Photoprotective function of photorespiration in several grapevine cultivars under drought stress

Four grapevine cultivars, i.e. Cabernet Sauvignon (a member of the Western Europe cultivar group), Rizamat (a member of the East cultivar group), Red Double Taste (a hybridized cultivar from Vitis vinifera L. and V. labrusca L.), and 1103Paulsen (a hybridized rootstock), were treated by three severity orders of drought stress for 25 d. Then net photosynthetic rate (P-N), maximal photochemical efficiency (F-v/F-m), actual photochemical efficiency (Phi(PS2)) of photosystern 2, total electron transport rate PT), and electron transport flows used in carboxylation (J(C)) and in oxygenation (J(O)) reactions catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase were determined. PN was determined again after re-watering for 2 d by gas exchange measurement. Along with the increase in severity of drought stress, P-N, F-v/F-m, Phi(PS2), J(T), and J(C) in all four cultivars decreased. The range of decrease differed among cultivars. J(O) expressed various trends from cultivar to cultivar. In Rizamat that received slight and moderate drought stress, P-N evidently decreased, but J(O) markedly increased, thus maintaining high values of J(T) and Phi(PS2). Prior to the moderate drought stress, the F-v/F-m was high in Rizamat, indicating that the photodamage had not happened ahead of the moderate drought stress given. Under the severe drought stress, the photorespiration rate in Rizamat decreased by 70 %, and J(T), Phi(PS2), and F-v/F-m also dropped to very low values, i.e. the photodamage of photosynthetic apparatus has taken place. This suggested that the photorespiration has consumed the excessive assimilatory power and the photo-protective function of photorespiration is very important for Rizamat. When Cabernet Sauvignon grew under drought stress, its J(O) decreased in a small range, thus maintaining higher values of J(C), J(T), Phi(PS2), and F-v/F-m; hence no serious photodamage occurred. Despite of the fact that P-N of cv. Red Double Taste decreased markedly under the slight drought stress, J(O) still increased under the severe drought stress. This suggests that photorespiration is important in photoprotection under drought stress. J(O) in cv. 1103Paulsen markedly decreased under slight stress. Accordingly, P-N, F-v/F-m, Phi(PS2), J(T), and J(C) decreased to extremely low values. Thus photorespiration effectively protects the photosynthetic apparatus from photo-damage under drought, assists in maintaining a relatively high Phi(PS2), and helps P-N to be rapidly recovered after re-watering.