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Flash-induced ATP synthesis was investigated in chloroplasts isolated from spinach varieties with a pronounced difference in their adenylate kinase activity. In chloroplasts with high adenylate kinase activity the first 4 flashes induced ATP hydrolysis which persisted for an extended dark period at which the membrane remained energized. In chloroplasts showing low adenylate kinase activity, ATP hydrolysis could only be induced by a successive flash train of at least 60 flashes or by a 30–60 s period of continuous pre-illumination. The activity of the adenylate kinase must be considered when the kinetics of initial ATP synthesis and of P515 in light flashes are studied.

The effect of uncouplers of photophosphorylation on the kinetics of the flash-induced P515 electrochromic bandshift was investigated in dark-adapted chloroplasts. It was found that the presence of low concentrations of carbonyl cyanide p -trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m -chloro-phenylhydrazone (CCCP) resulted in the selective suppression of the slow P515 absorbance change (reaction 2), whereas the fast change (reaction 1) was not influenced. In contrast, high concentrations of NH 4 Cl did not alter the P515 response with respect to reactions 1 and 2. These results indicate that reaction 2 is specifically sensitive towards uncouplers which exert their function as a proton carrier in the lipophilic phase of the membrane.