Comparative Ecophysiology and Photoprotection in Two Stem Succulent Euphorbia Species in Arid Habitat

Stem succulents Euphorbia fractiflexa and Euphorbia triaculeata (Euphorbiaceae) are common arido-active perennials in arid regions of Arabian Peninsula. Work presented in this paper aimed at investigating Crassulacean Acid Metabolism (CAM) in these two species by studying diurnal and seasonal changes in stomatal conductance and cell sap acidity. Chlorophyll fluorescence was measured to elucidate effect of drought and high irradiance on photosynthetic apparatus. Pigment profile analysis was carried out to provide comparative view of photoprotective mechanisms in these two species. Results indicated that these two Euphorbia species are obligate CAM plants that shift to CAM-idling in response to protracted drought. Results also revealed stress induced changes in activity and antenna size of Photosystem II. Changes in Photosystem II associated with increased non-photochemical quenching of chlorophyll fluorescence reflecting operation of photoprotective excess energy dissipation mechanisms. Pigment profile analysis revealed that photoprotection in E. fractiflexa relies on operation of xanthophyll cycle while that in E. triaculeata relies on anthocyanin accumulation.