3. Bending of Oat coleoptiles tip toward source of unilateral light of wavelength 454 is due to

(1) lateral distribution of auxin toward shaded area

(2) Polar transport of auxin

(3) Degradation of auxin toward light

(4) Synthesis of auxin in shaded area

Correct answer: (1) lateral distribution of auxin toward shaded area.

IAA (auxin) in oat coleoptiles under unilateral blue light (around 454 nm) redistributes from the illuminated side to the shaded side of the tip. Higher auxin on the shaded side stimulates more cell elongation there, so the coleoptile bends toward the light source.

Explanation of options:

• (1) Lateral distribution of auxin toward shaded area – Correct. Light is perceived at the tip, triggers a lateral auxin movement, and creates a gradient: more auxin on the shaded side, less on the illuminated side. Differential elongation causes curvature toward the light.

• (2) Polar transport of auxin – Polar (basipetal) transport is always present and is important for overall auxin movement down the coleoptile, but by itself it does not explain bending. The key for phototropism is lateral (side-to-side) redistribution, not just polar flow.

• (3) Degradation of auxin toward light – Auxin may be slightly reduced on the illuminated side, but classic phototropism in oats is explained primarily by auxin relocation rather than complete localized degradation. Saying bending is “due to degradation of auxin toward light” is an oversimplification and not the accepted explanation.

• (4) Synthesis of auxin in shaded area – Auxin is mainly synthesized in the coleoptile tip and then redistributed. There is no evidence that phototropic curvature is driven by new auxin synthesis specifically in the shaded flank; the critical step is redistribution of existing auxin.