PHOTOSYNTHETIC RESPONSES TO TEMPERATURE, LIGHT FLUX-DENSITY, CO2 CONCENTRATION AND VAPOR-PRESSURE DEFICIT IN EUCALYPTUS TETRODONTA CROWN UNDER CO2 ENRICHMENT

Seeds of Eucalyptus tetrodonta were sown under ambient or CO2 enriched (700 mu l 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 CO2 enrichment, on trees growing in the ground After 12 months exposure to CO2 enrichment, the rate of light-saturated assimilation (A(max)) of plants growing in the ground was determined. Responses of CO2 assimilation to variations in leaf temperature, leaf-to-air vapour,pressure deficit (LAVPD), Eight flux density and CO2 concentration were also measured in the laboratory using plants growing in large pots. There was no significant difference in A(max) between pot and ground located plants. Assimilation of E. tetrodonta was relatively insensitive to changes in LAVPD for both ambient and CO2 enriched plants but the temperature optimum of assimilation was increased in plants grown and measured under CO2 enrichment. Plants grown with CO2 enrichment had an increased rate of light-saturated assimilation and apparent quantum yield I-vas significantly inn eased by CO2 enrichment. rn contrast, carboxylation efficiency was decreased significantly by CO2 enrichment. After 18 months growth with CO2 enrichment, there was no sign of a decline in assimilation I ate compared to measurements undertaken after 12 months, At low LAVPD values, assimilation rate was not influenced by CO2 treatment but at moderate to high LAVPD, plants grown under CO2 enrichment exhibited a larger assimilation rate than control plants. Specific leaf area and chlorophyll contents decreased in response to CO2 enrichment, whilst foliar soluble protein contents and chlorophyll a/b ratios were unaffected by CO2 treatment. Changes in soluble protein and chlorophyll contents in response to CO2 enrichment did not account for changes in assimilation between treatments. After 30 months exposure to CO2 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 CO2 enrichment.