Role of acid efflux during growth promotion of primary leaves of Phaseolus vulgaris L. by hormones and light

Abstract

The white-light-(WL) induced enlargement of dicotyledonous leaf cells is known to occur via an acid-growth mechanism; i.e., WL causes leaf cells to excrete protons which lead to an increase in wall extensibility and thus cell enlargement. Gibberellic acid (GA 3 ) and N 6 -benzyladenine (BA) also induce leaf cell enlargement. To see if they also act via acid-induced cell wall loosening, a comparison has been made of WL-, GA 3 -and BA-induced growth of strips, taken from primary leaves of bean ( Phaseolus vulgaris L.) plants raised in continuous red light for 10 d. White light, GA 3 and BA all increased wall extensibility as measured by the Instron technique, and this change preceded the increase in growth rate. However, whereas WL induced significant proton excretion, neither GA 3 nor BA caused any acidification of the apoplast. Furthermore, neutral buffers, which effectively inhibited the growth induced by WL, were without effect on growth promoted by either GA 3 or BA. These results indicate that while WL, GA 3 and BA all initiate growth in bean leaves by altering cell-wall properties, GA 3 and BA do so through some wall loosening mechanism other than wall acidification. Neither gibberellin nor cytokinin is likely to play a major role in light-induced cell enlargement of dicotyledonous leaves.