Leaf thickness as key to the contrasting ecophysiology, including water-loss resistance, of eight arid-climate species

Shrubs in the succulent karoo of the southern Namib Desert survive an annual rainfall < 150 mm per annum so that their leaves must possess special adaptations to prolonged drought. Leaves of eight Namib species were assessed for their water potential (ψ) and storage over 12 months or when severed from the parent plant, water-use efficiency (WUE) via d13C, contents of N and P and cations (Na+ + K+). Four species were non-succulent (orthophylls), four were succulent (succophylls) and two exhibited CAM-type photosynthesis. Succophylls were distinguished by their thicker leaves; lower SLA; presence of water-storing parenchyma; higher levels of ‘utilizable’ water; slower rates of water loss; higher/less variable ψ; higher (Na+ + K+), N and P contents/leaf-area, –d13C and WUE. Water-loss resistance (a component of capacitance) – the change in water potential for a given change in relative water content when subjected to drought, was twice as high in the succophylls as the orthophylls under both laboratory and field conditions. Leaves of the CAM species stored most water, increased their ψ overnight, maintained least rates of water loss, had the highest N and P contents/leaf-area, and were the most water-use efficient. Cations may serve an osmotic balancing function among succophylls, whereas high N and P/area may help maintain metabolic functions when transpiration is limited. d13C/WUE relationships were functions of photosynthetic type, N/P contents/leaf-area and, (especially) leaf thickness. The special structural and physiological features of leaf succulents need to be recognized when developing any general theory about the water relations of plants.   Key-words: leaf succulents; nitrogen and phosphorus contents; saturated leaf water content; SLA; water potential; water-use efficiency.