Inhibition of powdery mildew development and activation of antioxidant enzymes by induction of oxidative stress with foliar application of a mixture of riboflavin and methionine in cucumber

Powdery mildew (Sphaerotheca fuliginea Pollacci) infection in cucumber was significantly reduced by foliar application of a mixture of riboflavin and methionine (RM). The effects of fungicidal activity on leaves applied with RM were detected through restriction of progress of colonies and disease severity compared with control plants. The initial response to foliar application of RM was abrupt generation of hydrogen peroxide in the leaves of cucumber plants. Activities of antioxidant enzymes such as SOD and POD were abruptly increased by foliar application of RM. However, activities of antioxidant enzymes in control plants were increased with disease development 9 d after pathogen inoculation. Cucumber leaves have six major SOD isoforms. When plants were foliar-applied with RM, densities of three SOD isozyme bands at SOD-1, SOD-2, and SOD-3 were increased 3 d after foliar application. Leaves of cucumber plants have three major POD isozyme bands. Densities of three POD isozyme bands were increased 3 d after foliar application with RM. Four major PPO isozyme bands were determined in cucumber leaves. Though the overall banding patterns of PPO in control and RM-applied plants were similar, the band profiles in leaves applied with RM were characterized by high densities of the three major isoforms. Activities of PPO in leaves applied with RM increased rapidly during the 3 d after foliar application, and then remained relatively constant for 15 d. Although activities of PPO in the leaves of control plants also abruptly increased after 9 d, it was lower than those of RM-applied plants during the whole time. The difference in lignin content between control and RM-applied plants was detected 9 d after foliar application; it was high in leaves applied with RM.