Question

Phototropic and geotropic movements are linked to

(a) gibberellins

(b) enzymes

(c) auxin

(d) cytokinins

Answer 1

Phototropic movement or phototropism is the movement of plant in response to light. This can be positive phototropism when the movement is towards light or negative when the movement is away from light.

Tropical movements which take place in response to gravity stimulus are called geotropic movements or geotropism. The primary root grows down into the soil and are positively geotropic. The secondary roots growing at right angles to the force of gravity are called diageotropic and the primary stems are negatively geotropic.

The hormone auxin is widely distributed in plant but its relative concentration differs in different parts of the plant. They are synthesized in growing tips or meristematic regions from where it is transported to different parts of the plant.

Auxins play an important role in phototropism. The shoot tip that has auxins aids in the directional movement of plant in response to sunlight. Sunlight destroys auxins, which means the shoot tip receiving direct sunlight will have least amount of auxin. Thus, the extra auxin present on the shaded side promotes more cell division and elongation causing the plant to bend towards sunlight.

The effect of gravity causes unequal distribution of auxin in the root tip i.e., more auxin concentrates on the lower side than the upper side. This in turn results in more growth on the upper side and less growth on the lower side and eventually a positive geotropic curvature is observed.

But in the case of stem, the higher concentration of auxin on the lower side promotes more growth on that side so that a negative geotropic curvature is observed.

Therefore, phototropic and geotropic movements are linked to auxin.

Option c is the answer.