The photoenzymatic cycle of NADPH: protochlorophyllide oxidoreductase in primary bean leaves (Phaseolus vulgaris) during the first days of photoperiodic growth.

The photoenzymatic cycle of the light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) was investigated in situ during early stages of development of bean leaves under light-dark cycles (LDC). In the experimental system used in this study, prolamellar bodies developed during night periods and disappeared during light periods. This was accompanied by changes in the photoactive to non-photoactive Pchlide ratio, which was higher at the end of the light period, and tended to increase with the number of LDC's. Flash-induced absorbance changes in the Chlide absorption region (700 nm) were used in order to monitor the formation of short- and long-wavelength forms of Chlide (C670-675 and C682-694), which correspond to free Chlide and aggregated Chlide-NADPH-LPOR complexes, respectively. The ratio of long-wavelength to short-wavelength Chlides after one flash increased with the number of LDC's, and was higher in leaves collected at the end of light periods, compared to leaves collected at the end of night periods. During light periods, photoactive Pchlide regeneration and Chlide phytylation were completed within 1 min after flash-induced formation of long-wavelength Chlide. The results show for the first time that the photoenzymatic LPOR cycle proceeds through similar steps, but at much faster rates, during photoperiodic greening than in the previously studied leaves of etiolated plants. In particular, the parallel formation of two Chlide species always occurs, but the ratio of the two species depends on the ratio of photoactive to non-photoactive Pchlide and on light or dark adaptation.