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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 (<3 mmol m–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.

1. The ecophysiological significance of Crassulacean acid metabolism (CAM) in the invasive aquatic macrophyte Crassula helmsii was studied in an English soft-water lake. The extent and the contribution of CAM to the carbon budget was examined in spring (April) and summer (July) along a depth gradient (0.5-2.2 m), covering the growth range of C. helmsii in the lake. 2. Significant in situ CAM activity (30-80 meq kg )1 FW) was present in all specimens, although it decreased with depth and hence correlated with the decline in photon irradiance. Potential CAM activity (60-161 meq kg )1 FW), measured after exposure to low concentrations of CO 2 in the day and high concentrations at night, were on average 2.7-times greater than in situ CAM activity. Overall CAM activity increased from April to July, which is consistent with higher potential carbon limitation caused by increased temperature and light availability. 3. CAM activity in C. helmsii appeared to be carbon-limited at night because night-time carbon-fixation increased at raised, compared to ambient, concentrations of CO 2 . 4. The high in situ CAM activity in C. helmsii was reflected in the contribution of CAM to the total carbon budget which, independent of depth and season, ranged from 18% to 42%. The amount of CO 2 taken up in the night via CAM was 0.74 to 2.94 times the amount of CO 2 lost in respiration, thus emphasizing the importance of CAM in refixation of potentially lost respiratory CO 2 . 5. The onset of decarboxylation in the morning appeared to be under circadian control as there was a delay of up to 5.5 h between the start of the light period and a decline in cell acidity level. 6. There was little variation in d 13 C content ()21.69 to 23.49&) with season or depth suggesting, along with the estimated contribution to the carbon-budget, that CAM is important for the whole population of C. helmsii. CAM may confer a competitive advantage in relation to growth, which may be one of the reasons for the invasiveness of this species.

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