Plant nutrition
Plant Nutrition
1. What is meant by “plant nutrition”
2. The chemical elements required by plants
3. How plants take up mineral elements from soil
4. Problems in plant nutrition
5. Nitrogen and the effects of soil organic matter on plant nutrition
6. Leaf senescence and withdrawal of nutrients to the plant
1. What is meant by “plant nutrition”
Uptake from the soil of mineral
elements
“Plant nutrition” specifically
does not refer to photosynthesis.
In this lecture the uptake of
nutrients from the soil directly
by roots
In the next lecture mutualistic
relationships between plants and
fungi and microrganisms
2.
The chemical elements required by plants
Plants require 13 mineral nutrient elements for growth.
The elements that are required or necessary for plants to
complete their life cycle are called essential plant nutrients.
Each has a critical function in plants and are required in varying
amounts in plant tissue, see table on next slide for typical
amounts relative to nitrogen and the function of essential
nutrients .
The nutrient elements differ in the form they are absorbed by the
plant, by their functions in the plant, by their mobility in the
plant and by the plant deficiency or toxicity symptoms
characteristic of the nutrient.
Name
Chemical
symbol
Primary macronutrients
Nitrogen
N
Phosphorus
P
Potassium
K
Relative
% in plant
to N
100
6
25
Function in plant
Proteins, amino acids
Nucleic acids, ATP
Catalyst, ion transport
Secondary macronutrients
Calcium
Ca
Magnesium
Mg
Sulfur
S
Iron
Fe
12.5
8
3
0.2
Cell wall component
Part of chlorophyll
Amino acids
Chlorophyll synthesis
Micronutrients
Copper
Manganese
Zinc
Boron
Molybdenum
Chlorine
0.01
0.1
0.03
0.2
0.0001
0.3
Component of enzymes
Activates enzymes
Activates enzymes
Cell wall component
Involved in N fixation
Photosynthesis reactions
Cu
Mn
Zn
B
Mo
Cl
3. How plants take up mineral elements from soil
A. Bulk flow: Uptake in the transpiration stream
Nutrients diffuse to regions of low
concentration and roots grow into and
proliferate in soil zones with high nutrient
concentrations (horse manure in sand).
Dominant in mineral soils:
B. Mycorrhizae: symbiotic relationship with fungi
Roots are slow growing but mycorrhizal fungi
proliferate and ramify through the soil. Symbiotic
relationship: carbon-nitrogen exchange.
Dominant in organic soils:
Mineral soils
Nutrients are available through WATER in the soil
Soil acidity determines how nutrients become available to plants
Mineral soils
Small quantities of water molecules dissociate:
H2O
OH- + H+
The concentration of dissociated water in freshly-distilled water is
10-7 M. This is used to describe acidity-alkalinity, originally called
the pouvoir Hydrogéne, which we know now as pH.
pH = - log [H+] = - log [10-7M] = 7 for fresh distilled water
Small values for acid, e.g., the water in Sphagnum bogs can be ~3
Large values for alkaline, e.g., soils on limestone ~8
A clay particle (much enlarged here) is covered
with negative charges, anions:
How clay particles provide nutrients
Opposites attract, so metal ions with positive
charge(s), cations, stick all over the surface of
the clay particle:
The root hair cells of plant roots secrete H+ into the water around
nearby clay particles. These smaller H cations replace the larger
macro- and micro-nutrient cations:
2H+
Ca2+
The released cations are now available for uptake into roots.
In this summary occurrence of H+ in soil water is
shown as the result of respiration of CO2 and
disassociation of carbonic acid H2CO3 that forms
Water flow
Summary of soil water chemistry
Single cell root hairs
Apoplastic and Symplastic Transport
Recall transport of sucrose from photosynthesizing cells to phloem
Water and cations can be taken up by roots:
1. apoplastically,
i.e. through the cell walls and
intercellular spaces,
2. symplastically,
i.e. from protoplast to protoplast via
plasmodesmata
However, at the endodermis the apoplastic pathway is blocked
by a waxy deposit of the wall called the Casparian strip.
In some plants is the Casparian strip located in the exodermis
so that the apoplastic barrier works sooner.
