PLANT GROWTH SUBSTANCES AND THEIR ROLES IN CROP PRODUCTION PPCP 202 DR. AKINTOKUN P.O

Growth is brought about by cell division, expansion and assimilation. Any factor that directly or indirectly affects those processes will influence growth. These are internal and external factors that influence growth of plants. Protein form the structural foundation of the body.

Nitrogen for the building block of protein. Plants growing in Nitrogen-deceit soils can’t manufacture enough protein, thus show started growth and development. Light play a major role in synthesis and action of chlorophyll without which photosynthesis cannot take place. Seedling growing in dark become etiolated: it is yellow due to lack of chlorophyll, the leaves fail and expand and the item is long and spindly.

Oxygen those not generally affect the growths of the above ground parts and plant, but it can have profound influence on the growth of roots. Internal factor affecting growth include the genetic constitution and the relative amount of hormones prevent in the body thus these two are related. Hence, plant growth and development are characterized by high degree of co-ordination and phasing. The growth of one part is closely related with the growth or activities in other parts of the plant.

Such internal coordination system is maintained by naturally occurring organic compounds referred to as plant hormones or phytohormones in the plant. This phyto-hormones influence the numerous physiological processes culminating in plant growth, development and death. Effects of environment are largely influenced by either the synthesis and or the general distribution of these substances within the plant.

Five basic classes of phyto-hormones have been recognized: (1).AUXINS (2). GIBBERELLINS (3).CYTOKININS (4).ABSSCISIC ACID(5).ETHYLENE These are the most important agents involved in coordinating the growth of the plant as a whole. Depending on the plant, they act either singly or in association.

Phyto-hormones are organic compounds which in small quantities promote, support, maintain or inhibit various metabolic activities within the plant system. Phyto-hormones are not carbohydrate in the sense that carbohydrates are produced and require by the plant in large quantities while phyto-hormones are produced in small quantities but their effect are pronounced.

Regulation of growth by is actually based on interaction and delicate balance amongst the various groups of phyto-hormones. AUXINS The word auxin is actually a generic term which applies to a group of compounds affecting the plant in some way or manner. The word include the internally occurring and the synthetic members of the group called Auxins.

The auxins are formed chiefly in the apical meristem of the stem and root, buds, flower or inflorescene on growing flower stalk- and transported from there to another to produce a particular physiological effect there. At low concentration, they stimulate growth while at high concentration they retard growth. They are characterized by causing cell enlargement and stem elongation. They are also active in development of branches in plants and are associated with apical dominance.

CYTOKININS The word for cytokinins is a generic name for all naturally occurring substances that are known to promote cell division. They are also known to delay senescence. The first naturally occurring cytokinin was found in corn and is referred to as ZEATIN. The most widely distributed cytokinins are the synthetic benzyladenine and 6-sulfuryl aminopurine or Kinetin

Zeatin, a naturally occurring cytokinin was isolated from Zea mays grains. Zeatin has since been found in other plants. Some closely related compounds have also been found to exhibit cytokinin activities. Such synthetic cytokinins include Benzyl ammo-purine. The natural cytokinin appears to be made principally in apical root meristem, inflorescences and developing fruits. Certain cytokinins have been found to be the constituent of certain transfer RNA molecules in a number of different organisms.

They are also involved in stimulation of organ formation e.g. formation of leave, fruit, buds, and branches. They tend to contract or overcome apical dominance and break dormancy. They also enhance seed germination and uniform flowering. Cytokinins regulate the transportation of metabolites in the phloem. Cytokinins are also useful in the preservation of flowers, fruits and leafy vegetables.

ABSCISIC ACID These were previously called Dormin or Abscisin mainly because of their regulatory effect on abscission and dormancy. This hormone is widespread in higher plants and is found in many different organs and tissues (both old and young) of plants. ABA induces abscission of the leaves of a wide variety of plants and fruits of some plant species. IAA and ethylene interact with ABA in the control of abscission.

ABA appears to be an internal factor inducing dormancy in the buds of at least some temperature zone woody plants. ABA prevents or delays the germination of many kinds of seeds. ABA retards the growth of a large variety of plant tissues and organs including leaves, colcoptiles, stems, hypocotyle and roots. It promotes senescence through leaf abscission, degeneration of excised leaves and acceleration of decomposition of chlorophyll. ABA inhibits flower induction in some long day plants.

ETHYLENE This is a simple gas that is produced in small quantities by many plant tissues and they serve as a very powerful regulator of growth and development. They are found very prominently in physiologically matured fruits undergoing ripening. (Ethylene is a ripening agent). Ethylene operates as a fruit-ripening hormone. Treatment of fruits such as banana, oranges when mature, but not yet fully ripened, with ethylene hasten their ripening.

This treatment is of commercial importance. Endogenously generated Ethylene plays a role in normal abscission processes especially in interaction with IAA and abscisic acid. The leaves of many species exhibit epinasty (downward bending) when exposed to very low concentration of ethylene.

GIBBERELLIN It was originally discovered as a product of fungus reffered to as GIBBERELLINFUJIKORA, which parasite rice thereby causing rice plants to grow unusually tall. Gibberellin promotes a wide variety of effects but it particular eliminates dwarfism or shortness in plants. The gibberellins are structurally a distinctive and closely-knit group of plant hormones. They are of wide occurrence in vascular plants and they occur also in very low concentration. The best known of gibberellins is Gibberellic acid (i.e. A 3 )

All know gibberellins have this basic structures of four rings fixed together called gibberellane ring. The gibberellins differ one from the other in the side chain. Gibberellins are known to induce the production of male flowers (in dioecious flowers or plants).It reverses stunting caused by virus.

It encourages leaf expansion on and acceleration of flowering especially in plants that will require two or three years to mature. It can also be used to induce or eliminate the need for cold treatment or photo-periodic induction of plant in certain plants e.g. horticultural crops. Excessive application can cause gigantism.