Plant Growth And Development

Prunning is the process of cutting shoot tips to promote lateral growth of branches. Removal of shoot tips involves removal of apical buds. In the shoot tips auxins are produced. Auxins are growth promoting phytohormones. They cause apical dominance by promoting the growth of apical buds and suppressing the growth of axillary buds. So when the auxins produced in the shoot tips are removed by decapitation it results in lateral growth and plants thus show bushy appearance. This is because of a relatively high concentration of auxin in the apical bud than in the lateral buds.

Skoog observed that coconut milk contained a substance which stimulated cell division. The substance was later on called cytokinin. The most widely occurring cytokinin in plants is isopentanyl adenine (IPA). IPA has been isolated from Pseudomonas tumefaciens.

Cytokinins (Letham, ) are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives, that promote cytokinesis ( cell division) either alone or in conjunction with auxin. Coconut milk is a rich source of cytokinin.

Abscisic acid is a growth hormone which functions as a general growth inhibitor by counteracting other hormones (auxin, gibberellins, cytokinins) or reactions mediated by them. It is also called stress hormone because the production of hormone is stimulated by drought, water logging and other adverse environmental conditions. Abscisic acid is known as dormin as it induces dormancy in buds, underground stems and seeds.

Answer (a) Spinach

Cytokinins (Gk, cytos-cell, kinesis-division) are mildly basic growth hormones that are usually amino purine derivatives and promote cell division in plants. Along with auxins, they are extensively used in tissue culture experiments where they promote differentiation of root and shoot. They have also been found to delay senescence, overcome apical dominance and promote cell expansion, etc. Roots are the main source of cytokinin synthesis, e.g., zeatin, ribosyl zeatin, dihydrozeatin, etc., are some naturally occurring cytokinins.