In the present study the effect of elevated CO2 on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi-arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south-eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 mumol mol(-1)) or elevated (700 mumol mol(-1)) CO2 concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO2. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole-plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO2, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO2 increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.

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.

The interactive effects of nitrate supply and atmospheric CO2 concentration on growth, N2 fixation, dry matter and nitrogen partitioning in the leguminous tree Acacia melanoxylon R.Br. were studied. Seedlings were grown hydroponically in controlled-environment cabinets for 5 weeks at seven 15N-labelled nitrate levels, ranging from 3 to 6400 mmol m–3. Plants were exposed to ambient (~350 µmol mol–1) or elevated (~700 µmol mol–1) atmospheric CO2 for 6 weeks. Total plant dry mass increased strongly with nitrate supply. The proportion of nitrogen derived from air decreased with increasing nitrate supply. Plants grown under either ambient or elevated CO2 fixed the same amount of nitrogen per unit nodule dry mass (16.6 mmol N per g nodule dry mass) regardless of the nitrogen treatment. CO2 concentration had no effect on the relative contribution of N2 fixation to the nitrogen yield of plants. Plants grown with ≥50 mmol m–3 N and elevated CO2 had approximately twice the dry mass of those grown with ambient CO2 after 42 days. The rates of net CO2 assimilation under growth conditions were higher per unit leaf area for plants grown under elevated CO2. Elevated CO2 also decreased specific foliage area, due to an increase in foliage thickness and density. Dry matter partitioning between plant organs was affected by ontogeny and nitrogen status of the plants, but not by CO2 concentration. In contrast, plants grown under elevated CO2 partitioned more of their nitrogen to roots. This could be attributed to reduced nitrogen concentrations in foliage grown under elevated CO2.

In this study we assessed the inherent relative growth rate (RGR) under controlled environment conditions of 10 contrasting Acacia species from semi‐arid and mesic environments. For several of the species, compound pinnate leaves produced early in the seedling stage, were gradually replaced by phyllodes (expanded petioles that form simple lamina). Other species either did not form phyllodes, or only did so to a minor degree by the end of the study. Phyllode production was dominant in the four slow‐growing Acacia species from semi‐arid environments ( A. aneura, A. colei, A. coriacea and A. tetragonophylla ), with leaf production being exclusive or dominant in five ( A. dealbata, A. implexa, A. mearnsii, A. melanoxylon and A. irrorata ) of the six faster‐growing species from mesic environments. The exception was A. saligna which was fast growing but did produce phyllodes. From a carbon economy perspective, slow growth in the semi‐arid species was not associated with lower net assimilation rates or less plant mass allocated to foliage. Rather, the primary factor associated with their slow growth was a smaller foliage area per unit foliage mass. This was true for comparisons based on the mean over all harvests or at set plant masses. The production of phyllodes by the semi‐arid species substantially reduced foliage area per unit foliage mass, as this was lower for phyllodes than leaves in all species. To assess the impact that phyllode production had on ontogenetic changes in RGR, we modelled the situation where only leaves were formed. This analysis showed that changing from leaves to phyllodes substantially reduced the RGR. There was little difference in plant nitrogen concentration or the ratio of foliage nitrogen to plant nitrogen between the species. This resulted in foliage nitrogen productivity (dry mass gain per unit foliage nitrogen and time) being directly proportional to foliage area per unit foliage mass between species. We concluded that a smaller foliage area per unit foliage mass and phyllode production are the primary factors associated with lower RGR in contrasting Acacia species.