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.