Responses of CAM species to increasing atmospheric CO2 concentrations

Crassulacean acid metabolism (CAM) species show an average increase in biom ass productivity of 35% in response to a doubled atmospheric CO2 concentrat ion. Daily net CO2 uptake is similarly enhanced, reflecting in part an incr ease 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 C-3 spec ies and much greater than those for C-4 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 phosphoenolpyruv ate carboxylase (PEPCase)-mcdiated night-time CO2 uptake, the latter result ing in increased nocturnal malate accumulation. Chlorophyll contents and th e activities of Rubisco and PEPCase decrease under elevated atmospheric CO2 , but the activated percentage for Rubisco increases and the K-M(HCO3-) for PEPCase decreases, resulting in more efficient photosynthesis. Increases i n 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 th e potential feedback inhibition of photosynthesis. Longer-term studies for several CAM species show no downward acclimatization of photosynthesis in r esponse to elevated atmospheric CO2 concentrations, With increasing tempera ture and drought duration, the percentage enhancement of daily net CO2 upta ke caused by elevated atmospheric CO2 concentrations increases, Thus net CO 2 uptake, productivity, and the potential area for cultivation of CAM speci es will be enhanced by the increasing atmospheric CO2 concentrations and th e increasing temperatures associated with global climate change.