APOPLASTIC PH OF INTACT LEAVES OF VICIA-FABA AS 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 imagin g technique. in dark-adapted leaves the apoplastic pH Varied depending on the leaf between 5.2 and 5.9. Red light (660 nm, 4-12 W m(-2)) lea ds to multiphasic responses: in the first seconds an alkalinization (s imilar to 0.3 pH units), and thereafter an acidification of the leaf a poplast (similar to 0.4 pH units) were observed. Both effects could be inhibited by DCMU. While Variation of CO2 concentration revealed no e ffect on light-induced apoplastic pH changes, a decrease in O-2 concen tration decreased the effect. On the basis of our data it is suggested that the influence of photosynthesis on plasmalemma H+ ATPase is resp onsible for the observed effects, rather than altered CO2 uptake.