The inhibition of photosynthesis after exposure of bean leaves to various low levels of CO2

Mature first leaves of Phaseolus vulgaris L. were expos-ed to low partial pressures of CO2 (7, 6 and 0 Pa COJ 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 trans-port from water to methyl viologen, through the whole chain, decreased in parallel with the decrease in CER meas-ured at 90 Pa CO2. One site of inhibition in leaves exposed for 24 h to 6 Pa CO2 appeared to be the intersystem elec-tron-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, respective-ly. Another inhibitory phenomenon appeared to be a neg-ative 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 [Ishibash