The Effect of Temperature on the Photosynthesis of Ryegrass Canopies

Abstract

In bright light, the rate of gross photosynthesis of ryegrass canopies increased with increasing temperature between 10 and 25 °C, in keeping with an increase in the photosynthesis of their constituent leaves. The increase in canopy gross photosynthesis occurred irrespective of the leaf area index, which would affect the proportion of the leaves in bright light. Because the response of gross photosynthesis to temperature was similar to that of respiration, net photosynthesis also increased with temperature, irrespective of the relative magnitude of the two processes. However, these increases in photosynthesis were observed while the saturation deficit of water vapour was kept small (less than 3 g m⁻³). The photosynthesis of both leaves and canopies was reduced when measured at a greater saturation deficit; indicating that the increasing saturation deficit, which frequently accompanies increasing temperature in the field, may obscure the effect of temperature on photosynthesis. Lack of control of saturation deficit as measurement temperature is increased may explain why some workers have failed to detect a positive effect of temperature on photosynthesis.