Effect of salicylic acid on the antioxidant system and photosystem II in wheat seedlings

To study the effects of application of salicylic acid (SA) on the antioxidant system and photosystem II (PS II) in wheat seedlings we used two different experiments. The first method was carried out by immersing roots in Hoagland's nutrient solution containing 0, 0.25, or 2.5 mM SA, and the second method was performed by spraying two-week-old seedlings with the same SA concentrations. After 24 h, chlorophyll fluorescence, thylakoid membrane proteins, antioxidant enzyme activities, and reactive oxygen species were measured. The low concentration of SA caused a significant increase in the antioxidant enzyme activities. However, the treatment with 2.5 mM SA resulted in an increase in the non-photochemical quenching coefficient and a decrease in the antioxidant enzyme activities, the quantum yield of PS II photochemistry, and the photochemical quenching, especially in the first method of application. All these results indicate that the effects of SA on PS II and the antioxidative defense system were dependent on the concentration used and the method of application.