Photosynthetic characteristics of 10 Acacia species grown under ambient and elevated atmospheric CO2

Ten contrasting Acacia species were grown in glasshouses with normal ambient CO2 or elevated to 700 mu L L-1. Plants were grown in sand with a complete nutrient solution, including 5 mM nitrate. Our objective was to determine the degree to which photosynthesis, and the efficiency of nitrogen and water use, were affected by growth under elevated CO2 in contrasting plant species that differ in specific foliage area (foliage area per unit foliage dry mass). Photosynthetic characteristics were measured at several stages. Growth and measurement of gas exchange under 700 mu L L-1 CO2 resulted in enhanced rates of CO2 assimilation per unit foliage area in nine of the species. The degree of enhancement was independent of specific foliage area. The exception was the slow-growing A. aneura, which had lower rates of CO2 assimilation when grown and measured at 700 mu L L-1 CO2 compared to plants grown and measured at 350 mu L L-1 CO2, at 50, 78 and 93 d after transplanting. Leaf conductance was reduced by growth in elevated CO2 in only six of the species. Overall, elevated CO2 improved the ratio of CO2 assimilation to conductance by 78% and increased CO2 assimilation per unit of foliage nitrogen by 30% at a given specific foliage area. Detailed study of A. saligna and A. aneura revealed that the effects of the CO2 treatment were similarly evident on all fully expanded phyllodes, regardless of their age. Intercellular CO2 response curves were analysed on four species and revealed no change in the ratio of electron transport to Rubisco activities. However, for A. aneura and A. melanoxylon, both electron transport and Rubisco activities were reduced per unit foliage nitrogen, by growth under elevated CO2. For A. saligna and A. implexa, these activities per unit nitrogen, were not altered by the elevated CO2 treatment. To relate CO2 assimilation rates to net assimilation rates (dry matter increment per unit foliage area per day) derived from growth analysis, between 30 and 50% of daily photosynthesis appeared to be consumed in respiration. This proportion was not altered by CO2 treatment for seven of the Acacia species, but appeared to be reduced in the other three. The increase in CO2 assimilation rate by growth under 700 compared to 350 mu L L-1 CO2 that was measured (26%, mean of all species from two surveys), matched the increase in net assimilation rate that had been derived from destructive sampling (30%). We conclude that the increase in CO2 assimilation rate in the selected Acacia species was independent of species, growth rate and foliage area per unit foliage dry mass.