A model analysis of the photosynthetic response of Vitis vinifera L. cvs Riesling and Chasselas leaves in the field: I. Interaction of age, light and temperature
The photosynthetic activity (A) of leaves of different ages on primary and secondary shoots of Riesling and Chasselas vines was measured under field conditions in relation to photon flux density (PFD) at various leaf temperatures. The data sets from 4 years and two locations (Geisenheim, Germany; Changins, Switzerland) were analysed using non-linear regression models to determine possible genetic and/or climate-induced differences in the light and temperature response between different leaf ages. A non-rectangular hyperbola with physiologically meaningful parameters was found to adequately describe the response to photon flux density. For both varieties, maximum photosynthetic rates were observed on leaves of primary shoots, opposite to the clusters, at a leaf temperature of 27-32 °C and at light saturation. Young leaves showed a less pronounced temperature optimum. The light response curves of photosynthesis of the two cultivars were similar over a temperature range of 20-30 °C. Below this temperature, Riesling showed higher values of A than Chasselas in most cases, whereas it was the reverse when leaf temperature exceeded 30 °C. This was particularly evident for leaves on secondary shoots and was related to differences in the photorespiration rate. Mature Riesling leaves had higher apparent quantum yields (a) and lower light saturation indices (Is) than Chasselas at leaf temperatures below 30-35 °C. Dark respiration (RD) and the light compensation point (Ic) responded strongly to temperature with differences between leaf ages but no consistent difference between varieties. Leaves on secondary shoots of both cultivars had the highest photo-synthetic activity during the ripening period of the fruit