Annals of Botany

To help understand carbon balance between shoots and developing roots, 41 bare-root crassulacean acid metabolism (CAM) plants native to the Sonoran Desert were studied in a glass-panelled sealable room at day/night air temperatures of 25/15 °C. Net CO₂ uptake by the community of Agave schottii, Carnegia gigantea, Cylindropuntia versicolor, Ferocactus wislizenii and Opuntia engelmannii occurred 3 weeks after watering. At 4 weeks, the net CO₂ uptake rate measured for south-east-facing younger parts of the shoots averaged 1·94 µmol m^-2 s^-1 at night, considerably higher than the community-level nocturnal net CO₂ uptake averaged over the total shoot surface, primarily reflecting the influences of surface orientation on radiation interception (predicted net CO₂ uptake is twice as high for south-east-facing surfaces compared with all compass directions). Estimated growth plus maintenance respiration of the roots averaged 0·10 µmol m^-2 s^-1 over the 13-week period, when the community had a net carbon gain from the atmosphere of 4 mol C while the structural C incorporated into the roots was 23 mol. Thus, these five CAM species diverted all net C uptake over the 13-week period plus some existing shoot C to newly developing roots. Only after sufficient roots develop to support shoot water and nutrient requirements will the plant community have net above-ground biomass gains.