Chapter 37
Plant Nutrition
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Outline of this chapter
• Nutritional requirement of plants
• The role of soil in plant nutrition
• The special case of nitrogen as a plant
nutrient
• Nutritional adaptations: symbiosis of plants
and soil microbes
• Nutritional adaptations: parasitism and
predation by plants
Nutritional requirement of plants
• The chemical composition of plants
provides clues to their nutritional
requirements.
• Plants require nine macronutrients and at
least eight micronutrients.
• The symptoms of a mineral deficiency
depend on the function and mobility of the
element.
The chemical composition of
plants provides clues to their
nutritional requirements
• Researchers can analyze the chemical composition
of plants after they are dried.
• More than 50 chemical elements have been
identified, but not all of them are essential.
• Organic substances (for example, carbohydrate)
account for about 95% of the dry weight, with
inorganic substances making up the remaining 5%.
• Nonwoody plants contain 80-85% water, and
plants grow mainly by accumulating water in the
central vacuoles of their cells.
Plants do not get nutrients from
one source
Plants require nine
macronutrients and at least eight
micronutrients
• Mineral nutients are essential chemical
elements absorbed from the soil in the form
of inorganic ions.
• Essential nutrient is a particular chemical
element required for a plant to complete the
life cycle.
• Hydroponic culture helps researchers to
determine which of the mineral elements
are actually essential nutrients.
Hydroponic culture
• Because most
research has
involved crop
plants, little is
known about the
specific
nutritional needs
of uncultivated
plants.
Essential nutrients in plants
Macronutrients and
micronutrients
• Macronutrients (C, O, H, N, S, P, K, Ca, Mg)
are elements required by plants in relatively
large amounts; micronutrients (Fe, Cl, Cu,
Mn, Zn, Mo, B, Ni) are elements required in
very small amounts.
The symptoms of a mineral
deficiency depend on the function
and mobility of the element
• Although iron is not a chemical element in
chlorophyll, iron deficiency still results in
chlorosis (yellowing of the leaves) because
it is required for chlorophyll biosynthesis.
• Mobility of the particular element will
determine where the symptoms first
showing up when it is not enough.
Deficiency of iron results in the
same symptoms as magnesium
deficiency
Diagnosis and treatment of
nutrient deficiency
• Chemical analysis and observation of
symptoms help plant physiologists and
farmers to diagnose the cause of nutrient
deficiency.
• Usually N, K, and P deficiencies are most
common.
• Treatment of micronutrient deficiencies
should be careful because overdose of some
micronutrients can be toxic to plants.
Hydroponics can ensure optimal mineral nutrient supplement
The role of soil in plant nutrition
• Soil characteristics are key environmental
factors in terrestrial ecosystems.
• Soil conservation is one step toward
sustainable agriculture.
Soil characteristics are key
environmental factors in terrestrial
ecosystems.
• Texture and composition of soils are major
factors in terrestrial ecosystems.
• Availability of soil water and minerals.
Texture and composition of soils
• Topsoil (see figure 37.5, p. 771) is where
plant grows. It is a mixture of weathering
solid rock, living organisms, and residue of
partially decayed organic material called
humus.
• Weathering of solid rock is achieved by
water, temperature, and organisms like
lichens, fungi, bacteria, mosses and the
roots of vascular plants.
Topsoil is where plant grows
The texture of topsoil depends on
the size of its particles
• The most
fertile soils
are usually
loams, made
up roughly
equal
amounts of
sand, silt, and
clay.
Loams
• Loamy soils have enough fine particles to provide
a large surface area for retaining minerals and
water, which adhere to the particles. But loams
also have enough coarse particles to provide air
spaces containing oxygen that can be used by
roots for cellular respiration.
• Humus is the decomposing organic material
formed by the action of bacteria and fungi on dead
organisms, feces, fallen leaves, and other organic
refuse. It prevents clay from packing together but
is still porous enough for the aeration of the roots.
It is also a reservoir of mineral nutrients that are
returned gradually to the soil as microorganisms
decompose.
The availability of soil water and
minerals
• Soil particles have
electrically charged
surfaces. Some of the
water adheres so
tightly to the
hydrophilic soil
particles that it cannot
be extracted by plants.
• The film of water
bound less tightly to
the particles is the
water generally
available to plants.
Minerals are attracted to soil
particles by electrical attraction
• Because the surfaces of
clay particles are
negatively charged,
positively charged
minerals like K+, Ca2+,
and Mg2+ are adhered to
the surface.
• Plant roots will secrete
H+ in the soil to
displace those ions.
Minerals are attracted to soil
particles by electrical attraction
• On the other hand,
negatively charged
minerals like nitrate
(NO3-), phosphate
(H2PO4-), and sulfate
(SO42-) are usually not
bound tightly to soil
particles and thus tend
to leach away more
quickly.
Soil conservation is one step
toward sustainable agriculture
• Because it may take centuries for a soil to
become fertile through the breakdown of
rock and the accumulation of organic
material, it is important to manage the soil
wisely.
• Agriculture is unnatural.
