The Inhibition of Photosynthesis after Exposure of Bean Leaves to Various Low Levels of CO₂

Mature first leaves of Phaseolus vulgaris L. were exposed to low partial pressures of CO₂ (7, 6 and 0 Pa CO₂) for 24 h. After exposure of leaves to 6 Pa CO₂ for 24 h, there was a reduction in the carbon exchange rate (CER) at all partial pressures of CO₂ at which measurements were made. After exposure to 7 Pa CO₂, the CER decreased only at high partial pressures of CO₂. The rates of electron transport from water to methyl viologen, through the whole chain, decreased in parallel with the decrease in CER measured at 90 Pa CO₂. One site of inhibition in leaves exposed for 24 h to 6 Pa CO₂ appeared to be the intersystem electron-transport chain since there were no significant changes in the activities of PSI and PSII, as determined from the level of P-700 and measurement of fluorescence, respectively. Another inhibitory phenomenon appeared to be a negative change in the activation state of Rubisco, while the level of Rubisco was unaffected by the exposure to 6 Pa CO₂. These decreases in photosynthetic activity caused by depletion of CO₂ explains at least in part, the inhibition of photosynthesis that is caused by rain treatment [Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431].