Ozone detoxification in the mesophyll cell wall during a simulated oxidative burst

With the aim to assess the effect of possible O-2(.-) generation during an oxidative burst on O-3 reduction in mesophyll cell walls due to the reactio n O-2(.-) + O-3 --> O-3(.-) + O-2 and subsequent formation of (OH)-O-., O-3 flow through this sequence was compared with O-2(.-) flow through the comp etitive sequences where H2O2 is formed. The two-electron reduction of O-3 v ia the direct reaction with ascorbate was also considered. The calculations were exemplified in an experiment where Phaseolus vulgaris L. leaves were exposed to 530 nl O(3)l(-1) in air for 3.5 h. During the exposure, H2O2 was assumed to be generated at peak rates observed in pathogen-elicited cell s uspensions. O-3 reduction through reaction with O-2(.-) was 25-44% of O-3 d etoxified in the direct reaction with ascorbate. More than 99% of O-2(.-) w as reduced to H2O2 via spontaneous disproportionation and reduction with as corbate, the disproportionation prevailing at pH 5 and reduction at the exp ense of ascorbate at pH 7. H2O2 was estimated to be channelled mostly to th e peroxidase-catalysed scavenging reaction. Calculated steady state H2O2 co ncentrations were 40-80 mu M. It is concluded that generation of H2O2 at th e postulated rate was too high and that the generation of O-2(.-) during an oxidative burst is ineffective in reducing O-3 through the network of reac tive oxygen species. Superoxide dismutase induction in the cell wall under O-3 is discussed.