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

Apoplastic ascorbate (ASC) is an important contributor to the detoxification of ozone (O). The objective of the study is to explore whether ASC is stimulated by elevated O concentrations. The detoxification of O by ASC 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 O treatments (E-O). ASC in the three species were significantly increased by E-O compared with the values in the filtered treatment. For all three species, E-O significantly increased the malondialdehyde (MDA) content and decreased light-saturated rate of photosynthesis (A), suggesting that high O has induced injury/damage to plants. E-O significantly increased redox state in the apoplast (redox state) for all species, whereas no effect on the apoplastic dehydroascorbate (DHA) was observed. In leaf tissues, E-O 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 O levels among the three species. Total antioxidant capacity in the apoplast (TAC) was significantly increased by E-O in tobacco and poplar, but leaf tissue TAC was significantly enhanced only in tobacco. Leaf tissue superoxide anion (O) in poplar and hydrogen peroxide (HO) in tobacco and soybean were significantly increased by E-O. The diurnal variation of ASC, 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 O on tobacco, soybean and poplar.