Seasonal photosynthetic temperature responses and changes in delta C-13under varying temperature regimes in leaf-succulent and drought-deciduousshrubs fromthe Succulent Karoo

摘要

Temperatures in the Succulent Karoo, South Africa, are predicted to increase by 1-3 degreesC 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 (delta C-13) 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.) Nod.]). Plants were not subjected to water stress, and thus operated in the C-3 photosynthetic mode. It was predicted that the photosynthetic temperature optima (Topt) and the delta C-113 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. delta C-13 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.