Photoinhibition of photosynthesis and photorespiration in Vitis vinifera under field conditions: Effects of light climate and leaf position

Abstract

Field-grown grapevines (Vitis vinifera cv. Cabernet Sauvignon) were examined for photoinhibition under field conditions. Attached leaves at different positions along the shoot were investigated and their net-assimilation, photorespiration and electron transport were measured. The photochemical efficiency of photosystem II was evaluated on detached, dark-adapted leaves by determining differences in chlorophyll fluorescence, using a portable fluorometer. The comparison of unshaded and artificially shaded plants allowed an estimate of direct photoinhibitory effects on their photochemical capacity. Photoinhibition was also quantified for dark-adapted leaves following exposure to moderate and high light. The immature apical leaves had a lower assimilation rate than mature leaves. This was attributable in part to greater photoinhibition in immature leaves due to greater non-photochemical quenching of fluorescence (qi) and commensurate inhibition of the photochemical efficiency of system II of photosynthesis (Fv/Fm). This inhibition coincided with the high levels of solar radiation at noon but became less during the afternoon. Shade-adapted leaves were more sensitive to photoinhibition than sun-adapted leaves but light acclimatisation effects were independent of photochemical quenching, as distinct from nonphotochemical quenching. The degree of photoinhibition was inversely related to photochemical quenching under both low and high light. Practical implication of fluorescence measurements are discussed.