Chapter 37
Plant Nutrition
Nutrient Reservoirs



Every organism
continually exchanges
energy and materials
with its environment
For plants…water and
minerals come from
the soil, while carbon
dioxide comes from
the air
The branching root
system and shoot
system of a vascular
plant ensure extensive
networking with both
reservoirs of inorganic
nutrients
Macronutrients and Micronutrients



Plants derive most of their organic mass
from the CO2 of air but they also depend on
soil nutrients
More than 50 chemical elements have been
identified among the inorganic substances in
plants, but not all of these are essential
A chemical element is considered essential
if it is required for a plant to complete a life
cycle
How would you identify an essential nutrient?

Hydroponic culture can be used to determine
which chemicals elements are essential
APPLICATION
In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic
culture is to identify essential elements in plants.
TECHNIQUE
Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides
the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is
essential.
Control: Solution
containing all minerals
Experimental: Solution
without potassium
RESULTS
If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and
discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral
deficiencies in soil.
Macronutrients and Micronutrients

Nine of the essential elements are called
macronutrients because plants require them in
relatively large amounts


C, O, H, N, K, Ca, Mg, P, S
The remaining eight essential elements are
known as micronutrients because plants need
them in very small amounts

Cl, Fe, Zn, Mn, B, Cu, N, Mo
Essential elements in plants
Mineral Deficiency

The symptoms of mineral deficiency
 Depend partly on the nutrient’s function
 Depend on the mobility of a nutrient within the plant

Deficiency of a mobile nutrient
 Usually affects older organs more than young ones (young tissue
can more efficiently draw minerals to it)

Deficiency of a less mobile nutrient
 Usually affects younger organs more than older ones (older tissue
has a store of minerals to fall back on when the mineral is in short supply)
Mineral Deficiency

The most common deficiencies

Are those of nitrogen, potassium, and phosphorus
Healthy
Phosphate-deficient
Reddish-purple margins
esp. on young leaves
Potassium-deficient
“Firing”…drying along
tips and margins of older
leaves
Nitrogen-deficient
Yellowing that starts at
the tip and moves along
the center of older leaves
Soil Characteristics


Soil quality is a major determinant of plant distribution and
growth
Along with climate



Texture…is the soil’s general structure (sandy, clayey, etc)
Composition…refers to the soil’s organic and inorganic
chemical components


The major factors determining whether particular plants can grow
well in a certain location are the texture and composition of the soil
Various sizes of particles derived from the breakdown of rock are
found in soil along with organic material (humus) in various stages
of decomposition
Topsoil… is the mixture of particles of rock and organic material
Soil Horizons

The topsoil and other distinct soil layers, or
horizons are often visible in vertical profile where
there is a road cut or deep hole
The A horizon is the topsoil, a mixture of
broken-down rock of various textures, living
organisms, and decaying organic matter.
A
B
The B horizon contains much less organic
matter than the A horizon and is less
weathered.
C
The C horizon, composed mainly of partially
broken-down rock, serves as the “parent”
material for the upper layers of soil.
Availability of Soil Water


After a rainfall, water drains away from the larger
spaces of soil but smaller spaces retain water
because of its attraction to surfaces of clay and
other particles.
The film of loosely bound water is usually
available to plants
Soil particle surrounded by
film of water
Root hair
Water available
to plant
Air space
Soil water. A plant cannot extract all the water in the soil because
some of it is tightly held by hydrophilic soil particles. Water bound
less tightly to soil particles can be absorbed by the root.
Cation Exchange

Acids derived from roots contribute to a
plant’s uptake of minerals when H+ displaces
mineral cations from clay particles
Soil particle
K+
–
–
Cu2+
–
–
K+
–
–
–
Mg2+
– +
K
–
Ca2+
H+
H2O + CO2
H2CO3
HCO3– + H+
Root hair
Cation exchange in soil. Hydrogen ions (H+) help make nutrients available
by displacing positively charged minerals (cations such as Ca 2+) that
were bound tightly to the surface of negatively charged
soil particles. Plants contribute H+ by secreting it from root hairs
and also by cellular respiration, which releases CO2 into the soil
solution, where it reacts with H2O to form carbonic acid (H2CO3).
Dissociation of this acid adds H+ to the soil solution.
Agriculture

