Responses of CAM species to increasing atmospheric CO₂ concentrations

Abstract

ABSTRACT Crassulacean acid metabolism (CAM) species show an average increase in biomass productivity of 35% in response to a doubled atmospheric CO 2 concentration. Daily net CO 2 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 CO 2 uptake in CAM species to increasing atmospheric CO 2 concentrations are similar to those for C 3 species and much greater than those for C 4 species. Increases in net daily CO 2 uptake by CAM plants under elevated atmospheric CO 2 concentrations reflect increases in both Rubisco‐mediated daytime CO 2 uptake and phosphoenolpyruvate carboxylase (PEPCase)‐mediated night‐time CO 2 uptake, the latter resulting in increased nocturnal malate accumulation. Chlorophyll contents and the activities of Rubisco and PEPCase decrease under elevated atmospheric CO 2 , but the activated percentage for Rubisco increases and the K M (HCO 3 − ) 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 CO 2 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 CO 2 concentrations. With increasing temperature and drought duration, the percentage enhancement of daily net CO 2 uptake caused by elevated atmospheric CO 2 concentrations increases. Thus net CO 2 uptake, productivity, and the potential area for cultivation of CAM species will be enhanced by the increasing atmospheric CO 2 concentrations and the increasing temperatures associated with global climate change.