Salinity treatment shows no effects on photosystem II photochemistry, but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemsitry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5°C, air temperature). CO₂ assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F_v/F_m), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F_v′/F_m′) and the actual PSII effciency (Φ_PSII), photochemical quenching (q_P), and non-photochemical quenching (q_N) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a ligh t-adapted state. With increasing temperature, CO₂ assimilation rate decreased significantly and no net CO₂ assimilation was observed at 47.5°C. Salinity treatment had no effect on the response of CO₂ assimilation to high temperature when temperature was below 40°C. At 45°C, CO₂ assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO₂ assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35°C, the declines in F_v/F_m, Φ_PSII, F_v′/F_m′, and q_P were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O₂-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F_i-F_o)/(F_p-F_o) which is an expression of the proportion of the Q_B-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O₂-evolving complex and the reaction centres of PSII to high temperature.