Control of Hypocotyl Hook Angle inPhaseolus mungoL. Role of Growth Substances

Abstract

Effects of growth hormones on the hook angle and light response of Phaseolus mungo L. hypocotyl hooks are described and the results are discussed with reference to the functions of other parts of the seedling in controlling the growth and shape of the hook. Apically applied IAA (indolyl acetic acid) prevented hook opening in decapitated seedlings in the dark and in all the red-irradiated seedlings. [14C]IAA experiments showed that only a small quantity of IAA (2-6 pg per hook) was required to produce these effects, and that trans port of IAA through the hook was negligible and unaffected by red irradiation. ABA (abscisic acid) had little effect on the hooks or their light response. Applied ethylene and IAA-induced ethylene slightly closed the hooks, but only slightly reduced light-induced opening. IAA reduced the effect of ethylene in the dark, but after ir radiation the hooks appeared more sensitive to the ethylene in the presence of IAA, resulting in light-induced hook closure. Basally applied kinetin (6-furfurylaminopurine) prevented decapitated hooks from opening in the dark, especially when GA3 (gibberellic acid) was also present. Some combinations of kinetin and GA3 (with high kinetin concentrations) also prevented light-induced opening, but combinations with lower kinetin concentrations allowed almost as much opening as was found in intact hooks. It is proposed that the terminal parts act by regulating the supply of cytokinins and gibberel lins from the basal parts, and that IAA does not mediate this function in this species. The results are compared with those reported for other species.