Benzothiadiazole-induced resistance modulates ozone tolerance.

Benzothiadiazole (BTH) treatment induced opposite effects on bean susceptibility to O3, depending on the time elapsed before fumigation. When this time was of only 1-2 days, bean plants were more susceptible to O3 than untreated controls, showing rapid and extensive cell death in both palisade and spongy mesophyll. Damages appeared to be closely correlated with the amount of H2O2 localized in the leaf tissues. In BTH pre-treated, but not fumigated plants, H2O2 accumulation occurred in epidermal cell waals and no dead cells were detected, whereas O3 fumigation of untreated plants produced H2O2 accumulation also inside some palisade mesophyll cells, causing their death. When BTH pre-treatments were carried out 5-7 days before fumigation, plants appeared more tolerant to O3 compared to untreated controls. Under these conditions, no visible symptoms of phytotoxicity were observed for at least 2 weeks after fumigation, and no H2O2 accumulation was detected.