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

Abstract

Ten contrasting Acacia species were grown in glasshouses with normal ambient CO₂ or ele-vated to 700 µL L^–1. Plants were grown in sand with a complete nutrient solution, including 5 mМ 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 mL 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 µL L^–1 CO₂ compared to plants grown and measured at 350 µL 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 assimi-lation 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 com-pared to 350 µL 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.