CAM VARIATIONS IN THE LEAF-SUCCULENT DELOSPERMA TRADESCANTIOIDES (MESEMBRYANTHEMACEAE), NATIVE TO SOUTHERN AFRICA

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides, a succulen t that grows in drought-prone microenvironnents in summer rainfall and all-year rainfall regions of southern Africa. When well-watered, this species exhibited Crassulacean acid metabolism (CAM)-cycling, but its carbon fixation pattern changed during the development of drought, sh ifting to either low-level CAM or to CAM-idling. The rate and pattern of this change depended on environmental conditions, duration of water stress and leaf age. At the onset of drought, diurnal malate fluctuat ion increased, but was strongly depressed (by ca 70%) as drought conti nued, and when leaf water content and water potential were low (ca 35 and 50% of the initial levels, respectively). When rewatered, rates of growth and photosynthesis, gas exchange and water status recovered fu lly to pre-stressed values within two days. Whole-shoot carbon uptake rates suggested that leaf growth had continued unabated during a short -term (approximate to one week) drought. This emphasises that CAM-idli ng allows the maintenance of active metabolism with negligible gas exc hange when soil water is limiting. It is possible;that old or senescen t leaves may provide water for the expansion of developing leaves duri ng initial periods of drought. Regardless of the water regime and envi ronmental conditions,leaf nocturnal malate accumulation and water cont ent were positively correlated and increased with leaf age. Thus the g radual loss of water from older mature leaves may induce CAM-idling, w hich reduces water loss. An important ecological consequence of this c ombination of CAM modes is the potential to switch rapidly between fas t growth via C-3 gas exchanges when well-watered to water-conserving C AM-idling during drought.