Ecology and Conservation , National Botanical Institu te . Private 8ag X7, Claremont. 7735 Repub lic of South Afnca Two Succulent Karoa shrubs, an evergreen leaf succulent, Zygophyllum pnsmatoca rpum E Mey ex . Send and a drought-decIduous species, Toptens sinuata (~C . ) were grown under well watered conditions ove r 12 months In th ree greenhouses with dis tinct temperature characteristics Temperature differences between greenhouses varied between 3-8°C depending on season It was predicted

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... t both species would respond \0 increases in growth (treatment) and seasonal temperature, but that T. smuata would be more active under favourable conditions and more sensitive to temperature Increases than Z pnsmatocarpum Net CO 2 asslmtlation rates (Al, stomatal conductances (g s) ' transpiration ra les (El and Instantane ou s leaf water use efficiency (WUE) were determined 10 winter. spring and summer Both species responded to Increasing growth (treatment) and seasonal temperature regimes , but their gas exchange characteristics differed sign ificantly T smuata had higher A, gs' E and WUE. In Z pnsmatocarpum, 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 increas ing treatment tempe rature in winter, but gs was sign ificantly reduced In the high temperature treatment In spnng and summer T. suwala generally had the highest A In the intermediate and high temperature treatments Winter was the most fa vourable season for A and gs Conductances decreased with increasing seasona l temperature under al l temperatu re treatments . Both species had the highest transpiration rates in winter. Optimum temperatu re for photosynthesis was 2S-29°C III both species Although A was not strongly depressed by high temperatures (35-3rC), Z. pnsmatocarpum had better control of transpi ration . Our resu lts are interpreted with respect to climatiC dIfferences 10 the distribution ranges of the two specIes, and the ir differing life-history strateg ies Implications of pred icted climate wa rm ing (+1 0-3 SoC in th e next 50 years) for Succulen t Karoo shrubs are discussed Siellenbosch. Private Bag X1. Matlela nd , 7602 Republic of South Africa . (E-maIl sjew@ land .sun ac za )