The Ecological Water-Use Strategies of Succulent Plants

Plants with pronounced succulent tissues present considerable morphological and phylogenetic diversity. One way to make sense of this diversity is to recognise the common elements comprising ecological strategies shared by diverse taxa and forms. We review two broad plant ecological strategies often accompanied by pronounced tissue succulence, drought avoidance and salt tolerance, and identify common elements and variations within each. Drought-avoiding succulence typically involves high-capacitance water storage tissues, which buffer the transpiration stream and extend carbon uptake during drought. In contrast, water storage in salt-tolerant succulence is thought to be largely a by-product of massive ionic accumulation in vacuoles, and we show preliminary results indicating that succulence in halophytes is not closely linked to tissue capacitance. We review the relationship between crassulacean acid metabolism (CAM) photosynthesis and succulence, identifying putative anatomical features that may explain the frequent association of these two traits. Furthermore, although a high adaptive value of CAM has been proposed for halophytes, it is infrequent in these plants, possibly because of conflict between malate and salt storage functions in vacuoles. This may explain the surprising rarity of evolutionary transitions between drought-avoiding and halophytic succulence. We also discuss the exceptional case of the Aizoaceae, a mostly drought-avoiding group that appears to have evolved a high degree of salt tolerance, possibly multiple times. Finally, we discuss the need for a widely applicable method of quantifying succulence as a continuous trait.