Differential Effects of Elevated Ozone on Two Hybrid Aspen Genotypes Predisposed to Chronic Ozone Fumigation. Role of Ethylene and Salicylic Acid

Abstract

The role of ethylene (ET) signaling in the responses of two hybrid aspen (Populus tremula L. × P.tremuloides Michx.) clones to chronic ozone (O₃; 75 nL L⁻¹) was investigated. The hormonal responses differed between the clones; the O₃-sensitive clone 51 had higher ET evolution than the tolerant clone 200 during the exposure, whereas the free salicylic acid concentration in clone 200 was higher than in clone 51. The cellular redox status, measured as glutathione redox balance, did not differ between the clones suggesting that the O₃ lesions were not a result of deficient antioxidative capacity. The buildup of salicylic acid during chronic O₃ exposure might have prevented the up-regulation of ET biosynthesis in clone 200. Blocking of ET perception with 1-methylcyclopropene protected both clones from the decrease in net photosynthesis during chronic exposure to O₃. After a pretreatment with low O₃ for 9 d, an acute 1.5-fold O₃ elevation caused necrosis in the O₃-sensitive clone 51, which increased substantially when ET perception was blocked. The results suggest that in hybrid aspen, ET signaling had a dual role depending on the severity of the stress. ET accelerated leaf senescence under low O₃, but under acute O₃ elevation, ET signaling seemed to be required for protection from necrotic cell death.