Seasonal gas exchange responses under three different temperature treatments in a leaf-succulent and a drought-deciduous shrub from the Succulent Karoo
Two Succulent Karoo shrubs, an evergreen leaf succulent, Zygophyllum prismatocarpum E. Mey. ex. Sond. and a drought-deciduous species, Tripteris sinuata (DC.) were grown under well watered conditions over 12 months in three greenhouses with distinct temperature characteristics. Temperature differences between greenhouses varied between 3-8°C depending on season. It was predicted that both species would respond to increases in growth (treatment) and seasonal temperature, but that T. sinuata would be more active under favourable conditions and more sensitive to temperature increases than Z. prismatocarpum. Net CO2 assimilation rates (A), stomatal conductances (g(s)), transpiration rates (E) and instantaneous leaf water use efficiency (WUE) were determined in winter, spring and summer. Both species responded to increasing growth (treatment) and seasonal temperature regimes, but their gas exchange characteristics differed significantly. T. sinuata had higher A, g(s), E and WUE. In Z. prismatocarpum, A showed low variability across treatments and seasons, but was slightly higher in the high temperature treatment. Spring was the most favourable season for A and WUE. Conductances increased with increasing treatment temperature in winter, but g(s) was significantly reduced in the high temperature treatment in spring and summer. T. sinuata generally had the highest A in the intermediate and high temperature treatments. Winter was the most favourable season for A and g(s). Conductances decreased with increasing seasonal temperature under all temperature treatments. Both species had the highest transpiration rates in winter. Optimum temperature for photosynthesis was 25-29°C in both species. Although A was not strongly depressed by high temperatures (35-37°C), Z. prismatocarpum had better control of transpiration. Our results are interpreted with respect to climatic differences in the distribution ranges of the two species, and their differing life-history strategies. Implications of predicted climate warming (+1.0-3.5°C in the next 50 years) for Succulent Karoo shrubs are discussed.