Control of the Hypocotyl Hook Angle inPhaseolus mungoL.: the Role of Parts of the Seedling

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Abstract
Phaseolus mungo L. seedlings were grown in the dark for 5 d. They were either kept in the dark or exposed to 5 min red light. The location of the light-sensitive region (the hook) and the anatomy of light-induced opening are described. The response was phytochrome-mediated. The time course of light-induced opening and its far-red reversal are described and the implications for the mechanism of light-induced opening discussed. The lag phase between irradiation and the start of opening was >8 h, for 3 h of which the response remained completely photoreversible. This implies that some critical level of a factor is required before hook opening can be initiated. The terminal parts (leaves and cotyledons) kept the hooks shut in the dark, but did not directly affect the light response. The necessity for distinguishing between treatment effects on hook angle and on light-induced opening is noted. We hypothesize that the terminal parts produced a hook-maintaining factor whose level declined after their removal. This factor was dispersed or inactivated following irradiation, or the hook cells were rendered less sensitive to it. Differences between the responses of P. mungo and those reported for other species are discussed.

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