The role of acidification in gibberellic acid- and fusicoccin-induced elongation growth of lettuce hypocotyl sections

Abstract

The roles of gibberellic acid (GA3) and fusicoccin (FC) in the elongation growth and acidification of the medium by excised hypocotyl sections of lettuce (Lactuca sativa L.) were investigated. Hypocotyl sections incubated in buffer without GA3 elongate optimally at pH 4.0-4.25 while sections incubated with GA3 show the same growth between pH 4.25 and 6.0. Preincubation of sections at pH 6.0 for 6 h does not affect the subsequent elongation response to acidic medium (pH 4.25); however, the sections become refractory to further acid treatment after their initial burst of growth in response to pH 4.25. Sections made refractory to acid are responsive to GA3 application, however, and the rate of growth in response to GA3 of sections pretreated for 6 h at pH 4.25 is 85% of that of sections pretreated at pH 6.0. Although preincubation of sections for 48 h in medium at pH 6.0 abolishes the GA3 response, it does not affect the response to buffer at pH 4.25. FC stimulates elongation growth in letuce hypocotyls at an optimal concentration of 1 μM, and pretreatment of sections at pH 4.25 does not affect this elongation response. Although both GA3 and FC increase elongation of the section, neither causes appreciable acidification of the medium. Addition of KCl or NaCl to FC-treated sections causes rapid medium acidification but addition of salts to GA3-treated tissue does not cause acidification. Abrasion of the hypocotyl to remove the cuticle does not enhance acidification of the medium by the sections nor deos it affect elongation of the sections in response to GA3 or FC. Medium acidification by the sections is not a passive process since it is abolished both by low temperature (2° C) and metabolic inhibitors (carbonyl cyanide-m-chlorophenyl-hydrazone, azide). The acidification of the medium by barley (Hordeum vulgare L.) roots in response to FC is also dependent on the presence of KCl. We conclude that the acid-growth hypothesis does not explain GA3- or FC-induced elongation in lettuce hypocotyls.