Responses of plant processes to temperature may vary according to the time scale on which they are measured. In this study, both short-term and seasonal responses of photosynthesis to temperature were examined. A field study of seasonal changes in the temperature response of photosynthesis was conducted on two provenances, French and Moroccan, of mature maritime pine (Pinus pinaster Ait.). Measurements were made every 2 months over a 1-year period and used to parameterize a mechanistic model of photosynthesis. Temperature responses of maximum Rubisco activity, Vcmax, and potential electron transport rate, Jmax, were obtained for each measurement period, as was the response of stomatal conductance, gs, to water vapour pressure deficit (VPD). Absolute values of Vcmax and Jmax at 25°C were related to needle nitrogen content, Narea. Naarea, and thus Vcmax and Jmax, were negatively correlated with the mean minimum temperature in the month preceding measurements. The ratio of Jmax: Vcmax at 25°C varied between 1 and 1.7 but did not show any seasonal trend. Nor was there any seasonal trend in the relative temperature response of Vcmax, which had an activation energy Ha of approximately 57kJ mol-1 throughout the experiment. The activation energy of Jmax was also close to constant throughout the experiment, averaging 39 kJ mol-1. For the French provenance, the optimal temperature of Jmax was positively correlated with the maximum temperature of the previous day, but no such correlation was found for the Moroccan provenance. The response of gs to VPD also varied seasonally, with much stronger stomatal closure in winter months. Taken together, these results implied a translational shift downwards of the photosynthetic temperature response curve with increasing Tprev, and a shift in the temperature optimum of photosynthesis of 5-10°C between summer and winter. These results illustrate that the short-term temperature response of photosynthesis varies significantly on a seasonal basis.