| Summary: | This study investigated the protective effects of exogenous ascorbic acid (AsA, 0.5 mmol·L<sup>−1</sup>) treatment on salt-induced photosystem inhibition in tomato seedlings under salt stress (NaCl, 100 mmol·L<sup>−1</sup>) conditions with and without the AsA inhibitor lycorine. Salt stress reduced the activities of photosystem II (PSII) and PSI. AsA treatment mitigated inhibition of the maximal photochemical efficiency of PSII (<i>F</i><sub>v</sub>/<i>F</i><sub>m</sub>), maximal P700 changes (<i>P</i><sub>m</sub>), the effective quantum yields of PSII and I [Y(II) and Y(I)], and non-photochemical quenching coefficient (<i>NPQ</i>) values under salt stress conditions both with and without lycorine. Moreover, AsA restored the balance of excitation energy between two photosystems (<i>β/α</i>-1) after disruption by salt stress, with or without lycorine. Treatment of the leaves of salt-stressed plants with AsA with or without lycorine increased the proportion of electron flux for photosynthetic carbon reduction [<i>J</i>e(PCR)] while decreasing the O<sub>2</sub>-dependent alternative electron flux [<i>J</i>a(O<sub>2</sub>-dependent)]. AsA with or without lycorine further resulted in increases in the quantum yield of cyclic electron flow (CEF) around PSI [Y(CEF)] while increasing the expression of antioxidant and AsA–GSH cycle-related genes and elevating the ratio of reduced glutathione/oxidized glutathione (GSH/GSSG). Similarly, AsA treatment significantly decreased the levels of reactive oxygen species [superoxide anion (O<sub>2</sub><sup>−</sup>) and hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>)] in these plants. Together, these data indicate that AsA can alleviate salt-stress-induced inhibition of PSII and PSI in tomato seedlings by restoring the excitation energy balance between the photosystems, regulating the dissipation of excess light energy by CEF and <i>NPQ</i>, increasing photosynthetic electron flux, and enhancing the scavenging of reactive oxygen species, thereby enabling plants to better tolerate salt stress.
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