Photosynthetic temperature responses of Eucalyptus globulus and Eucalyptus nitens

Steady-state photosynthetic responses to leaf temperature of 4-year-old Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden trees were measured between 10 and 35°C at approximately monthly intervals from early spring to midwinter. The photosynthetic temperature optimum of recently expanded leaves in the sun canopy was linearly related to the average temperature of the preceding week during the 9-month measurement period. The optimum temperature for net photosynthesis of E. globulus increased from 17 to 23°C as the mean daily temperature increased from 7 to 16°C. Similarly, the optimum temperature for net photosynthesis of E. nitens increased from 14 to 20°C as the mean daily temperature increased from 7 to 19°C. The temperature for maximum photosynthetic response of E. globulus and E. nitens was similar at each measurement time, but the photosynthetic performance of E. nitens was less sensitive to temperatures above and below this optimum than that of E. globulus. In December, the apical shoots of branches of E. globulus had a net photosynthetic temperature optimum of between 10 and 15°C. The corresponding values for expanding leaves, fully expanded leaves from the current year's growth, and fully expanded leaves from the previous year's growth were 15, 20 and 20-25°C, respectively. In a second experiment, E. globulus clones taken from four mother plants originating from climatically dissimilar locations within Tasmania were acclimated at day/night temperatures of 10/15, 18/23 and 25/30°C in temperature-controlled greenhouses. Another set of clones was acclimated in a shade-house where temperatures ranged between 10 and 25°C and with a mean daily temperature of approximately 15°C. Plants grown at 25/30°C had significantly lower net photosynthetic rates when measured at 10 and 20°C than plants grown at lower temperatures. Plants grown at 10/15°C had significantly lower net photosynthetic rates when measured at 30°C than plants grown at higher temperatures. Plants grown at the ambient conditions prevailing in midautumn in Hobart had significantly higher net photosynthetic rates at 20°C than plants raised in the greenhouses and were equal best performers at 10 and 30°C. A comparison of the light response curves of the plants showed that the maximum rate of net photosynthesis was affected by the growth temperature, whereas the apparent quantum efficiency remained unchanged. There were no significant differences in the photosynthetic temperature responses of the four genotypes derived from climatically dissimilar locations within Tasmania. A comparison of temperature response models for E. globulus indicated that incomplete acclimation (defined by a slope value of less than 1 for the linear relationship between the temperature optimum for photosynthesis and the growth temperature) generally resulted in a greater daily carbon uptake than complete acclimation (slope value of 1).