[Exogenous NO application effectively alleviates the allelochemical stress on cucumber root border cells caused by Eupatorium adenophorum extracts.]

Using suspension cultures of cucumber (Cucumis sativus) cultivar Jinyou 35, we investigated the effects of allelochemical stresses induced by Eupatorium adenophorum extracts on root border cells (RBC), and the role of exogenous NO application in alleviation of the damage of root tips exposed to E. adenophorum extracts. The results showed that, 1000 mg·L-1 E. adenophorum extracts had significant damage to the cucumber root tip, resulting in severe tissue damage, exfoliated surface cells and irregular arrangement of inner cells, while those damages could be effectively alleviated by spraying exogenous NO. Compared with the control, E. adenophorum extracts (ZL) markedly reduced RBC numbers and survival rates by 54.5% and 97.2%, respectively, the RBC apoptosis rates were 12.3 times higher, the thicknesses of RBC adhesive layers were increased by 31.4%, and the root cap PME activities were markedly increased. Compared with the ZL treatment, exogenous NO application (ZN) significantly increased RBC numbers and survival rates by 72.4% and 146.0%, respectively, reduced the corresponding RBC apoptosis rates and the thicknesses of RBC adhesive layers by 30.7% and 15.0%, respectively, and inhibited the PME activities by 14.3% upon treatment for 72 hours. Our findings revealed that E. adenophorum extracts showed toxic effects on the cucumber RBC, resulting in cell apoptosis, abolishment of the RBC protection on root tips, and the destruction of root tip structure. Exogenous NO application, to some extent, could prevent the root tip and RBC from cell damage caused by E. adenophorum extracts.