Biology for ( Grade 10 )

9.3 (b) Gibberallins :

Gibberellins have a unique property of increasing the height of plants

but they do not cause curvature. The gibberellins were firs discovered in

Japan by Yabuta and Sumuki. E. Kurosawa obtained extracts from rice

plants which were infected with fungus Gibberella fujikuroi. These

rice plants were taller and sterile.

(i) Functions of gibberellins : The physiological effects of gibberellins

on plant growth are as follows :

(A) Cell elongation : Gibberellins cause stem elongation and

expansion of leaves in intact plants.

(B) Stem elongation : These induce stem elongation in genetically

dwarf varieties (pea, maize & Cabbage) It is called bolting.

(C) Seed germination

(D) Flowering

(E) Parthenocarpy

(F) Counteract dormancy : Natural dormancy of buds, tubers,

rhizomes and some seeds can be over come by gibberellins.

(G) Induction of maleness : GA 3 induces the formation of male

flowers on genetically female plants in Cucurbits.

(H) Induction of aerial stem

(I) Increase in size of fruits : Application of gibberellins increase

bunch length and fruit size in grapes.

9.3 (c) Cytokinins :

The cytokinins are chemically basic growth hormones which promote

cell division in plants. This group of phytohormones was discovered