Increase of apoplastic ascorbate induced by ozone is insufficient to remove the negative effects in tobacco, soybean...

Apoplastic ascorbate (ASC_apo) is an important contributor to the detoxification of ozone (O₃). The objective of the study is to explore whether ASC_apo is stimulated by elevated O₃ concentrations. The detoxification of O₃ by ASC_apo was quantified in tobacco (Nicotiana L), soybean (Glycine max (L.) Merr.) and poplar (Populus L), which were exposed to charcoal-filtered air (CF) and elevated O₃ treatments (E-O₃). ASC_apo in the three species were significantly increased by E-O₃ compared with the values in the filtered treatment. For all three species, E-O₃ significantly increased the malondialdehyde (MDA) content and decreased light-saturated rate of photosynthesis (A_sat), suggesting that high O₃ has induced injury/damage to plants. E-O₃ significantly increased redox state in the apoplast (redox state_apo) for all species, whereas no effect on the apoplastic dehydroascorbate (DHA_apo) was observed. In leaf tissues, E-O₃ significantly enhanced reduced-ascorbate (ASC) and total ascorbate (ASC+DHA) in soybean and poplar, but significantly reduced these in tobacco, indicating different antioxidative capacity to the high O₃ levels among the three species. Total antioxidant capacity in the apoplast (TAC_apo) was significantly increased by E-O₃ in tobacco and poplar, but leaf tissue TAC was significantly enhanced only in tobacco. Leaf tissue superoxide anion (O₂^•-) in poplar and hydrogen peroxide (H₂O₂) in tobacco and soybean were significantly increased by E-O₃. The diurnal variation of ASC_apo, with maximum values occurring in the late morning and lower values experienced in the afternoon, appeared to play an important role in the harmful effects of O₃ on tobacco, soybean and poplar.