Effect of light and CO2 on inorganic carbon uptake in the invasive aquatic CAM-plant Crassula helmsii

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 upCO2 in the night in addition to the daytime. The effect of light andCO2 on the regulation ofCAMand 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 mmol photonm–2 s–1), C. helmsii displayed 2.8 times higher CAM at low compared with highCO2 (22 v. 230 mmolm–3).CAMwas absent in plants grown at low light (23 mmol photonm–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 mmol O2 g–1DWh–1 at CO2 concentrations of 3 and 22 mmolm–3 CO2, which, together with meanfinalpHvalues of 9.01 in thepHdrift, indicate a lowCO2 compensation point (<3 mmolm–3) but do not indicate use of bicarbonate as an additional source of exogenous inorganic carbon. The relatively high photosynthetic rates during the entire daytime were caused by internally derived CAM-CO2 and uptake from the external medium. During decarboxylation, CO2 generated from CAM contributed up to 29% to photosynthesis, whereas over a day the contribution to the carbon balance was 13%. The flexible adjustment of CAM and the ability to maintain photosynthesis at very low external CO2 concentrations, partly by making use of internally generated CO2 via CAM, may contribute to the broad ecological niche of C. helmsii.