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The effect of salicylic acid treatment (SA) on tobacco seedlings tolerance to the alternation of light/dark stress (AL) (16/8 min light/dark cycles and a photosynthetic photon flux density (PPFD) of 50 μmol photons m−2 s−1 for three days) was studied in this experiment. The photosynthesis efficiency was determined by mean of chlorophyll a and JIP-test. The data showed that the AL treatment decreased PSII activity and increased the level of hydrogen peroxide (H2O2) and lipid peroxidation as evaluated by the accumulation of malonyldialdehyde (MDA). The SA treatment enhanced the growth parameters. The SA pretreatment reduced the inhibitory effect of AL on the PSII activity and photosynthesis, as indicated by the increase in the maximum quantum efficiency of PSII (Fv/Fm), the performance index on the absorption basis (PIabs), the observed rate of QA reduction (TR0/RC), the rate of electron transport beyond QA¯ (ET0/RC) and the efficiency in which an absorbed photon results in electron transport beyond QA¯ (ET0/ABS), and as well the decrease in the energy dissipation as heat (DI0/RC) and the H2O2 and MDA accumulations. Therefore, the observed oxidative damages provoked by AL were noticeably reduced when seedlings were pretreated with SA. [ABSTRACT FROM AUTHOR]