Physiological effects of azoxystrobin and epoxiconazole on senescence and the oxidative status of wheat

The impact of two fungicides, azoxystrobin and epoxiconazole, on the senescence process of spring wheat (Tritictun aestivum L. cv. `Nandu') grown under greenhouse conditions until maturity was investigated. The senescence process could be well described by the decrease in total leaf protein content and the increase in electrolyte leakage from leaf tissue. The changes in these two senescence factors coincided with an increase in the level of superoxide (O-2(.-)) and a decrease in the activity of the antioxidant enzyme superoxide dismutase (SOD) during senescence. The senescence was significantly delayed by application of azoxystrobin and epoxiconazole, which induced an increase in total SOD activity and a reduction of O-2(.-) levels, particularly at mature growth stages. The activity of peroxidase in fungicide-treated plants was about two times higher in flag leaves and three to four times higher in f-1 leaves than in untreated plants. Additionally, levels of H2O2 were significantly elevated in fungicide-treated plants. Paraquat induced a substantial increase in O-2(.-) production at growth stages later than GS 65/69, being much delayed and reduced by azoxystrobin and epoxiconazole. At later growth stages (GS 59/61), azoxystrobin and epoxiconazole showed similar effects in delaying senescence of wheat plants, but azoxystrobin was more efficient when applied at early growth stages (GS 31/32). The results suggest that the fungicide-induced delay of senescence is due to an enhanced antioxidative potential protecting the plants from harmful active oxygen species. Thus, ethylene reduction may not be the primary mechanism by which strobilurins or triazoles interfere with the senescence process, as previously suggested, but may be rather a consequence of reduced oxidative stress in the plant tissue. Elevated levels of H2O2 possibly play a key role as second messengers in inducing the expression of antioxidant genes in the fungicide-treated plants. (C) 2001 Academic Press.