EXTERNAL CALCIUM REQUIREMENTS FOR LIGHT INDUCTION OF CHLOROPHYLL ACCUMULATION AND ITS ENHANCEMENT BY RED-LIGHT AND CYTOKININ PRETREATMENTS IN EXCISED ETIOLATED CUCUMBER COTYLEDONS

Excised etiolated cucumber (Cucumis sativus L.) cotyledons that were depleted of external Ca2+ by equilibration with a Ca2+ buffer, which maintained the free Ca2+ concentration at 10(-8) M, failed to accumulate chlorophyll upon a 2-h exposure to white light. Increasing amounts of chlorophyll accumulation occurred at increasing external Ca2+ concentrations within the range of 10(-7)-10(-3) M. Preillumination with red light or pretreatment with benzyladenine, which enhanced the rate of light-induced chlorophyll accumulation in control cotyledons, did not overcome the block to light-induced chlorophyll accumulation caused by the depletion of external Ca2+. Etiolated cotyledons that were treated with the Ca2+ ionophore, A23187, and then equilibrated with 10(-5) M Ca2+, accumulated significantly more chlorophyll during exposure to light than did untreated cotyledons. The enhancing effect of A23187 was approximately equal to that caused by red-light pretreatment. Etiolated cotyledons that were exposed to the Ca2+ channel-blocking agent, Nd3+ (neodymium), in the presence of 10(-5) M Ca2+, did not exhibit an enhancement of chlorophyll accumulation by red-light pretreatment, although they accumulated control levels of chlorophyll upon exposure to light and showed control levels of enhancement of chlorophyll accumulation by cytokinin pretreatment. Conversely, etiolated cotyledons that were equilibrated with 10(-5) M Ca2+ in the presence of nifedipine, a blocker of some Ca2+ channels, did not exhibit an enhancement of chlorophyll accumulation by cytokinin pretreatment, although they accumulated control levels of chlorophyll upon exposure to light and showed control levels of enhancement of chlorophyll accumulation by red-light pretreatment. These results indicate that external Ca2+ is required for chlorophyll accumulation by excised etiolated cucumber cotyledons during the first 2 h of light exposure, and that an influx of external Ca2+ is required for the enhancing effect of red-light and cytokinin. The differential abilities of Nd3+ and nifedipine to block the effects of red-light and cytokinin pretreatments suggests that enhancement of chlorophyll accumulation by red-light and cytokinin may involve different classes of Ca2+ channels.