Apoplastic pH of intact leaves of Vicia faba as influenced by light

The fluorochrome FITC-dextran was used to measure the effectof light on the apoplastic pH of intact Vicia faba leaves withthe ratio imaging technique. In darkadapted leaves the apoplasticpH 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 ({small tilde}0.3 pHunits), and thereafter an acidification of the leaf apoplast({small tilde}0.4 pH units) were observed. Both effects couldbe inhibited by DCMU. While variation of CO₂ concentration revealedno effect on light-induced apoplastic pH changes, a decreasein O₂ concentration decreased the effect. On the basis of ourdata it is suggested that the influence of photosynthesis onplasmalemma H⁺ ATPase is responsible for the observed effects,rather than altered CO₂ uptake. Key words: Leaf apoplast, apoplastic pH, light, ratio imaging, pH-sensitive fluorescent dye, Vicia fab