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

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 (GA3) 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-, GA3-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, GA3 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 GA3 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 GA3 or BA. These results indicate that while WL, GA3 and BA all initiate growth in bean leaves by altering cell-wall properties, GA3 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.