Conventional agriculture


In contrast to natural ecosystems agriculture depletes the
mineral content of the soil, taxes water reserves, and
encourages erosion
Sustainable agriculture




Is ecologically sound
Is economically viable
Is socially just
Is humane.
Fertilizers


Commercially produced fertilizers contain
minerals that are either mined or prepared by
industrial processes
“Organic” fertilizers are composed of manure,
fishmeal, or compost
Irrigation


Is a huge drain on water resources when
used for farming in arid regions
Can change the chemical makeup of soil

Salinization (salt buildup)
drip
Ditch…trench
sprinkler
Erosion

Topsoil from thousands of acres of farmland

Is lost to water and wind erosion each year in the
United States
The U.S. Soil
Conservation
Service reports that
more than 4 million
acres of cropland
are being lost to
erosion in this
country every year.
That's an area
greater than the
size of Connecticut.
Our annual topsoil
loss amounts to 7
billion tons. That is
60,000 pounds for
each member of
the population.
Erosion on conventionally
tilled field
Prevention of topsoil loss


Strip cropping: practice of growing field crops in narrow strips
either at right angles to the direction of the prevailing wind, or
following the natural contours of the terrain to prevent wind and
water erosion of the soil
Contour tillage (slows water runoff and erosion)
Prevention of topsoil loss

Terraces

Conservation
tillage (Min-till)
A minimum tillage system may
involve quicker and fewer passes
at a shallower depth

Cover Crops
Cover crop in an
orchard
Cover crop in vegetable garden
Soil Reclamation

Some areas are unfit for agriculture


Because of contamination of soil or groundwater with
toxic pollutants
Phytoremediation: is a biological, nondestructive
technology that seeks to reclaim contaminated
areas by using the ability of some plants to remove
soil pollutants
Nitrogen


Nitrogen is often the mineral that has the
greatest effect on plant growth
Plants require nitrogen as a component of
proteins, nucleic acids, chlorophyll, and a
host of other important organic molecules
Soil Bacteria and Nitrogen Availability

Nitrogen-fixing bacteria convert atmospheric N2
to nitrogenous minerals that plants can absorb
as a nitrogen source for organic synthesis
Atmosphere
N2
N2
Atmosphere
Soil
N2
Nitrogen-fixing
bacteria
Denitrifying
bacteria
H+
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
(From soil)
Soil
+
NH4
NH3
(ammonia)
–
+
NH4
(ammonium)
Nitrifying
bacteria
NO3
(nitrate)
Ammonifying
bacteria
Organic
material (humus)
Root
The Role of Bacteria in Symbiotic
Nitrogen Fixation


Symbiotic relationships with nitrogen-fixing
bacteria provide some plant species with a
built-in source of fixed nitrogen
From an agricultural standpoint the most
important and efficient symbioses between
plants and nitrogen-fixing bacteria occur in the
legume family (peas, beans, and other similar
plants)
Root Nodules



