Our previous study revealed that salicylic acid (SA) accumulates in salt-stressed rice (Oryza sativa L. cv.Nipponbare) seedlings, and we hypothesized that the accumulation of SA might potentiate oxidativeinjury in rice seedlings since the inhibition of SA synthesis alleviated the growth inhibition under highsalinity. To further clarify the action of SA under salt stress, we investigated the changes in the SA content,the activities of the antioxidative enzymes, and the effects of exogenous SA on barnyardgrass(Echinochloa crus-galli Beauv. var. formosensis Ohwi), a gramineous weed which shows lower SA contentand is more salt tolerant than rice. In E. crus-galli seedlings exposed to high salinity, neither free norconjugated SA content showed any increase, while the fresh weight of the shoot and chlorophyllfluorescence (FPSII) slightly decreased. When E. crus-galli seedlings were treated with salt after foliarapplication of SA, the absorbed SA resulted in the enhancement of the salt-induced growth inhibition anda striking reduction of the FPSII value. Catalase (CAT) and superoxide dismutase (SOD) activities of E. crusgalliseedlings were induced by the salt treatment. However, SA pre-treatment suppressed such aninduction of CAT activity and further promoted SOD activity, both of which led to the elevation of the leafhydrogen peroxide (H2O2) level. The present results suggested that enlargement of the cellular SA poolfacilitates the generation of H2O2 through the suppression of CAT activity and through a remarkablepromotion of SOD activity, and thereby enhances the oxidative injury caused by salt stress
