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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 (CO 2 , H 2 O)
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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
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
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
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
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
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 acquiredby 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
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 nutrient-depleted zones in the soil near the root
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
