Shifts in the Carbon Metabolism of Xerosicyos danguyi H. Humb. (Cucurbitaceae) Brought About by Water Stress

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Abstract
Xerosicyos danguyi H. Humb. (Cucurbitaceae) is an unusual leaf succulent endemic to Madagascar. Under well-watered conditions the plant exhibited Crassulacean acid metabolism (CAM), as characterized by large diurnal changes in titratable acidity, predominantly nighttime stomatal opening and CO2 uptake, and high δ13C values. When plants were exposed to water stress for a minimum of a month, they shifted to a mode of carbon metabolism previously labeled CAM-idling. Under this mode of metabolism, the plants exhibited reduced stomatal opening, reduced CO2 uptake, dampened diurnal fluctuations in titratable acidity, and no apparent changes in the δ13C values. Additionally, investigations showed that the stress hormones 1-aminocyclopropane-1-carboxylic acid (an ethylene precursor) and abscisic acid increased as much as 6-fold in the water-stressed plants. The results are discussed in relation to physiological significance and evolution of the CAM-idling mode of metabolism.