Cinnamic acid causes oxidative stress in cucumber roots, and promotes incidence of Fusarium wilt

To ascertain whether oxidative stress plays a role in promoting the incidence of Fusarium wilt of cucumber, the activities of several antioxidant enzymes, namely superoxide dismutase (SOD), guaiacol-dependent peroxidase (GPX), ascorbate peroxidase (APX) and catalase (CAT) together with contents of H2O2, MDA and O2−, were determined in cucumber root after exposure to cinnamic acid and prior to the inoculation of Fusarium oxysporum (Schlechtend,:Fr) f. sp. cucumerinum. Results showed that plant growth was greatly inhibited by cinnamic acid at 0.05–0.25 mM, accompanied by a higher incidence of Fusarium wilt. Effects increased with the concentration of cinnamic acid. The increase in SOD, APX, CAT and GPX activities as well as the H2O2 and O2− contents became increasingly apparent with increasing concentration of cinnamic acid. Exposure to cinnamic acid resulted in enhanced membrane peroxidation and decreased plasma membrane H+-ATPase, vacuolar H+-ATPase and PPase activities. Moreover, it also increased the activities of phenylalanine ammonia-lyase and polyphenolase together with increased levels of phenolics and flavonoids. It is likely that the enhanced incidence of Fusarium wilt is associated with oxidative stress triggered by cinnamic acid.