Saline and alkaline stresses affect more than 10% of the World’s arable land, limiting agricultural production. Salt-induced stress may affect the photosystem II (PSII) function, altering fluorescence emission. Therefore, changes in fluorescence are used to quantify and analyze abiotic stress responses in plants. So far, no study has focused on the response of PSII to saline, alkaline and saline–alkaline stresses in the model legume Lotus japonicus. For the saline, alkaline and saline–alkaline treatments, plants of the L. japonicus ecotypes MG20 and Gifu-129 were cultivated in sand with nutrient solution, added with NaCl and NaHCO₃ in different proportions. Growth, gas exchange, and chlorophyll a fluorescence transient kinetic and OJIP parameters were measured, and chlorophyll a and b were determined. The analysis of the kinetic of chlorophyll a fluorescence showed that NaCl-derived stress sources affect the photochemical events in PSII in both ecotypes, being this effect more evident under higher pH condition, whereas alkalinity per se has a mild or no effect on these events. The saline–alkaline stress induced a more severe effect on Gifu B-129, compared with Miyakojima MG20, whereas NaCl improved primary photochemistry in MG20. Our results allow us to accept the hypothesis that both ecotypes deploy differential responses under the three stressful treatments and that the saline–alkaline stress causes higher damage levels than saline and alkaline stresses alone in relation with structures and sub-processes of the PSII.
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