Seasonal photosynthetic temperature responses and changes in δ13C under varying temperature regimes in leaf-succulent and drought-deciduous shrubs from the Succulent Karoo
Temperatures in the Succulent Karoo, South Africa, are predicted to increase by 1-3°C over the next 50 years. Four species from the semi-arid Succulent Karoo were grown under differing temperature regimes in order to determine their photosynthetic acclimation potential, using gas exchange and carbon stable isotope composition (δ13C) techniques. Three species are evergreen leaf succulent shrubs (Zygophyllum prismatocarpum E. Mey. ex. Sond., Stoeberia utilis (L. Bolus) Van Jaarsveld and Ruschia spinosa (L.) Dehn), and one species is a drought-deciduous shrub (Tripteris sinuata (DC.) [previously Osteospermum sinuatum (DC.) Norl.]). Plants were not subjected to water stress, and thus operated in the C3 photosynthetic mode. It was predicted that the photosynthetic temperature optima (Topt) and the δ13C values would increase with an increase in growth temperature (treatment) and seasonal temperature regimes in the evergreen, but not in the drought-deciduous species. However, both growth forms showed photosynthetic acclimation (increasing Topt) in response to increasing temperature. δ13C also increased with increasing temperature in all species, indicating improved intrinsic water use efficiency. It is unlikely that photosynthetic capacity of these shrubs will be negatively impacted by increasing temperatures alone and their responses to climate change may depend more on simultaneous changes in water supply.