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 (<em>Hordeum vulgare‘</em>Bahman’ ) seedlings using chlorophyll (Chl) <em>a</em> fluorescence transient (<em>OJIP</em>) 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 <em>b</em> and carotenoid contents in barley seedlings. In contrary, 5 days after 5 mM SA treatment, the maximal quantum efficiency of PSII (<em>F</em><sub>v</sub>/<em>F</em><sub>m</sub>) and the Performance Index (PI<sub>ABS</sub>), as an indicator of PSII structure and functioning, were significantly decreased. This lower <em>F</em><sub>v</sub>/<em>F</em><sub>m</sub> and PI<sub>ABS</sub> coupled with lower levels of Chl <em>b</em> and carotenoids, and lower values of photosynthetic electron transport chain components including the electron transport flux (<em>φ</em>E<sub>o</sub>) and the inferred oxygen evolving complex activity (<em>F</em><sub>v</sub>/<em>F</em><sub>o</sub>). By monitoring the chlorophyll <em>a</em> fluorescence rise kinetics, from the initial “<em>O</em>” level to the “<em>P</em>” (the peak) level, a dramatic increase in “<em>OJ</em>” phase was detected, which coincides with an increased photo-reduction of Q<sub>A</sub> 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.
High salicylic acid concentration alters the electron flow associated with photosystem II in barley
