Photosynthetic acclimation to temperature of four Eucalyptus species and Sequoia sempervirens

Abstract

The 3-PG physiological/mensurational hybrid model is a useful forest management tool capable of producing accurate growth results across a number of parameterised species. The temperature data used in the model are the average maximum and minimum values for photosynthesis above the compensation point (Landsberg and Sands 2011). There is a minimum temperature below which positive net CO₂ exchange will not occur, a maximum temperature above which it will not occur and an optimum temperature at which it is maximised. These parameters are used in the 3-PG physiological model of forest production. However, a species’ photosynthetic response to short-term variation may differ from one season to another as species acclimate to temperatures over periods of a few weeks. In this study, acclimation responses of four species of eucalypt and Sequoia sempervirens to long-term temperatures were studied over a wide range of short-term temperature changes in order to identify the minimum, optimum and maximum temperatures of CO₂ assimilation for physiological/mensurational hybrid modelling, and also to identify the sites for which the species would be best suited. In order to achieve the aims of this study, a growth chamber experiment was established. Seedlings of four eucalypt species and Sequoia sempervirens were grown at base-line day/night temperatures of 30/16, 22/12 and 10/5ºC in controlled environment chambers for three months and leaf gas exchange measurements were made of the species at seven short-term temperature levels (5, 10, 15, 20, 25, 30 and 35ºC). The optimum and the maximum temperatures for net photosynthesis increased with an increase in base-line temperature for all species. The highest optimum temperature and net photosynthetic rates recorded were in plants grown at 30/16ºC and the lowest were in those grown at 10/5ºC. The maximum rate of net CO₂ assimilation increased with the temperature at which plants were grown partly because of acclimation in key photosynthetic processes in the Calvin cycle. Responses of maximal carboxylation rate (Vcmax) and also the maximal light-driven electron flux (Jmax) to short-term temperature change varied with base-line temperature for all species studied. Net photosynthesis and photosynthetic parameters measured did not vary significantly with effects of nitrogen, phosphorus and their interaction (p = 0.1468). The ratio of Jmax to Vcmax decreased with increasing leaf temperatures for all species (p < 0.001). These results indicate that the species studied will adapt to long-run changes in temperature, and the parameters obtained from these studies can be used for models that simulate the physiology and growth of the species.