Transcript Mineral Nutrition - Lectures For UG-5

Plant Physiology
Mineral Nutrition
Mineral Nutrition in plants
• Plants are:
• Capable of making all necessary organic compounds
from inorganic compounds and elements in the
environment (autotrophic)
• Supplied with all the carbon, hydrogen, and oxygen
they could ever need (CO2, H2O)
• Required to obtain all other elements from the soil
so in a sense plants act as soil miners.
Mineral Nutrition in plants
• The study of how plants obtain, distribute,
metabolize, and utilize mineral nutrients.
• “Mineral”: An inorganic element
– Acquired mostly in the form of inorganic ions from the
soil
• “Nutrient”: A substance needed to survive or
necessary for the synthesis of organic compounds
Classifying mineral nutrients
• Amount required or present in plant tissue
• Metabolic need for the mineral nutrient
• Biochemical function(s) for the mineral
nutrient
• Mobility within the plant
Mineral macronutrients
Mineral micronutrients
Essentiality of mineral
nutrients
Essential: Universal for all plants
• – Absence prevents completion of life cycle
• – Absence leads to deficiency
• – Required for some aspect of mineral nutrition
• Beneficial: Often limited to a few species
• – Stimulates growth and development
• – May be required in some species
• – Examples: Na, Si, Se. Cobalt required for N-fixing
species only for development of nodules
Essentiality of mineral nutrients
• There are four basic groups:
• Group one:
– Forms the organic components of plants
– Plants assimilate these nutrients via biochemical
reactions involving oxidation and reduction
• Group two:
– Energy storage reactions or maintaining structural
integrity
– Present in plant tissue as phosphate, borate or silicate
esters
Biochemical functions of mineral
nutrients
Essentiality of mineral nutrients
• Group three:
– Present in plant tissue as either free ions or ions
bound to substrates such as the pectin component of
the plant cell wall
– Of particular importance are their roles as
– Enzyme cofactors
– In the regulation of osmotic potentials
– e.g. Ca2+ secondary messenger-amount
determines state of stress
Biochemical functions of mineral
nutrients
Essentiality of mineral nutrients
• Group four:
– This last group has important roles in reactions
involving electron transfer.
– Some also involved in the formation/regulation of plant
growth hormones – Zinc
– The light reaction of photosynthesis - Copper
Biochemical functions of mineral
nutrients
Techniques used to study
plant nutrition
Hydroponic and Aeroponic systems for
growing plants in nutrient solutions
in which composition and pH can be
automatically controlled.
(A) In a hydroponic
system, the roots are immersed in the nutrient
solution, and air is bubbled
through the solution.
(B) An alternative hydroponic system, often
used in commercial
production, is the nutrient film growth system, in
which the nutrient solution is pumped as a thin
film down a shallow trough surrounding the plant
roots. In this system the composition and pH of
the nutrient solution can be controlled
automatically.
(C) In the aeroponic system, the roots are
suspended over the nutrient solution,
which is whipped into a mist by a motor-driven
rotor.
Nutrient deficiencies
Mineral nutrient deficiencies occur when the concentration of a
nutrient decreases below this typical range
• Deficiencies of specific nutrients lead to specific visual, often
characteristic, symptoms reflective of the role of that nutrient in plant
metabolism
Chlorosis
Necrosis
Nutrient deficiency v. sufficiency
Patterns of deficiency
• The location where a
deficiency reflects the
mobility of a nutrient
• Nutrients are
redistributed in the
phloem
• Old leaves = mobile
• Young = immobile
Patterns of deficiency
Older leaves on celery turning yellow while the growing points in the center
remain green.
How are mineral nutrients acquired
by plants?
Uptake through the leaves
• Artificial: called foliar application. Used to apply
iron, copper and manganese.
• Associations with mycorrhizal fungi
• Fungi help with root absorption
• Uptake by the roots
The soil affects nutrient
absorption
• pH affects the growth of plant roots
and soil microbes
• Root growth favors a pH of 5.5 to
6.5
• Acidic conditions weathers rock
and releases potassium,
magnesium, calcium, and
manganese.
• The decomposition of organic
material lowers soil pH.
