Riesling and Müller-Thurgau vines cultivated in a glasshouse at maximal 400 µmol quanta·m-2s-1 were exposed to water logging and subsequently to decreasing soil moisture contents (SMC) until severe drought stress occurred. In the course of the daily applied light treatments, consisting of stepwise increases of light intensity from 0 to 3200 µmol·m-2s-1 with intermittent stabilisation phases, the Fv/Fm of dark-adapted and the Fv/F'm ratios of light-adapted leaves were determined by chlorophyll fluorescence measure ments. At moderate SMC (21 %) quantum yield of PSII (Y) of Muller-Thurgau leaves decreased when photosynthetic active radiation (PAR) increased beyond 400 µmol·m-2s-1. Water stress (water logging and drought) led to a more precipitous decrease, indicating an increased sensitivity of Y to the combined stresses. The deviation of the electron transport rates from (theoretical) maximum quantum yield ("excessive PAR") increased with increasing PAR and was higher in water-stressed than in moderately irrigated vines. Nonphotochemical quenching started to increase at relatively low PAR (400 µmol·m-2s-1) and reached saturation at 1600 µmol·m-2s-1; values were higher in water-stressed vines than in moderately irrigated vines. Riesling and Müller-Thurgau vines showed photoinhibition of photosynthesis at high PAR; water logging and drought intensified this tendency. The extent of photoinhibition was higher in Müller-Thurgau than in Riesling leaves. Even under most severe stress conditions photoinhibition was transient, suggesting complete overnight repair of PSII in both varieties.

VITIS - Journal of Grapevine Research, Vol. 37 No. 1 (1998): Vitis