The aim of this study was to investigate whether the root system of Mesembryanttieinuin crystattimim (L.) plays a role in triggering the induction of crassulacean acid metabolism (CAM) during water stress. Depriving wellirrigated plants of water, by allowing the soil surrounding the roots to dry, caused increased daily losses in leaf relative water content (RWC) and mesophyll cell turgor pressure. The RWC of the roots also declined. Subsequently plants exhibited physiological characteristics of CAM photosynthesis (i.e. diurnal fluctuations in leaf titratable acidity and nocturnal net CO2 fixation). When the root system of plants was divided equally between two soil compartments and one half deprived of water, plants exhibited physiological characteristics of CAM without prior changes in leaf RWC content or mesophyll cell turgor pressure. Only the RWC of the water-stressed portion of the roots was reduced. These data suggest that in waterstressed plants daily changes in leaf water relations greater than those observed in well-irrigated plants, are not essential to trigger CAM expression. It is probable that a reduction in soil water availability can be perceived by the roots of M. crystattiiium and that this information is conveyed to the leaves triggering the transition from C, to CAM photosynthesis.Kev-worcts: Mesetnbryatititcttttttn ctystattiiuitn; crassulacean acid metabolism; root signalling; water stress.