Quantum yields of photosynthesis at temperatures between −;2°C and 35°C in a cold‐tolerant C₃ plant (Pinus sylvestris) during the course of one year

Quantum yields of photosynthetic CO 2 uptake by Pinus sylvestris (L.) shoots were measured at temperatures between −; 2°C and 35°C from September 1984 to September 1985. The ratio of variable to peak fluorescence of photosystem II ( F v / F p ) was also measured. Quantum yield measured at 25°C varied with time from a low winter value of 0.017 to a high summer value of 0.057. This variation was strongly correlated to variation in F v F p ( r 2 = 0.91). The response of quantum yield to temperature changed with season. During winter, quantum yield was essentially constant between 0°C and 35°C. The constancy above 5°C was associated with a strong increase in intercellular space CO 2 (C i ) with temperature. In June, the quantum yield peaked at 5°C, decreased sharply below 5°C, and was rather constant between 25°C and 35°C. This insensitivity to increased temperature above 25°C was attributed to a large increase in C i In contrast, by September, the quantum yield was less sensitive to temperature below 5°C and more sensitive above 25°C, despite an unchanged C i response with increasing temperatures as compared with June. In August, quantum yields were lowered at 0, 5 and 15°C, apparently as a result of high carbohydrate levels in the leaves. Overall, the results suggest that there are sites other than in photosystem II or at ribulose bisphosphate carboxylase/oxygenase at which the quantum yield of photosynthetic CO 2 uptake is affected. Possible causes for the changes in efficiency are discussed.