Responses of Subterranean Clover Communities to Temperature. III. Effects of Temperature on Canopy Photosynthesis

Subterranean clover communities were grown at 12, 16, 20 and 24°C in naturally lit temperature- controlled glasshouses. Rates of net CO2 exchange of communities varying in leaf area index (LAI) from 1 to 8 were determined at several levels of irradiance at the temperature at which the plants were grown and at other measurement temperatures. When measured at each of the four growth temperatures and at an irradiance of 250 W m-2, the net CO2 exchange rate increased with increase in LAI up to 3. Higher values of LAI affected the rate only slightly. At each growth temperature and an irradiance of only 50 W m-3, net CO2 exchange decreased with increase in LA1 over the range 2-8. Net CO2 exchange rate followed a hyperbolic curve with increase in irradiance at all LAI values, the asymptote being largely independent of temperature and of LAI within the range 3-8. The net CO2 exchange rates of a community, determined at different measurement temperatures, were also directly related to measurement temperature at high irradiance but, owing to increased respiration rate at high temperature, inversely related to temperature at low irradiance. Differences in net CO2 exchange rates of a community at high irradiance due to different measurement temperatures were greater than the differences due to different growth temperatures.