Comparison of CAM expression, photochemistry and antioxidant responses in Sedum album and Portulaca oleracea under combined stress.

Abstract

Previous studies of crassulacean acid metabolism (CAM) pathway during stress have been directed at individual drought and salinity stress, here, we studied the effects of a combination of drought and salt on CAM expression, chlorophyll fluorescence and antioxidant parameters in the C3 -CAM facultative Sedum album and C4 -CAM facultative Portulaca oleracea plants. While salinity alone was not able to induce functional CAM expression in P. oleracea leaves, we showed that salinity induced low level of nocturnal acid accumulation in S. album species. After 20 d of exposure to the combination of simultaneous salt and drought stress, P. oleracea plants exhibited more resistance to photoinhibition as compared to S. album plants. The decrease of maximum quantum yield (Fv /Fm ) in S. album leaves under combined stress was in parallel with the largest suppression of CAM expression of >50%, probably displaying the withdrawal of functional CAM back to C3 pathway. However, under drought treatment alone, S. album plants exhibited higher photosynthetic flexibility, which was associated with the up-regulation of antioxidant enzymes activities and maintenance of glutathione (GSH) pool, and consequently higher photochemical functioning. The levels of nitric oxide (NO) correlated well with CAM expression, which was observed only in S. album, suggesting that NO acts in a different way in C3 and C4 species during CAM induction. Additionally, in both species, over the course of CAM induction, the changes in CAM expression parameters exhibited a similar pattern to that of antioxidant capacity and photochemical functioning parameters.