Responses of CAM species to increasing atmospheric CO2 concentrations
Crassulacean acid metabolism (CAM) species show an average increase in biomass productivity of 35% in response to a doubled atmospheric CO2 concentration. Daily net CO2 uptake is similarly enhanced, reflecting in part an increase in chlorenchyma thickness and accompanied by an even greater increase in water-use efficiency. The responses of net CO2 uptake in CAM species to increasing atmospheric CO2 concentrations are similar to those for C3 species and much greater than those for C4 species. Increases in net daily CO2 uptake by CAM plants under elevated atmospheric CO2 concentrations reflect increases in both Rubisco-mediated daytime CO2 uptake and phosphoenolpyruvate carboxylase (PEPCase)-mediated night-time CO2 uptake, the latter resulting in increased nocturnal malate accumulation. Chlorophyll contents and the activities of Rubisco and PEPCase decrease under elevated atmospheric CO2, but the activated percentage for Rubisco increases and the KM(HCO3−) for PEPCase decreases, resulting in more efficient photosynthesis. Increases in root:shoot ratios and the formation of additional photosynthetic organs, together with increases in sucrose-Pi synthase and starch synthase activity in these organs under elevated atmospheric CO2 concentrations, decrease the potential feedback inhibition of photosynthesis. Longer-term studies for several CAM species show no downward acclimatization of photosynthesis in response to elevated atmospheric CO2 concentrations. With increasing temperature and drought duration, the percentage enhancement of daily net CO2 uptake caused by elevated atmospheric CO2 concentrations increases. Thus net CO2 uptake, productivity, and the potential area for cultivation of CAM species will be enhanced by the increasing atmospheric CO2 concentrations and the increasing temperatures associated with global climate change.