Physiological Characterization of the Response of Excised Etiolated Cucumber Cotyledons to Benzyladenine: Expansion Begins at 30 Minutes

Benzyladenine (BA) and other cytokinins are known to induce a three-part physiological response in excised, dark-grown cucumber (Cucumis sativus L.) cotyledons consisting of a) increased size and fresh weight, b) more prominent veins, and c) increased yellowing due to carotenoid biosynthesis. Based upon an analysis of long-term kinetics, the three elements of this response occurred roughly in parallel rather than sequentially. KCl (20 mmol · L−1 optimum) promoted all three elements, especially when supplied with BA, but RbCl had no effect. As measured by increase in cotyledon width, BA-induced expansion was extrapolated to begin at approximately 30 min with linear kinetics for at least the first 2 h. KCl promoted the growth rate (1.7-fold at 2 h) but failed to decrease the lag time. This lag time, far shorter than any previous determination using cytokinin-treated cotyledons, is in the range reported for auxin- and gibberellin-responsive elongating systems. Fusicoccin promoted growth even more quickly and with greater initial rate than BA (2.9-fold at 2 h). As found in many plant tissues responsive to auxin and gibberellin, but in contrast to cytokinin-treated radish cotyledons, cell wall synthesis in cucumber cotyledons was promoted by BA (1.5-fold at 48 h). 2,6-dichlorobenwnitrile, a potent inhibitor of cellulose biosynthesis, had no effect on overall BA-induced cotyledon growth as measured at 24 h but completely abolished vein enlargement. As discussed, these results indicate that auxin-, gibberellin-, and cytokinin-induced cell expansion have apparent physiological similarities in terms of early kinetics and cell wall synthesis.