Casparian strip
Cross section of Smilax root
showing heavily thickened
endodermis walls
Cross section of endodermis
with the Casparian strip
stained pink. The Casparian
strip contains suberin and
lignin
Cross section of Zea mays root using
fluorescence microscopy showing
thickened cell walls on the inside of
endodermis
Uptake of water and nutrients by roots
See Equivalent Fig. 32.2B
Water uptake by the root
The ions that have passed through the endodermis are contained
within the vascular tissue.
Water can then be drawn into the root from the soil by osmosis, the
endosmotic root pressure. This can be sufficient to force water up
through the xylem and may be particularly important when there is
not a strong water potential gradient due to transpiration
Some plants have hydathodes at their leaf margins that secrete water
as droplets, a process called guttation.
Film clip
4.Problems in plant nutrition
Plant Nutrient Type
Nitrogen
Deficiency
Excess
Phosphorus
Deficiency
Excess
Potassium
Deficiency
Excess
Visual symptoms
Light green to yellow appearance of leaves, especially
older leaves; stunted growth; poor fruit development.
Dark green foliage which may be susceptible to
lodging, drought, disease and insect invasion. Fruit and
seed crops may fail to yield.
Leaves may develop purple coloration; stunted plant
growth and delay in plant development.
Excess phosphorus may cause micronutrient
deficiencies, especially iron or zinc.
Older leaves turn yellow initially around margins and
die; irregular fruit development.
Excess potassium may cause deficiencies in magnesium
and possibly calcium.
Excess frequently operates through imbalance
W.F. Bennett (editor), 1993. Nutrient Deficiencies & Toxicities in Crop Plants, APS Press, St. Paul, Minnesota.
5. Nitrogen and the effects of soil organic matter
on plant nutrition
Nitrogen is the element most required by plants, in terms of weight.
It is not a product of weathering of soil particles.
There are two sources: fixation of atmospheric nitrogen by
bacteria
decomposition of organic matter,
usually decaying plant material.
N-fixing bacteria
Most uptake from the soil is in the form of nitrate
Fig. 32.13
Organic material is important in agricultural soils both as a
source of nitrogen and because it can increase water holding
capacity, e.g. biosolids application effects
A characteristic of non-agricultural
soils is accumulation of organic
material and acidification of the soil.
Such soils typically develop a very
distinct stratification, with organic
mater at the top.
Spodic soil
The organic layers in such soils can
have a considerable total quantity of
nitrogen but little may be available
due to the high acidity, and sometimes
lack of oxygen, in the organic layer.
6. Leaf senescence and withdrawal of
nutrients to the plant
Senescence
is a
term for the collective
process leading to the
death of a plant or plant
part, like a leaf. Leaf
senescence is a part of
the process by which a
plant goes into dormancy
and is induced by a
change in day length.
Changing leaf color
As daylength decreases, the plants ability
to synthesize chlorophyll becomes
reduced.
Yellow and orange carotinoids and
xanthophylls, always present within the
leaf, begin to show.
Water and nutrients are drawn into the
stems and from the leaves.
Senescing cells also produce other
chemicals, particularly anthocyanins,
responsible for red and purple colors.
Some species, particularly oaks, contain
high quantities of tannins in the leaves
which are responsible for brown colors.
Nutrient retention during senescence
In deciduous tree species some 60 – 70% of N, 60 – 70% of P,
30% of K, 25% of Mg, and 15% of Ca are withdrawn from
leaves prior to them being shed. Storage is in the bark and
elements are re-mobilized in spring
A decline in photosynthesis with aging can be prevented by the
decapitation of the plant above the leaf in question. This implies a
regulatory action by the growing point. Results from primary leaves
of bean (Das, 1968).
Leaf Abscission
The final stage in leaf senescence is abscission ("cutting off")
Abscission is controlled by a special layer of cells at the
base of the petiole, the abscission layer.
This layer releases ethylene gas that stimulates production
of cellulase. This in turn breaks down cells walls so that
eventually the leaf is held on to the plant only by xylem
fibers. Wind eventually weakens these and leaf falls
Another special layer of cells adjacent to the abscission layer
produces cells impregnated with suberin. These form a
protective layer, which is seen as the leaf scar
Tyloses, as well as gums are formed inside the vessels and
plug them up before abscission occurs
Vascular tissue
Leaf
Abscission layer
Developing leaf scar
Axilliary
bud
Stem
Sections you need to have read
32.6 32.7 32.8 32.9 32.13
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory