toward the
center of the ring
– Allow studies to be conducted at the
ecosystem level
– Extensive studies have yielded complex
results
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• As CO2 levels increase, relatively less
nitrogen and other macronutrients are
found in leaves
– Herbivores need to eat more biomass to
obtain adequate nutrients, particularly protein
– Protein deficiencies in human diets could
result from decreased nitrogen in crops
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Phytoremediation
• Use of plants to concentrate or breakdown
pollutants
• Phytodegradation – contaminant is taken
up from soil and broken down
• Phytovolatilization – contaminant is taken
up from soil and released through stomata
• Phytoaccumulation – contaminant is taken
up from soil and concentrated in shoots
– These are later harvested
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• Trichloroethylene (TCE)
– May be removed from the soil by poplar trees
– Degraded into CO2 and chlorine
– A fraction moves rapidly through the xylem
and is released through stomata
• Trinitrotoluene (TNT)
– May be removed from soil and degraded by
poplar and bean plants
– But at high concentrations, it is toxic to these
plants
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• Heavy metals, including arsenic, cadmium,
and lead, are toxic to animals in even
small quantities
• 400 plant species have the ability to
hyperaccumulate toxic metals from soil
• However, a concern is that animals eating
these plants will be exposed to high
concentrations of toxic compounds
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• Phytoremediation is a promising technique
– Costs are 50–80% lower than cleanup
methods involving mechanical removal of
contaminated soil
• An illustrative example comes from the
1998 accident at the Aznalcóllar mine in
Spain
– Dam broke, releasing 5 million cubic meters
of sludge, composed of arsenic, cadmium,
lead, and zinc, over 4300 hectares of land
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5 million cubic meters of
black sludge containing
heavy metals was
released into a national
park and the Guadiamar
River
Large amounts of
sludge were removed
mechanically
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• Since the original spill, 3 plant species with the
potential to hyperaccumulate some of the metals
have begun growing in the area
• These plants are fairly large and can accumulate
a substantial amount of metal
• They also offer the advantage of being native
species
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