Apoplastic pH of intact leaves ofVicia fabaas influenced by light

The fluorochrome FITC-dextran was used to measure the effect of light on the apoplastic pH of intact Vicia faba leaves with the ratio imaging technique. In darkadapted leaves the apoplastic pH varied depending on the leaf between 5.2 and 5.9. Red light (660 nm, 4–12 W m−2) leads to multiphasic responses: in the first seconds an alkalinization (˜0.3 pH units), and thereafter an acidification of the leaf apoplast (˜0.4 pH units) were observed. Both effects could be inhibited by DCMU. While variation of CO2 concentration revealed no effect on light-induced apoplastic pH changes, a decrease in O2 concentration decreased the effect. On the basis of our data it is suggested that the influence of photosynthesis on plasmalemma H+ ATPase is responsible for the observed effects, rather than altered CO2 uptake.