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 m easurements were made, the first two after 12 months, on trees growing either in pots or planted in the ground. The third and fourth set wer e made after 18 and 30 months exposure to CO2 enrichment, on trees gro wing in the ground After 12 months exposure to CO2 enrichment, the rat e of light-saturated assimilation (A(max)) of plants growing in the gr ound was determined. Responses of CO2 assimilation to variations in le af temperature, leaf-to-air vapour,pressure deficit (LAVPD), Eight flu x density and CO2 concentration were also measured in the laboratory u sing plants growing in large pots. There was no significant difference in A(max) between pot and ground located plants. Assimilation of E. t etrodonta was relatively insensitive to changes in LAVPD for both ambi ent and CO2 enriched plants but the temperature optimum of assimilatio n was increased in plants grown and measured under CO2 enrichment. Pla nts 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 decrease d significantly by CO2 enrichment. After 18 months growth with CO2 enr ichment, there was no sign of a decline in assimilation I ate compared to measurements undertaken after 12 months, At low LAVPD values, assi milation rate was not influenced by CO2 treatment but at moderate to h igh LAVPD, plants grown under CO2 enrichment exhibited a larger assimi lation rate than control plants. Specific leaf area and chlorophyll co ntents decreased in response to CO2 enrichment, whilst foliar soluble protein contents and chlorophyll a/b ratios were unaffected by CO2 tre atment. Changes in soluble protein and chlorophyll contents in respons e to CO2 enrichment did not account for changes in assimilation betwee n treatments. After 30 months exposure to CO2 enrichment, the rate of light-saturated assimilation was approximately 50% larger than control s and this enhancement was larger than that observed after 18 months e xposure to CO2 enrichment.