Variability of CAM in leaf-deciduous succulents from the Succulent Karoo (South africa)

Seasonal or drought-induced deciduousness is a common adaptational strategy in aboveground persistent succulent species native to the Succulent Karoo (South Africa). This feature may appear together with crassulacean acid met abolism (CAM) in stems and/or leaves. Due to differences in morphology or l ife cycle, different species may exhibit different degrees of flexibility i n the expression of CAM. The potential for CAM plasticity has not been inve stigated in leaf-deciduous succulents of the Succulent Karoo, Diurnal gas e xchange and nocturnal organic acid accumulation were measured to investigat e whether the expression of CAM may vary in Tylecodon paniculatus (L.F.) To elken (Crassulaceae), Monilaria moniliformis (Mesembryanthemaceae) and Cera ria fruticulosa H. Pearson & Stephens (Portulacaceae), a selection of leaf- deciduous succulents. These species differ both in leaf and stem morphology . In the seasonal deciduous Tylecodon paniculatus only obligate CAM was fou nd, regardless of the plant water status. When droughted Monilaria monilifo rmis switches directly from CAM-cycling (C-3-like diurnal gas exchange patt erns combined with nocturnal acid accumulation) to CAM-idling (gas exchange completely ceased while acid accumulation still continued). This seasonal deciduous species has also both succulent leaves and stem. A highly flexibl e CAM expression (CAM-cycling, full-CAM and CAM-idling) was observed in the drought deciduous Ceraria fruticulosa (non-succulent stem showing no CAM) in response to changes in water availability. The ecological significance o f the observed diversity in the flexibility of CAM expression was discussed in terms of morphological traits, and the relative ecological significance of both metabolic adaptation and leaf-deciduousness.