The state of the photosynthetic apparatus in leaves as analyzed by rapid gas exchange and optical methods: the pH of the chloroplast stroma and activation of enzymes in vivo

摘要

The exchange of CO2and O2was measured in leaves using specially constructed equipment capable of responding to rapid transients. Optical measurements provided information on cytochromefand P 700 oxidation in the light. The following results were obtained: i) The solubilization of CO2was used to calculate the pH of the chloroplast stroma in darkened leaves. Values ranged from pH 7.8 to pH 8.0 in different C3plants. ii) Illumination of predarkened leaves ofHelianthus annuusL. resulted in three distinct phases of O2evolution that illustrate the complexity of light activation of the photosynthetic apparatus. A first burst of O2is attributed to the reduction of electron carriers of the electron-transport chain. While plastoquinone was reduced, cytochromefwas oxidized. Appreciable oxidation of P 700 became possible only during the second O2burst, which indicates the reduction of the phosphoglycerate pool. Extensive oxidation required the opening of an electron gate on the reducing side of photosystem I. The subsequent slow rise in O2evolution towards a steady state reflects activation of the Calvin cycle and is the result of CO2assimilation. iii) Light-dependent CO2uptake by predarkened leaves occurred in four phases, three of them based on pH changes in the chloroplast stroma. Initial CO2uptake was small and probably caused by protonation of reduced plastoquinone. In the second phase, which coincided with the reduction of the pool of phosphoglycerate, the initial alkalization of the chloroplast stroma was substantially increased. In the third phase, the stroma alkalization decreased, and the fourth phase was dominated by CO2assimilation. iv) Respiratory CO2production was partially suppressed in the light during the second phase of O2evolution while phosphoglycerate was being reduced.