Physiological And Biochemical Functions Of Plant Nutrients

Nitrogen is a building block for amino acids, amides, proteins, and alkaloids. Adequate N is essential for normal flowering, fruit set, and vegetative and fruit growth, but too much N induces excessive vegetative growth, poor color and quality of fruit, and reduced storage and shelf life.

Calcium plays an important role in binding polysaccharides and proteins that form the cell wall, stabilizing cell membranes, and regulating the activities of several enzymes in the cytoplasm, including those involved in the regulation of respiration rate. Low levels of Ca in fruit are associated with several physiological disorders of apple, pear, and other fruit, elevated susceptibility to postharvest diseases, and generally poor storage fruit quality.

Potassium is involved in protein synthesis, enzyme activation, stomatal movement, photosynthesis, and transport of photosynthates. The element acts as an osmoticum and maintains tur-gidity and growth of plant cells. Potassium applications to trees deficient in this element improve tree growth, fruit size, and apple red color, but an excess of K may exacerbate bitter pit in apples.

Phosphorus is an important structural element of deoxyribonu-cleic acid (DNA) and ribonucleic acid (RNA) and of phospholipid membranes. Also, it plays a role in hydrolysis of ATP to ADP and in the formation of ATP. Inorganic P regulates a number of enzymatic processes in the cell. Because fruit trees are very efficient in acquiring P from the soil, the deficiency of this element is usually not observed in orchards. Apple P correlates positively with fruit firmness and negatively with low-temperature breakdown in cold storage.

Magnesium occupies a central position in the chlorophyll molecule and also activates ribulose biphosphate carboxylase that plays a prominent role in photosynthesis. Magnesium forms a bridge between ATP and an enzyme, thus enabling phosphorylation and dephosphorylation, which are responsible for transfer of energy and activation of enzymatic processes. Through its effect on ribosomes, Mg plays a role in protein synthesis. Applications of Mg increase vegetative growth, fruit set, and fruit size in trees affected by severe deficiency of this element. Magnesium decelerates ripening and senescent breakdown of apples but also increases the incidence of bitter pit.

Iron is a constituent of cytochromes and nonheme iron proteins involved in photosynthesis, nitrite reduction, and respiration. Iron deficiency chlorosis occurs on fruit trees grown in neutral or high pH calcareous soils. Affected trees are less vigorous and unproductive.

Boron forms complexes with sugar derivatives and other constituents of cell walls. The element is involved in nucleic acid metabolism and in the process of cell division and elongation. Trees low in B suffer from poor fruit set because of the death of flowers, a condition known as "blossom blast." Deficient trees are less vigorous and develop small, deformed, and cracked fruit.

Manganese activates a number of important plant enzymes, some of which protect tissues from the deleterious effect of free oxygen radicals. Deficiency of Mn is usually associated with high pH soils and leads to leaf chlorosis and even tree defoliation. On acidic soils, where Mn availability is high, the element may be absorbed in excessive amounts and lead to the development of a physiological disorder on 'Delicious' apple trees known as internal bark necrosis or "measles."

Copper is a component of a number of important plant enzymes, particularly those involved in redox reactions. Copper deficiency may lead to severe shoot dieback, but such cases are rather rare.

Zinc is a component of a number of enzymes and acts as an enzyme cofactor. The element is required for the formation of tryptophan, which is a precursor for the auxin indole-3-acetic acid. Severe Zn deficiency drastically reduces shoot growth, narrowing and decreasing the size of terminal leaves and causing them to be bunched together at the shoot tips. The condition is known as "little leaf " or "rosette." Trees grown in high pH soils or in highly leached sandy soils are more likely to develop the deficiency.

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