Temperature response of parameters of a biochemically based model of photosynthesis.: I.: Seasonal changes in mature maritime pine (Pinus pinaster Ait.)

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, V-cmax, and potential electron transport rate, J(max), were obtained for each measurement period, as was the response of stomatal conductance, g(s), to water vapour pressure deficit (VPD). Absolute values of V-cmax and J(max) at 25 degreesC were related to needle nitrogen content, N (area.) N-area, and thus V-cmax and J(max), were negatively correlated with the mean minimum temperature in the month preceding measurements. The ratio of J(max):V-cmax at 25 degreesC 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 V-cmax, which had an activation energy H-a of approximately 57 kJ mol(-1) throughout the experiment. The activation energy of J(max) was also close to constant throughout the experiment, averaging 39 kJ mol(-1). For the French provenance, the optimal temperature of J(max) was positively correlated with the maximum temperature of the previous day, but no such correlation was found for the Moroccan provenance. The response of g(s) 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 T-prev, and a shift in the temperature optimum of photosynthesis of 5-10 degreesC between summer and winter. These results illustrate that the short-term temperature response of photosynthesis varies significantly on a seasonal basis.