• Rainfall leaches ions through soil
to form alkaline conditions
The soil affects nutrient
absorption
• Negatively charged soil particles
affect the absorption of mineral
nutrients
• Cation exchange occurs on the
surface of the soil particle
• Cations (+ve charged ions) bind
to soil as it is –ve charded
• If potassium binds to the soil it
can displace calcium from the soil
particle and make it available for
uptake by the root
Roots and Mycorrhizae
Plant Physiology
Root Systems
Tap root and Fibrous Root
Comparison of Root Systems
Plant roots – the primary route
for mineral nutrient acquisition
• Meristematic zone
– Cells divide both in direction of
root base to form cells that will
become the functional root and
in the direction of the root apex
to form the root cap
• Elongation zone
– Cells elongate rapidly, undergo
final round of divisions to form
the endodermis. Some cells
thicken to form casparian strip
• Maturation zone
– Fully formed root with xylem
and phloem – root hairs first
appear here
Root absorbs different mineral
ions in different areas
• Calcium
– Apical region
• Iron
– Apical region (barley)
– Or entire root (corn)
• Potassium, nitrate, ammonium,
and phosphate
– All locations of root surface
• In corn, elongation zone has max K
accumulation and nitrate absorption
– In corn and rice, root apex absorbs
ammonium faster than the
elongation zone does
– In several species, root hairs are the
most active phosphate absorbers
Why should root tips be the
primary site of nutrient uptake?
• Tissues with greatest need for nutrients
– Cell elongation requires Potassium, nitrate, and chlorine to increase osmotic
pressure within the wall
– Ammonium is a good nitrogen source for cell division in meristem
– Apex grows into fresh soil and finds fresh supplies of nutrients
• Nutrients are carried via bulk flow with water, and
water enters near tips
• Maintain concentration gradients for mineral
nutrient transport and uptake
Root uptake soon depletes
nutrients near the roots
• Formation of a nutrient
depletion zone in the region
of the soil near the plant root
– Forms when rate of nutrient
uptake exceeds rate of
replacement in soil by
diffusion in the water
column
– Root associations with
Mycorrhizal fungi help
the plant overcome this
problem
Mycorrhizal associations
• Not unusual
– 83% of dicots, 79% of monocots and
all gymnosperms
• Ectotrophic Mycorrhizal fungi
– Form a thick sheath around root.
Some mycelium penetrates the
cortex cells of the root
– Root cortex cells are not penetrated,
surrounded by a zone of hyphae
called Hartig net
– The capacity of the root system to
absorb nutrients improved by this
association – the fungal hyphae are
finer than root hairs and can reach
beyond nutrient-depleted zones in
the soil near the root
Mycorrhizal associations
• Vesicular arbuscular
mycorrhizal fungi
– Hyphae grow in dense
arrangement , both within the root
itself and extending out from the
root into the soil
– After entering root, either by root
hair or through epidermis hyphae
move through regions between
cells and penetrate individual
cortex cells.
– Within cells form oval structures
– vesicles – and branched
structures – arbuscules (site of
nutrient transfer)
– P, Cu, & Zn absorption improved by
hyphae reaching beyond the nutrientdepleted zones in the soil near the
Nutrients move from fungi
to root cells
• Ectotrophic Mycorrhizal
– Occurs by simple diffusion from the hyphae in the hartig
net to the root cells
• Vesicular arbuscular mycorrhizal fungi
– Occurs by simple diffusion from the arbuscules to the
root cells
– Also, as arbuscules are degenerating as new ones are
forming, the nutrients may be released directly into the
host cell
Vesicular-arbuscular mycorrhiza:Highly
colonized root of maize dyed with trypan
blue. Mycorrhizal formations are clearly
visible: 1) vesicles; 2) arbuscules
Ectomycorrhiza: root tip of Pinus
nigracolonised by ectomycorrhizal fungus
Manipulating mineral transport
in plants
• Increase plant growth and yield
• Increase plant nutritional quality and density
• Increase removal of soil contaminants (as in
phytoremediation)
Further Readings
• Mineral Nutrition, Chapter 5, Plant
Physiology by Taiz and Zeiger