The quantum requirement of photo phosphorylation and its stoichiometric relation to reduction in anacystis nidulans in vivo
Zeitschrift fuer Pflanzenphysiologie 85(5): 393-402
Rates of ATP synthesis in the blue-green alga A. nidulans were measured as a function of light intensity up to 20 kerg .cntdot. cm-2 .cntdot. s-1 during the transition from dark to light of 632 nm and of 719 nm. Rates of 14CO2 fixation were determined under the same light conditions. The absorption of light by the cells was measured at both wavelengths up to light saturation of phorophosphorylation. ATP synthesis reached light saturation at 3 kerg .cntdot. cm-2 .cntdot. s-1 (632 nm) or at 1 kerg .cntdot. cm-2 .cntdot. s-1 (719 nm). By contrast CO2 fixation was not saturated even at intensities of 20 kerg .cntdot. cm-2 .cntdot. s-1 (632 nm). At this intensity of 719 nm light, CO2 fixation was very low and did not exceed 1.8 .mu.mol .cntdot. mg chl-1 .cntdot. h-1. The quantum requirement of ATP synthesis was determined. A plot of the rates v.s. light intensity yielded a requirement of 2 quanta per ATP at zero light intensity for both wavelengths. ATP synthesis was inhibited by about 50% by 5 .mu.M DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] or by 10 .mu.M desapidin suggesting an equal contribution of both cyclic and non-cyclic electron transport to the total photophosphorylation. Evidence is presented that the open-chain photophosphorylation occurs in part by non-cyclic and by pseudo-cyclic electron transport, the proportion of which is estimated from the results.