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 parti al pressures of CO2 (7, 6 and 0 Pa CO2) for 24 h. After exposure of le aves to 6 Pa CO2 for 24 h, there was a reduction in the carbon exchang e rate (CER) at all partial pressures of CO2 at which measurements wer e made, After exposure to 7 Pa CO2, the CER decreased only at high par tial pressures of CO2. The rates of electron transport from water to m ethyl viologen, through the whole chain, decreased in parallel with th e decrease in CER measured at 90 Pa CO2. One site of inhibition in lea ves exposed for 24 h to 6 Pa CO2 appeared to be the intersystem electr on-transport chain since there were no significant changes in the acti vities of PSI and PSII, as determined from the level of P-700 and meas urement of fluorescence, respectively, Another inhibitory phenomenon a ppeared to be a negative change in the activation state of Rubisco, wh ile the level of Rubisco was unaffected by the exposure to 6 Pa CO2. T hese decreases in photosynthetic activity caused by depletion of CO2 e xplains at least in part, the inhibition of photosynthesis that is cau sed by rain treatment [Ishibashi and Terashima (1995) Plant Cell Envir on. 18: 431].