Fluorescence induction and activity of ferredoxin-NADP reductase in Bryopsis chloroplasts

Effects of medium osmolarity on the rate of CO₂ fixation, the rate of the NADP⁺-Hill reaction, and the DPS₁ transient of chlorophyll fluorescence were measured in intact Bryopsis chloroplasts. Upon decreasing the sorbitol concentration from 1.0 M (the isoosmotic conditions) to 0.25 M, the envelopes of the chloroplasts became leaky to small molecules, resulting in a considerable depression of the CO₂-fixation rate and a higher rate of the NADP⁺-Hill reaction whereas the DPS₁ transient was unaffected. This DPS₁ transient of chlorophyll fluorescence is thought to be caused by the photoactivation of electron flow on the reducing side of Photosystem I at a site occurring after ferredoxin and probably before the reduction of NADP⁺ (Satoh, K. and Katoh, S. (1980) Plant and Cell Physiol. 21, 907–916). Little effect of NADP⁺ on the DPS₁ transient and a marked lag in NADP⁺ photo-reduction in dark-adapted (inactivated) chloroplasts support the hypothesis that the site of dark inactivation is prior to the reduction site of NADP⁺, and therefore, that ferredoxin-NADP⁺ reductase is inactivated in the dark and activated in the light. Moreover, at 0.25 M sorbitol, the activity of ferredoxin-NADP⁺ reductase itself (2,6-dichlorophenolindophenol reduction by NADPH) was shown to increase according to dark-light transition of the chloroplasts. At low osmolarities (below 0.1 M sorbitol), the difference in the diaphorase activity between dark-and light-adapted chloroplasts and the lag time observed in the NADP⁺ photoreduction were lowered. This may correspond to a less pronounced DPS₁ transient at low concentrations of sorbitol. The mechanism of the photo-activation is discussed.