Potassium mitigates salt-stress impacts on photosynthesis by alleviation of the proton diffusion potential in thylakoids

Addition of K+ improves the tolerance of the photosynthetic apparatus to salt stress, but the primary, underlying mechanism is unclear, prompting our study. The main aim of this study was to elucidate the primary mechanism by which supplemental K+ alleviates the effects of salinity stress on photosynthetic parameters in mulberry seedlings. K+ contents in leaves and chloroplasts, plasma-membrane ATPase activity, photosynthetic properties and expression of genes encoding photosynthetic proteins and ion channels/transporters were measured. K+ addition during salt stress improved H+-ATPase activity in plasma-membranes, restored photosynthetic linear and cyclic electron flow, restored the activity of key Photosystem I (PSI) and PSII proteins, and increased dark-adapted PSII photochemical yield Fv/Fm, while reducing non-photochemical quenching. Among the genes encoding ion channels/transporters, that of the Two-Pore-K+ channel in thylakoid membranes (TPK3) was notably responsive to NaCl/K+ treatments: relative to the control (CK) treatment, it decreased slightly in the CK + K treatment, but increased in the NaCl treatment and increased further in the NaCl + K treatment. The above observations are consistent with our hypothesis that K+ supplementation in salt stress restores a high stromal [K+] which, by utilizing TPK3, minimizes a transient, proton diffusion potential that otherwise develops as protons exit the thylakoid lumen through the ATP synthase; thus, rapid charge compensation by K+ movement accelerates both the proton efflux from the lumen and the associated ATP synthesis to enhance carbon assimilation, thereby improving linear as well as cyclic electron flow. The hypothesis pinpoints the primary physicochemical mechanism by which potassium mitigates salt-stress impacts on photosynthesis. It rationalizes both the existence of a K+ channel in thylakoid membranes and the maintenance of a high stromal [K+].