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In this study, the effects of exogenously applied salicylic acid (0.5 and 5 mM SA) on the rates of photosystem II (PSII) activity was analysed in 4-week-old barley (Hordeum vulgare'Bahman') seedlings using chlorophyll (Chl) a fluorescence transient (OJIP) measurements. No evident changes in Chl and carotenoid contents as well as chlorophyll fluorescence transient curves were observed in either of the studied concentrations after 24 h of SA application. After 5 d, low SA concentration (0.5 mM) increased PSII activity, Chl b and carotenoid contents in barley seedlings. In contrary, 5 days after 5 mM SA treatment, the maximal quantum efficiency of PSII (F v /F m ) and the Performance Index (PI ABS ), as an indicator of PSII structure and functioning, were significantly decreased. This lower F v /F m and PI ABS coupled with lower levels of Chl b and carotenoids, and lower values of photosynthetic electron transport chain components including the electron transport flux (φE o ) and the inferred oxygen evolving complex activity (F v /F o ). By monitoring the chlorophyll a fluorescence rise kinetics, from the initial "O" level to the "P" (the peak) level, a dramatic increase in "OJ" phase was detected, which coincides with an increased photoreduction of Q A as a result of blockage of electron flow. This study provided the evidence that the high concentration of SA induced damage to different sites of the PSII. . Pri spremljanju povečanja fluorescence klorofila a od začetne "O" na največjo vrednost "P" je bilo opazno njeno dramatično povečanje v fazi "OJ", kar je soupadalo s povečano fotoredukcijo Q A kot posledica blokade fotosintezenega elektronskega pretoka. Raziskava dokazuje, da večja koncentracija salicilne kisline povzroči poškodbe na večih mestih PSII.

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