CAM variations in the leaf‐succulent Delosperma tradescantioides (Mesembryanthemaceae), native to southern Africa

Abstract

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides, a succulent that grows in drought‐prone microenvironments 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, shifting 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 fluctuation increased, but was strongly depressed (by ca 70%) as drought continued, 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 fully to pre‐stressed values within two days. Whole‐shoot carbon uptake rates suggested that leaf growth had continued unabated during a short‐term (≅ one week) drought. This emphasises that CAM‐idling allows the maintenance of active metabolism with negligible gas exchange when soil water is limiting. It is possible that old or senescent leaves may provide water for the expansion of developing leaves during initial periods of drought. Regardless of the water regime and environmental conditions, leaf nocturnal malate accumulation and water content were positively correlated and increased with leaf age. Thus the gradual loss of water from older mature leaves may induce CAM‐idling, which reduces water loss. An important ecological consequence of this combination of CAM modes is the potential to switch rapidly between fast growth via C3 gas exchanges when well‐watered to water‐conserving CAM‐idling during drought.