Carbon assimilation patterns and growth of the introduced CAM plant Opuntia inermis in Eastern Australia.

The daily course of CO2 and H2O exchange in cladodes of Opuntia inermis was studied at four sites in Eastern Australia. On most occasions cladode water contents were high and nocturnal stomatal opening resulted in substantial uptake of CO2 and synthesis of about 130 μ equiv cm-2 of malic acid during the night. Under water stress nocturnal stomatal opening was confined to the latter part of the night and acid synthesis was reduced to about 40 μ equiv cm-2. Night temperature had little effect on acid synthesis, which responded primarily to rainfall and changed from the stressed condition within 2-3 days in irrigation experiments. On many occasions following summer rainfall stomata opened for 4 h in the late afternoon permitting net CO2 fixation which may contribute about 25% of the total carbon assimilated. This CO2 fixation was insufficient to have a marked impact on the δ13C value of the Opuntia cladodes. CO2 fixation in the light occurred in conjunction with maximum dark CO2 fixation under mesic conditions. Dark CO2 fixation rates were 3 to 5 times greater than those recorded in desert cacti under favorable conditions. Relative growth rates calculated on the basic of CO2 exchange correspond to measured relative growth rates of 0.05 g g-1 dry wt day-1 which prevailed for 60-90 days in summer. The capacity for very active CO2 fixation in the dark and light following summer rainfall and the capacity to persist at low levels of metabolic activity through summer drought are discussed in relation to the success of this introduced species in this habitat.