THREE different photosynthetic options have been identified in plants1,2: (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.
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