Assertion (A) : If the oat seedling are exposed to unilateral illumination, the stem bends towards the light. Reason (R) : An auxin gradient is established in the coleoptile by the destruction of auxin in the lighted side and extra synthesis of auxin in the darker side.

Phototropism (bending toward light) in oat coleoptiles is caused by an auxin gradient. Current understanding indicates that auxin redistribution occurs through lateral transport (PIN proteins) rather than differential destruction/synthesis. The dark side accumulates more auxin, causing elongation and bending toward light. While the reason mentions auxin gradients (which is correct), the mechanism described (destruction in light, extra synthesis in dark) is outdated. However, for the level of this question, it may be accepted as explaining the gradient.