In order to understand the role of nitric oxide (NO) in plant cell death,we investigated the toxic mechanism of SNP on Vicia faba guard cell by using epidemal strips assays together with nitric oxide donor sodium nitroprusside (SNP) and some inhibitors of the NO signaling pathways.The experimental results showed that:(1)SNP treatment significantly decreased cell viability and induced cell death at concentrations from 0.5 to 9 mmol·L^-1.(2)Two types of specific caspase inhibitors,Z-Asp-2,6 -dichlorobenzoyloxymethylketone (Z-Asp-CH₂-DCB) and Nα-tosyl-Lys-chloromethyl keton (TLCK),significantly blocked SNP-induced cell death.(3)Antioxidants ascorbic acid (AsA) and catalase (CAT) also inhibited SNP-induced guard cell death.Both Ca²⁺ chelating agent ethylene glycol bis (2-aminoethyl) tetraacetic acid (EGTA) and Ca²⁺ channel blocker LaCl₃ decreased the cell death rate caused by SNP.(4)Application of NO scavenger c-PTIO,and inhibitors of mitogen-activated protein kinase (MAPK) (PD98059) or guanylate cyclase (ODQ) significantly inhibited SNP-induced cell death.The results of the present study showed that SNP caused programmed cell death in Vicia guard cells.Intracellular NO,ROS and Ca²⁺ as well as MAPK and cGMP molecules participated in the process of SNP-induced programmed cell death.