Photosynthetic responses to temperature, light flux-density, CO2 concentration and vapour pressure deficit in Eucalyptus tetrodonta grown under CO2 enrichment

Seeds of Eucalyptus tetrodonta were sown under ambient or CO(2) enriched (700 microl litre(-1)) conditions in tropical Australia. Four sets of measurements were made, the first two after 12 months, on trees growing either in pots or planted in the ground. The third and fourth set were made after 18 and 30 months exposure to CO(2) enrichment, on trees growing in the ground. After 12 months exposure to CO(2) enrichment, the rate of light-saturated assimilation (Amax) of plants growing in the ground was determined. Responses of CO(2) assimilation to variations in leaf temperature, leaf-to-air vapour pressure deficit (LAVPD), light flux density and CO(2) concentration were also measured in the laboratory using plants growing in large pots. There was no significant difference in Amax between pot and ground located plants. Assimilation of E. tetrodonta was relatively insensitive to changes in LAVPD for both ambient and CO(2) enriched plants but the temperature optimum of assimilation was increased in plants grown and measured under CO(2) enrichment. Plants grown with CO(2) enrichment had an increased rate of light-saturated assimilation and apparent quantum yield was significantly increased by CO(2) enrichment. In contrast, carboxylation efficiency was decreased significantly by CO(2) enrichment. After 18 months growth with CO(2) enrichment, there was no sign of a decline in assimilation rate compared to measurements undertaken after 12 months. At low LAVPD values, assimilation rate was not influenced by CO(2) treatment but at moderate to high LAVPD, plants grown under CO(2) enrichment exhibited a larger assimilation rate than control plants. Specific leaf area and chlorophyll contents decreased in response to CO(2) enrichment, whilst foliar soluble protein contents and chlorophyll a/b ratios were unaffected by CO(2) treatment. Changes in soluble protein and chlorophyll contents in response to CO(2) enrichment did not account for changes in assimilation between treatments. After 30 months exposure to CO(2) enrichment, the rate of light-saturated assimilation was approximately 50% larger than controls and this enhancement was larger than that observed after 18 months exposure to CO(2) enrichment.