Variation in photosynthesis related parameters in two kinds of Sedum after cadmium treatment

Hyperaccumulator plants are recognized as promising phytoremediation candidates owing to their ability to accumulate substantial heavy metal ions without obvious signs of poisoning. Two ecotypes of Sedum alfredii (hyperaccumulating, HE and non-hyperaccumulating, NHE) were employed for hydroponic experiments with a modified culture solution. The chlorophyll content, photosynthetic parameters, and chlorophyll fluorescence characteristics were analysed. The results indicated that the chlorophyll a (Chla) and chlorophyll b (Chlb) content of the HE plants significantly decreased (P<0.05) by 46.91% and 43.09%, respectively, compared with non-Cd treated samples after stress for 7 days, whereas the Chla and Chlb contents of NHE were significantly decreased by 50.9% and 49.98%, respectively. Under the stress, the decrease in net photosynthetic rate (Pn) in HE was less than that in NHE. The Pn of HE was 31.97% less than that of the control and 50.69% less in NHE; both changes were statistically significant. The transpiration rate (Tr) of HE decreased by 50.58% after stress for 4 days and then increased, whereas the Tr of NHE decreased by 60.47% after stress for 1 day and then increased. The initial fluorescence (F0) of HE increased promptly and then decreased, whereas the F0 of NHE gradually increased during stress for 4 days. The maximum fluorescence (Fm) in HE was always higher than NHE. The actual photochemical efficiency (ΦPSII), photochemical quenching coefficient (qP), and apparent photosynthetic electron transport rate (ETR) of HE firstly decreased and then increased, whereas the ΦPSII and ETR of NHE were stable after a decrease. The non-photochemical quenching coefficient (NPQ) of HE gradually increased, whereas NPQ and qP tended to be stable. Thus, the response of the two ecotypes of Sedum to cadmium treatment was completely different. This study also investigated the physiological responses of the two S. alfredii ecotypes to cadmium stress, which would provide further evidence for the HE S. alfredii restoration of soil cadmium pollution.