Differential effects of neutral and alkaline salt stress on wheat photosystem II photochemistry

Soil salinization poses a major threat to global crop productivity, yet the differential effects of neutral (NaCl) versus alkaline (NaHCO 3 ) salts on photosystem II (PSII) thylakoid reactions remain poorly understood. To address this gap, we systematically compared the photosynthetic responses of two wheat cultivars (Xinong 388 and Linhan 6) to equimolar concentrations (100 mM) of NaCl and NaHCO 3 stress, using both OJIP transient and PAM chlorophyll a fluorescence analyses. Specifically, we investigated the effects of these two salt types on thylakoid reactions, and explored the associated photoinhibition sensitivity and nonphotochemical quenching (NPQ)-mediated photoprotection. The results indicated NaHCO 3 stress was associated with greater reductions in shoot and root biomass than NaCl in both cultivars. This growth inhibition coincided with a pronounced impairment of PSII function. This was indicated by significant decreases in the maximum quantum yield of PSII photochemistry (Fv/Fm) and the performance index (PIabs). Energy flux analysis indicated electron transport flux per reaction center (ETo/RC) was reduced under NaHCO 3 in both cultivars. In contrast, dissipated energy flux per reaction center (DIo/RC) increased only in Linhan 6. Moreover, Linhan 6 failed to increase NPQ under stress, which exacerbated photoinhibition. Xinong 388 exhibited increased NPQ but still showed decreased Fv/Fm and PIabs. This suggested that the upregulation of NPQ alone was insufficient to protect PSII from photoinhibition. A possible explanation, which remains to be tested, is that impaired D1 protein repair or inadequate antioxidant capacity underlies this response. These contrasting responses reflect intrinsic cultivar differences in stress tolerance. Overall, alkaline stress impairs PSII function more severely than neutral salt stress. Both PIabs and Fv/Fm were responsive to this damage, supporting their use as indicators of photosynthetic impairment.