Simultaneous measurements on the effect of oxygen concentration on water vapor and carbon dioxide exchange in leaves.

Transpiration rate was unaffected by O2 concentration in the range 1-80% when illuminated leaves ofAtriplex patula andA. rosea were kept at low CO2 concentrations. Moreover, the marked increase in the rate of light-saturated photosynthesis that takes place in many plant species when O2 concentration is reduced from 21% to a few percent was not accompanied by any change in transpiration rate inA. patula andSolanum dulcamara. The results indicate that higher plant photosynthesis in normal air and saturating light is not determined by physical barriers to gas diffusion alone but that it is markedly limited by biochemical processes.