Combined effects of NaCl and Cd2+ stress on the photosynthetic apparatus of Thellungiella salsuginea.

Plants show complex responses to abiotic stress while, the effect of the stress combinations can be different to those seen when each stress is applied individually. Here, we report on the effects of salt and/or cadmium on photosynthetic apparatus of Thellungiella salsuginea. Our results showed a considerable reduction of plant growth with some symptoms of toxicity, especially with cadmium treatment. The structural integrity of both photosystems (PSI and PSII) was mostly maintained under salt stress. Cadmium induced a considerable decrease of both PSI and PSII quantum yields and the electron transport rate ETR(I) and ETR(II) paralleled by an increase of non-photochemical quenching (NPQ). In addition, cadmium alone affects the rate of primary photochemistry by an increase of fluorescence at O-J phase and also the photo-electrochemical quenching at J-I phase. A positive L-band appeared with (Cd) treatment as an indicator of lower PSII connectivity, and a positive K-band reflecting the imbalance in number of electrons at donor and acceptor side. In continuity to our previous studies which showed that NaCl supply reduced Cd2+ uptake and limited its accumulation in shoot of divers halophyte species, here as a consequence, we demonstrated the NaCl-induced enhancement effect of Cd2+ toxicity on the PSII activity by maintaining the photosynthetic electron transport chain as evidenced by the differences in ψO, φEo, ABS/RC and TR0/RC and by improvement of performance index PI(ABS), especially after short time of treatment. A significant decrease of LHCII, D1 and CP47 amounts was detected under (Cd) treatment. However, NaCl supply alleviates the Cd2+ effect on protein abundance including LHCII and PSII core complex (D1 and CP47).