Along a legumes roots are
swellings called nodules
composed of plant cells that
have been “infected” by
nitrogen-fixing Rhizobium
bacteria
The bacteria of a nodule obtain
sugar from the plant and supply
the plant with fixed nitrogen
Each legume is associated with
a particular strain of Rhizobium
Nodules
Roots
Pea plant root. The bumps on
this pea plant root are nodules
containing Rhizobium bacteria.
The bacteria fix nitrogen and
obtain photosynthetic products
supplied by the plant.
Development of a soybean root nodule
1 Roots emit chemical
signals that attract
Rhizobium bacteria.
The bacteria then emit
signals that stimulate
root hairs to elongate
and to form an
infection thread by an
invagination of the
plasma membrane.
Infection
thread
Infected
root hair
Rhizobium
bacteria
Dividing cells
in root cortex
Bacteroid
Dividing cells in
pericycle
1
2
2 The bacteria penetrate the
cortex within the Infection
thread. Cells of the cortex
and pericycle begin
dividing, and vesicles
containing the bacteria
bud into cortical cells from
the branching infection
thread. This process
results in the formation of
bacteroids.
Developing
root nodule
3
3 Growth continues in the
affected regions of the
cortex and pericycle,
and these two masses
of dividing cells fuse,
forming the nodule.
4
4 The nodule develops
vascular tissue that
supplies nutrients to the
nodule and carries
nitrogenous compounds
into the vascular
cylinder for distribution
throughout the plant.
Bacteroid
Bacteroid
Nodule
vascular
tissue
Symbiotic Nitrogen Fixation and Agriculture


The agriculture benefits of symbiotic nitrogen
fixation are the basis for crop rotation
In this practice a non-legume such as maize is
planted one year, and the following year a
legume is planted to restore the concentration
of nitrogen in the soil
Mycorrhizae and Plant Nutrition

Mycorrhizae: are modified roots consisting of
mutualistic associations of fungi and roots

The fungus benefits from a steady supply of sugar
donated by the host plant

In return, the fungus increases the surface area of
water uptake and mineral absorption and supplies
water and minerals to the host plant

Agricultural importance: Farmers and foresters often
inoculate seeds with spores of mycorrhizal fungi to
promote the formation of mycorrhizae
Ectomycorrhizae

In ectomycorrhizae the mycelium of the fungus
forms a dense sheath over the surface of the root
Epidermis
a Ectomycorrhizae. The mantle
(a)
of the fungal mycelium
ensheathes the root. Fungal
hyphae extend from the mantle
into the soil, absorbing water
and minerals, especially
phosphate. Hyphae also
extend into the extracellular
spaces of the root cortex,
providing extensive surface
area for nutrient exchange
between the fungus and its
host plant.
Cortex
Mantle
(fungal
sheath)
100 m
Endodermis
Mantle
(fungal sheath)
Fungal
hyphae
between
cortical
cells
(colorized SEM)
Endomycorrhizae

In endomycorrhizae the microscopic fungal
hyphae extend into the root
(b)
2 Endomycorrhizae. No mantle
forms around the root, but
microscopic fungal hyphae
extend into the root. Within
the root cortex, the fungus
makes extensive contact with
the plant through branching of
hyphae that form arbuscules,
providing an enormous
surface area for nutrient
swapping. The hyphae
penetrate the cell walls, but
not the plasma membranes,
of cells within the cortex.
Epidermis
Cortex
Cortical cells
10 m
Endodermis
Fungal
hyphae
Vesicle
Casparian
strip
Root
hair
Arbuscules
(LM, stained specimen)
Epiphytes, Parasitic Plants, and
Carnivorous Plants
Epiphytes use
a host for
support but do
not extract
nutrients from
the host
EPIPHYTES

Some plants
have nutritional
adaptations
that use other
organisms in
nonmutualistic
ways
Staghorn fern, an epiphyte
PARASITIC PLANTS
Host’s phloem
Dodder
Haustoria
Mistletoe, a photosynthetic parasite
Dodder, a nonphotosynthetic
parasite
Indian pipe, a nonphotosynthetic parasite
CARNIVOROUS PLANTS
Venus’ flytrap
Pitcher plants
Sundews
Carnivorous plant movie
Improving the Protein Yield of Crops



Plant breeding research has resulted in new
varieties of maize, wheat, and rice that are
enriched in protein
Such research addresses the most widespread
form of human malnutrition: protein deficiency
Many of the projects creating GMOs
(genetically modified organisms) are aimed at
protein enrichment of crops.
High lysine corn