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

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 (q1) 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 non-photochemical quenching. The degree of photoinhibition was inversely related to photochemical quenching under both low and high light. Practical implication of fluorescence measurements are discussed