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 O-2(center dot-), 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 O-2(center dot-) contents became increasingly apparent with increasing concentration of cinnamic acid. Exposure to cinnamic acid resulted in enhanced membrane peroxidation and decreased plasma membrane W-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. (c) 2005 Elsevier B.V. All rights reserved.