COMBINED EFFECTS OF HORMONES AND LIGHT DURING GROWTH PROMOTION IN PRIMARY LEAVES OF PHASEOLUS-VULGARIS

Cell enlargement in primary leaves of bean is promoted by bright white light, gibberellic acid, or the cytokinin N6-benzyladenine. I examine the combined effects of light and hormones on growth, cell wall prope rties, and osmotic parameters during growth over 24 h. Applied alone, benzyladenine (10 muM), gibberellic acid (10 muM), and white light pro duced similar increases in the length and fresh weight of excised leaf strips over 24 h. The combined effects of hormones and light on growt h were much less than additive. Individually, all three treatments sig nificantly increased cell wall plastic extensibility over 24 h. Howeve r, benzyladenine combined with white light were additive in effect on plastic extensibility, and gibberellic acid combined with white light were synergistic. The differences in effects of hormones in white ligh t on growth versus plastic extensibility indicate a decrease in growth potential, which is attributable in part to hormonal effects on osmot ic concentration. Although white light alone increased osmotic concent ration, both benzyladenine and gibberellic acid greatly decreased it, with or without white light. Furthermore, because growth potential is a function of both osmotic potential and wall yield threshold, it appe ars that yield threshold does not decline in parallel with osmotic pot ential in hormone-treated bean leaf strips. Finally, both benzyladenin e and gibberellic acid inhibit the increase in osmotic solutes normall y produced by white light. This effect, coupled with water uptake duri ng cell expansion, would produce the observed decreases in osmotic con centration in hormone-treated strips. Hence, both benzyladenine and gi bberellic acid interfere with light-induced growth, primarily through effects on the apparent ability of light to direct solute accumulation .