The effect of Alternaria leaf spot on the antioxidant system of cucumber seedlings

Cucumber (Cucumis sativus L.) is one of the most popular Cucurbitaceae vegetable worldwide. Alternaria leaf spot, caused by Alternaria cucumerina (A. cucumerina), is a major pathogenic fungus of cucumber in China. In order to explore the response of the antioxidant system to the stress of Alternaria leaf spot in leaves of cucumber seedlings with different resistance ratings, the first true leaves of D1322 (resistant cucumber) and BJ204 (susceptible cucumber) were sprayed with A. cucumerina suspension. We determined chlorophyll (Chl), malondialdehyde (MDA), superoxide anion (O₂⁻), hydrogen peroxide (H₂O₂), ascorbic acid (AsA), glutathione (GSH) content and protective enzyme activities at 1 day post inoculation (dpi), 2 dpi, 3 dpi, 4 dpi, 5 dpi and 6 dpi. The results showed that, under A. cucumerina infection, Chl content decreased, and MDA, O₂⁻, H₂O₂ production increased, compared with the uninfected control. Most of the time the decrease or increase of Chl, MDA, O₂⁻, H₂O₂ content in infected D1322 was less than in infected BJ204. The content of AsA kept a steady content in infected D1322, but decreased in infected BJ204, compared with each control. Superoxide dismutase (SOD) activity of infected D1322 increased, but SOD activity of infected BJ204 decreased. In addition, compared with each control, catalase (CAT), ascorbate peroxidase (APX) and monodehydroascorbate (MDHAR) activities in infected D1322 were higher than those in infected BJ204. Gene expression of copper/zinc superoxide dismutase (CsCu-ZnSOD), CsCAT, CsAPX and CsMDHAR showed the same changes as enzyme activities. These results demonstrate that the over-accumulation of ROS, a result of A. cucumerina infection, appears to be responsible for chlorotic and membrane lipid peroxidation. Disease resistance is associated with high activities of ROS detoxification enzymes--SOD, APX, CAT and steady levels of non-enzymic antioxidant—AsA, which scavenge excess ROS.