5. A young dicot seedling (e.gsoyabean) is subjected tc gravity stimulus by laying it horizontally on a surface the shoot bends upwards and root bends downward. Indicate the reason.
(1) Redistribution of auxin throughout the seedlings is responsible for stimulatory unequal growth in shoots and roots.
(2) Redistribution of auxin in shoots while cytokinine in roots is responsible for stimulatory unequal growth.
(3) Redistribution of auxin in roots while cytokinine in shoot is responsible for stimulatory unequal growth.
(4) Redistribution of cytokinine throughout the seedlings is responsible for stimulatory unequal growth in shoots and roots.

Correct answer: (1) Redistribution of auxin throughout the seedling is responsible for stimulatory unequal growth in shoots and roots.

Auxin (IAA) redistributes under gravity so that it accumulates more on the lower side of both shoot and root. In shoots, higher auxin on the lower side stimulates cell elongation, so the lower side grows faster and the shoot bends upwards (negative gravitropism). In roots, the same higher auxin on the lower side inhibits elongation there, while the upper side cells elongate more, so the root bends downward (positive gravitropism). This opposite response to the same auxin gradient explains why both organs curve appropriately using a single hormone.

Why other options are wrong:

• (2) and (3) invoke cytokinin, but classic primary gravitropic bending in young seedlings is regulated mainly by auxin redistribution and differential auxin sensitivity, not by a shoot–root cytokinin swap.

• (4) Cytokinin redistribution alone cannot account for the characteristic opposite curvature patterns seen in shoots and roots; experimental work consistently identifies auxin gradients as the key signal.