• Fertilizers, Irrigation, Erosion, and
Phytoremediation are all important for soil
conservation.
Fertilizers – chemical fertilizers
• Chemical fertilizers are minerals mined or
prepared by industrial processes. They are
usually enriched in nitrogen, phosphate and
potassium, the three mineral elements most
commonly deficient in farm and garden
soils.
• A fertilizer marks “10-12-8” is 10%
nitrogen, 12% phosphorus, and 8%
potassium.
Fertilizers – organic fertilizers
• Organic fertilizers are of biological origin
and contain organic material that is in the
process of decomposing.
• Manure, fishmeal, and compost are organic
fertilizers.
• They release minerals gradually. On the
other hand, chemical fertilizers are available
immediately but may not be retained by the
soil for long.
How to use fertilizers wisely
• Monitoring soil pH can help farmers to use
fertilizers wisely.
• Soil pH not only affects cation exchange but
also influences the chemical form of all
minerals.
• Adding sulfate to lower the pH or using
calcium carbonate or calcium hydroxide
(liming) to raise the pH are the usual
practices for farmers.
Irrigation
• Irrigation in arid region can gradually make
the soil so salty that it becomes completely
infertile (salinization). Irrigation in arid
region will also dry the river.
• Drip irrigation instead of flooding fields
will reduce the risks of running out of water
or losing farmland to salinization. Crops
require less water are also being bred.
Erosion
• Water and wind erosion make thousands of
acres of topsoil lost every year.
• Windbreaks (rows of trees dividing fields)
and contour tillage (planting crops around
the hills) will prevent the loss of topsoil by
wind or rain.
Phytoremediation
• Plants like Thlaspi
caerulescens can
accumulate zinc in its
shoots at concentrations
that are 300 times the
level that most plants can
tolerate, so it can be used
to remove zinc in an area
polluted with zinc.
The special case of nitrogen as a
plant nutrient
• The metabolism of soil bacteria makes
nitrogen available to plants
• Improving the protein yield of crops is a
major goal of agricultural research
The metabolism of soil bacteria
makes nitrogen available to
plants
• Although the atmosphere is nearly 80%
nitrogen, plants cannot use it until it is
converted to ammonium (NH4+) or nitrate
(NO3-).
• Nitrate (NO3-) is the form plant acquire
mostly.
Nitrogenase reaction:
N2+8e-+8H++16ATP  2NH3+H2+16ATP+16Pi
Improving the protein yield of
crops is a major goal of
agricultural research
• Many plants have a low proteins content,
and the proteins that are present may be
deficient in one or more of the amino acids
that humans need from their diet.
• Breed new variety of crops that are enriched
in protein or improve the productivity of
symbiotic nitrogen fixation will help to
improve the protein yield of crops.
Nutritional adaptations:
symbiosis of plants and soil
microbes
• Symbiotic nitrogen fixation results from
intricate interactions between roots and
bacteria
• Mycorrhizae are symbiotic association of
roots and fungi that enhance plant nutrition
Symbiotic nitrogen fixation
• Members of the legume family have
swellings called nodules composed of plant
cells that contain nitrogen-fixing bacteria of
the genus Rhizobium (root living), which
will take the form of bacteroids inside the
nodules.
Symbiotic nitrogen fixation
• The symbiotic relationship between a legume and
nitrogen-fixing bacteria is mutualistic. Plant
provides bacteria with carbohydrates and other
organic compounds and bacteria supplies plant
with fixed nitrogen.
Symbiotic nitrogen fixation
• Leghemoglobin makes
the nodules pink.
Molecular biology of root nodule
formation
Mycorrhizae
• Mycorrhizae also forms mutualistic
relationship with plant.
• Mycorrhizae acquires sugar from the host
plant and plant extends its areas for water
and other mineral (for example, phosphate)
uptake.
• There are two types of mycorrhizae:
ectomycorrhizae and endomycorrhizae.
Ectomycorrhizae
• The mycelium (mass
of branching hyphae)
of ectomycorrhizae
forms a dense sheath
over the surface of
the root. They do not
penetrate the root
cells.
• It is especially
common in woody
plants.
Endomycorrhizae
• Endomycorrhizae do
not form a dense
mantle around the
root. It penetrates root
cell walls, grows into
a tube formed by
invagination of the
root cells’s membrane.
Agricultural importance of
mycorrhizae
Maize seedlings grow
less well without
mycorrhizae.
Mycorrhizae and root nodules
may have an evolutionary
relationship
• Because mutations in these early nodulin
genes block development of both root
nodules and mycorrhizae in legumes that
form both structures, researchers suspected
mycorrhizae and root nodules are somehow
related in evolution.
Nutritional adaptations:
parasitism and predation by
plants
• Parasitic plants extract nutrients from other
plants
• Carnivorous plants supplement their
mineral nutrition by digesting animals.
mistletoe
Carnivorous plants
• Carnivorous plants usually
live in acid habitats where
soil is poor in nitrogen, so
they have to obtain some
nitrogen and
minerals by
killing and
digesting
insects and
other small
animals.