PHOTOSYNTHETIC TEMPERATURE RESPONSES OF EUCALYPTUS-GLOBULUS AND EUCALYPTUS NITENS

Steady-state photosynthetic responses to leaf temperature of 4-year-ol d Eucalyptus globulus Labill and E. nitens (Deane and Maiden) Maiden t rees were measured between 10 and 35 degrees C at approximately monthl y intervals from early spring to midwinter. The photosynthetic tempera ture optimum of recently expanded leaves in the sun canopy was linearl y related to the average temperature of the preceding week during the 9-month measurement period. The optimum temperature for net photosynth esis of E. globulus increased from 17 to 23 degrees C as the mean dail y temperature increased from 7 to 16 degrees C. Similarly, the optimum temperature for net photosynthesis off. nitens increased from 14 to 2 0 degrees C as the mean daily temperature increased from 7 to 19 degre es C. The temperature for maximum photosynthetic response of E. globul us and E. nitens was similar at each measurement time, but the photosy nthetic performance of E. nitens was less sensitive to temperatures ab ove and below this optimum than that of E. globulus. In December, the apical shoots of branches of E. globulus had a net photosynthetic temp erature optimum of between 10 and 15 degrees C. The corresponding valu es for expanding leaves, fully expanded leaves from the current year's growth, and fully expanded leaves from the previous year's growth wer e 15, 20 and 20-25 degrees C, respectively. In a second experiment, E. globulus clones taken from four mother plants originating from climat ically dissimilar locations within Tasmania were acclimated at day/nig ht temperatures of 10/15, 18/23 and 25/30 degrees C in temperature-con trolled greenhouses. Another set of clones was acclimated in a shade-h ouse where temperatures ranged between 10 and 25 degrees C and with a mean daily temperature of approximately 15 degrees C. Plants grown at 25/30 degrees C had significantly lower net photosynthetic rates when measured at 10 and 20 ''C than plants grown at lower temperatures. Pla nts grown at 10/15 ''C had significantly lower net photosynthetic rate s when measured at 30 OC than plants grown at higher temperatures. Pla nts grown at the ambient conditions prevailing in midautumn in Hobart had significafitly higher net photosynthetic rates at 20 OC than plant s raised in the greenhouses and were equal best performers at 10 and 3 0 degrees C. A comparison of the light response curves of the plants s howed that the maximum rate of net photosynthesis was affected by the growth temperature, whereas the apparent quantum efficiency remained u nchanged. 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 resp onse models for E. globulus indicated that incomplete acclimation (def ined by a slope value of less than 1 for the linear relationship betwe en the temperature optimum for photosynthesis and the growth temperatu re) generally resulted in a greater daily carbon uptake than complete acclimation (slope value of 1).