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 metabolism (CAM) in stems and/or leaves. Due to differences in morphology or life cycle, different species may exhibit different degrees of flexibility in the expression of CAM. The potential for CAM plasticity has not been investigated in leaf-deciduous succulents of the Succulent Karoo, Diurnal gas exchange and nocturnal organic acid accumulation were measured to investigate whether the expression of CAM may vary in Tylecodon paniculatus (L.F.) Toelken (Crassulaceae), Monilaria moniliformis (Mesembryanthemaceae) and Ceraria 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 found, regardless of the plant water status. When droughted Monilaria moniliformis switches directly from CAM-cycling (C(3)-like diurnal gas exchange patterns 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 flexible 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 of 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.