Watering converts a CAM plant to daytime CO₂ uptake

Abstract

THREE different photosynthetic options have been identified in plants^1,2: (1) most plants have the reductive pentose phosphate or C₃ pathway, where CO₂ is incorporated into ribulose-1,5-diphosphate (RuDP) to yield two molecules of 3-phosphoglyceric acid, a three-carbon compound; (2) the C₄ mode, where the first photosynthetic products are four-carbon dicarboxylic acids like oxaloacetate and malate formed following CO₂ incorporation into phosphoenolpyruvate (PEP); and (3) crassulacean acid metabolism (CAM), found in many succulent plants growing in arid regions. In the last, stomatal opening and net CO₂ uptake occur at night, CO₂ being incorporated by way of PEP carboxylase into organic acids. The tissue acidity decreases as the organic acids are decarboxylated during the day, when the internally released CO₂ is prevented from leaving by the closed stomata. The water vapour concentration difference between the tissue and ambient air is less at night, and thus the night-time stomatal opening of CAM plants leads to overall water conservation. For example, the water lost per CO₂ fixed averages about sixfold higher for C₄ plants and tenfold higher for C₃ ones than for CAM plants in natural conditions². The net daily CO₂ uptake by CAM plants is less than for C₃ or C₄ plants, so CAM plants tend to be relatively slow growing.