A model analysis of the photosynthetic response of Vitis vinifera L. cvs Riesling and Chasselas leaves in the field: I. Interaction of age, light andtemperature

The photosynthetic activity (A) of leaves of different ages on primary and secondary shoots of Riesling and Chasselas vines was measured under field c onditions in relation to photon flux density (PFD) at various leaf temperat ures. The data sets from 4 years and two locations (Geisenheim, Germany; Ch angins, Switzerland) were analysed using non-linear regression models to de termine possible genetic and/or climate-induced differences in the light an d temperature response between different leaf ages, A non-rectangular hyper bola with physiologically meaningful parameters was found to adequately des cribe the response to photon flux density. For both varieties, maximum phot osynthetic rates were observed on leaves of primary shoots, opposite to the clusters, at a leaf temperature of 27-32 degrees C and at light saturation . Young leaves showed a less pronounced temperature optimum. The light resp onse curves of photosynthesis of the two cultivars were similar over a temp erature range of 20-30 degrees C. Below this temperature, Riesling showed h igher values of A than Chasselas in most Eases, whereas it was the reverse when leaf temperature exceeded 30 degrees C, This was particularly evident for leaves on secondary shoots and was related to differences in the photor espiration rate. Mature Riesling leaves had higher apparent quantum yields (alpha) and lower light saturation indices (I-s) than Chasselas at leaf tem peratures below 30-35 degrees C. Dark respiration (R-D) and the light compe nsation point (I-c) responded strongly to temperature with differences betw een leaf ages but no consistent difference between varieties. Leaves on sec ondary shoots of both cultivars had the highest photosynthetic activity dur ing the ripening period of the fruit.