Light and Hormone Interaction in Apical Dominance in Phaseolus vulgaris L.

Abstract

Gibberellic acid (GAS), kinetin, and indole-3yl-acetic acid (IAA) each at four concentrations (°> °'5> 5, and 50 /xM) were applied alone and in all possible combinations to roots of Phaseolus vulgaris L. grown under four different light regimes (7000, 14 000, 21 000, and 28 000 lx). GA8 increased growth of main stem and laterals but reduced apical dominance, especially in the absence of, or at low kinetin concentrations. A high level of kinetin lowered GAS induced growth of main stems and, to a lesser extent, laterals. Kinetin greatly reduced apical dominance, especially in the absence of, or at low GA8 concentrations. IAA slightly reduced growth of main stems and laterals and slightly increased apical dominance. Generally the magnitude of the IAA effects were less than those of GAS or kinetin and there were less interaction between IAA and other factors than between GA3 or kinetin and other factors. Light affected growth of both main stem and laterals but the effect was dependent on GA3 and kinetin levels and the interactions were complex. Generally a hormone balance seems to be operative with gibberellinpromoting growth of main stem and laterals and cytokinins and possibly auxins preventing excessive elongation. Differential responses between main stem and lateral may be due to different local hormone concentrations and over-all responses may be tempered by light intensity. INTRODUCTION In an earlier paper (Shein and Jackson, 1971) the response of main stems and laterals of Phaseolus vulgaris L. to root-applied gibberellic acid (GA3), kinetin, and indole3yl-acetic acid (IAA) was reported. It was found that GA3 promoted elongation of main stem and laterals but this effect could be modified by kinetin, IAA, and light intensity. Promotion by GA3 of stem growth under reduced light (5950 as against il 900 lx) was reduced if IAA and kinetin were present; promotion of lateral growth under reduced light was reduced if IAA was added and eliminated if kinetin or kinetin plus IAA were added. To our knowledge these are the only experiments in which root-applied hormones have been investigated for their effect on apical dominance. However, Shein and Jackson (1971) used soil media and the varying light regime of a glasshouse. The present paper describes experiments where a wider range of hormone concentrations were applied to roots of dwarf bean grown in water culture under precisely controlled conditions of humidity, temperature, and light.