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 ° C 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 ° 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 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 ° C between summer and winter. These results illustrate that the short-term temperature response of photosynthesis varies significantly on a seasonal basis. Key-words : acclimation; maritime pine; model parameters; photosynthesis; ribulose-1,5bis phosphate carboxylaseoxygenase; ribulose-1,5bis phosphate regeneration; stomatal conductance; temperature.