Photosynthesis, photosystem II efficiency and the xanthophyll cycle in the salt-adapted halophyte Atriplex centralasiatica

Here, the effects of salinity (0-400 mM NaCl) on photosynthesis, photosystem II (PSII) efficiency and the xanthophyll cycle were investigated in the halophyte Atriplex centralasiatica grown under outdoor conditions. Leaf sodium and chloride in leaves increased considerably whereas CO2 assimilation rate decreased. PSII efficiency (Phi(PSII) ) and the efficiency of excitation energy capture by open PSII reaction centres decreased whereas nonphotochemical quenching (NPQ) increased significantly. There was no change in photochemical quenching (q(P) ) in salt-adapted plants. Salinity induced no changes in the maximum efficiency of PSII photochemistry (F (v) /F (m) ) measured either at midday or at predawn. However, F (v) /F (m) values were c. 20% lower at midday than at predawn. Contents of chlorophyll (a + b), neoxanthin, lutein and beta-carotene were unchanged with increasing salt concentration, but zeaxanthan increased significantly, at the expense of violaxanthin. There was a linear relationship between the de-epoxidation state of the xanthophyll cycle and Phi(PSII) , , and NPQ. Our results suggest that A. centralasiatica shows high tolerance to both high salinity and photoinhibition and that the xanthophyll cycle played an important role in protecting photosynthetic apparatus from photoinhibitory damage. Tolerance of PSII to salinity and photoinhibition can be viewed as an important strategy for A. centralasiatica to grow in very high saline soil during the summer season with high irradiance.