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Crassula helmsii (T. Kirk) Cockayne is an invasive aquatic plant in Europe that can suppress many native species because it can grow at a large range of dissolved inorganic carbon concentrations and light levels. One reason for its ecological success may be the possession of a regulated Crassulacean Acid Metabolism (CAM), which allows aquatic macrophytes to take up CO2 in the night in addition to the daytime. The effect of light and CO2 on the regulation of CAM and photosynthesis in C. helmsii was investigated to characterise how physiological acclimation may confer this ecological flexibility. After 3 weeks of growth at high light (230 µmol photon m–2 s–1), C. helmsii displayed 2.8 times higher CAM at low compared with high CO2 (22 v. 230 mmol m–3). CAM was absent in plants grown at low light (23 µmol photon m–2 s–1) at both CO2 concentrations. The observed regulation patterns are consistent with CAM acting as a carbon conserving mechanism. For C. helmsii grown at high light and low CO2, mean photosynthetic rates were relatively high at low concentrations of CO2 and were on average 80 and 102 µmol O2 g–1 DW h–1 at CO2 concentrations of 3 and 22 mmol m–3 CO2, which, together with mean final pH values of 9.01 in the pH drift, indicate a low CO2 compensation point ( |
