Functions Iron

• Iron is a constituent of two groups of proteins, viz. (a) Heme proteins containing Fe porphyrin complex as a prosthetic group: Cytochrome oxidase, catalase, peroxidase. leghemoglobin, and (b) Fe-S proteins in which Fe is coordinated to the thiol group of cysteine or to inorganic S: Ferrodoxin
• It activates a number of enzymes, including aminolevolinic acid synthetase and coproporphyrinogen oxidase.
• It plays an essential role in the nucleic acid metabolism.
• It is necessary for synthesis and maintenance of chlorophyll in plants.

Manganese

• Manganese is an integral component of the water-splining enzyme associated with photosystem II. Because of this role. Mn-deficiency is associated with adverse effects on photosynthesis and O2 evolution.
• It is a constituent of superoxide dismutase (Mn-SOD). Role of Mn assumes criticality because Mn-SOD (present in mitochondria, peroxisomes, and glyoxysomes) protects cells against the deleterious effects of superoxide free radicals.
• Manganese has a role in tricarboxylic acid cycle (TCA) in oxidative and non-oxidative decarboxylation reactions.

Zinc

Zinc is involved in many enzymatic activities such as dehydrogenase, proteinase, peptidase etc. Zinc is involved in the synthesis of indole acetic acid, metabolism of gibberellic acid and synthesis of RNA .

• Because of its preferential binding to sulphydryl group. Zn plays an important role in the stabilization and structural orientation of the membrane proteins.
• Zinc influences translocation and transport of P in plants. Under Zn-deficiency. excessive translocation of P occurs resulting in P-toxicity.

Copper

• Copper is a constituent of number of enzymes.
• Copper is important in imparting disease resistance to the plants.
• It enhances the fertility of male flowers.

Molybdenum

• Molybdenum is a component of nitrate reductase. nitrogenase, xanthine oxidase/dehydrogenase and sulphite oxidase.
• Biological nitrogen fixation (BNF) is catalysed by the Mo-containing enzyme, nitrogenase (essentially comprising of Mo-Fe-S protein and a Fe-S cluster protein) which directly transfers electrons to N2- Because of its involvement in BNF.
Mo requirement of nodulated legumes is particularly high.
• Nitrate is reduced by nitrate reductase (NR) enzyme in
cytoplasm by transfer of electrons from Mo to NO3". Owing
to close relationship between Mo supply, nitrate reductase activity (NRA) and plant growth, NRA has been used as an indicator of status of Mo in plants.
• Molybdenum is involved in protein biosynthesis through its effect on ribonuclease and alanine aminotransferase activity.
• Molybdenum affects the formation and viability of pollens and development of anthers.

Boron

• It is responsible for the cell wall formation and stabilization,. lignification and xylem differentiation. As a consequence. B-deficiency causes changes in chemical composition and ultrastructure of cell wall, accumulation of toxic phenols. inhibition of lignin synthesis and a decrease in the production of indole acetic acid (IAA)(Figure 2). Decrease in IAA is responsible for the induction of Ca-deficiency.
• It imparts drought tolerance to the crops. Regular boric acid sprays help in mitigating harmful effects of drought.
• Boron plays a role in pollen germination and pollen tube growth.
• It facilitates ion uptake by way of increasing the activities of plasma-membrane bound H+-ATPase (H+-adenosine triphosphatase).
• It facilitates transport of K in guard cells as well as stomatal opening.

Nickel

• Nickel is associated with nitrogen metabolism by way of influencing urease activity. In systems where urea is used as the sole N fertilizer for foliar spray and Ni supply is poor,
lower urease activity causes urea toxicity to the foliage and leads to severe necrosis of the root tips.
• In free-living Rhizobia, adequate Ni supply ensures optimum hydrogenase activity.
• It facilitates transport of nutrients to the seeds or grains. 

Chlorine

• It plays a major role in osmoregulation (cell elongation,stomatal opening) and charge compensation in higher plants.
• It acts as a cofactor in Mn-containing water splitting enzymeofphotosystemll.
• Chlorine in abundance suppresses the plant diseases, viz.grey leaf spot in coconut palms, take-all and common root rot in wheat, common root rot and Fusahum root rot in barley, stalk rot in corn, stem rot and sheath blight in rice, hollow heart and brown centre in potatoes, Fusarium yellows in celery, and downy mildew in millet. 

• Chlorine supply improves the nutritional quality of vegetables by preferentially lowering the NO3" -N concentration in tissues.

•  PGRs: Plant growth regulators defined as organic compounds other than nutrients that affect the physiological processes of growth   and  development   in  plants  when  applied   in  low concentrations.  Plant  growth regulators  are  active  at  low concentrations (1-10 ng / nl)  in promoting,  inhibiting or . modifying growth and development.

• They are either natural or synthetic compounds that are applied directly to a target plant to alter its life processes or its structure to improve quality, increase yields, or facilitate harvesting. In modern agriculture, people have established the benefits of extending the use of plant hormones to regulate growth of other plants.   When  natural  or  synthetic  substances used  in this manner, they are called Plant Growth Regulators.

Role: 

• Plant hormones are produced naturally by plants and are essential   for   regulating   their   own   growth.   The}'   act   by controlling  or  modifying  plant  growth  processes,   such  as formation   of   leaves   and   flowers,   elongation   of   stems, development and ripening of fruit etc.

• Plant hormones rarely act alone, and for most processes-- atleast those that are observed at the organ level-man)' of these regulators have interacted in order to produce the final effect.
  Examples:
• (a) Classical plant hormones (auxins, cytokinins. gibberellins, abscisic acid, ethylene) and growth regulatory substances with similar biological effects.

•  (b) More recently discovered natural growth substances that have    phytohormonal-like    regulatory    roles    (polyamines, oligosaccharins,         salicylates,        jasmonates,         sterols, brassinosteroids,     dehydrodiconiferyl     alcohol     glucosides. turgorins,    systemin,    unrelated    natural    stimulators    and inhibitors), as well as myoinositol. Many of these growth active substances have not yet been examined in relation to growth and organized development in vitro.
  

• Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), nickel (Ni) and chlorine (CI) are classified as plant micronutrients.Plant Growth Regulators Naturally Found:
• 
  The plant hormones are identified as promoters (auxins,gibberellin and cytokinin), inhibitors (abscissic acid and ethylene) and other hypothetical growth substance (florigen,
  flowering hormone, etc,,)
  More recently discovered natural growth substances that havephytohormonal-like regulator}' roles (polyamines, oligosaccharins, salicylates, jasmonates, sterols, brassinosteroids, dehydrodiconiferyl alcohol glucosides, turgorins, systemin. unrelated natural stimulators and inhibitors), as well as myoinositol.

Plant Growth Regulators which are produced industrially are listed in Table 1.