Comparative effects of water potential stress induced by salt, alkali and drought on photosynthetic electron transport and apparatus in Hordeum jubatum seedlings

Context. Photosynthetic electron transport and apparatus are highly susceptible to abiotic stressors during photosynthesis. Hordeum jubatum L. is an ornamental grass with excellent salt tolerance, used for soil improvement and ecological restoration in Northeast China. However, the comparative effects of salt, alkali and drought conditions on the photosynthetic apparatus of H. jubatum under uniform water potential are little known. Aims. We explored the photosynthetic response of H. jubatum to uniform levels of water potential stress induced by three different stressors: salt, alkali and drought. Methods. Seedlings of H. jubatum were irrigated with three concentrations of NaCl, NaHCO3 and PEG-6000 to induce water potentials of -0.21, -0.47 and -0.82 MPa. Transient fluorescence of chlorophyll a in the seedlings was measured, and JIP-test parameters were obtained. Key results. Lower values of J-I phase fluorescence intensity and appearance of a K-band under stress treatments indicated restricted electron transfer from the oxygen-evolving complex in the donor of photosystem II. In addition, the values of Delta I-band and parameters M-O and N increased, whereas those of S-m, phi(Eo) and psi(O) decreased, indicating interruption of electron flow between electron acceptors pheophytin and NADP(+). Serious adverse effects of alkali stress on the acceptors were observed; the low water potential induced by NaHCO3 damaged the thylakoid membrane on the chloroplast, resulting in increased degradation of D1 protein. Conclusions. The results suggest that H. jubatum seedlings have greater tolerance to salt and drought stress, but are highly sensitive to alkali stress in the photosynthetic system.