The elucidation of factors that contribute to cell viability loss is presently compromised by the lack of a universal measure that quantifies "stress." We have investigated mechanisms of viability loss in plant seeds to find a reliable marker of stress response. Oxidative damage has previously been correlated with degenerative processes and death, but how exactly this contributes to viability loss is unknown. We show in four species subjected to ageing or desiccation that seed viability decreased by 50% when the half-cell reduction potential of glutathione (E(GSSG/2GSH)), a major cellular antioxidant and redox buffer, increased to -180 to -160 mV. We then conducted a metaanalysis of data representative of 13 plant and fungal orders to show that plant stress generally becomes lethal when E(GSSG/2GSH) exceeds -160 mV. We put forward that this change in E(GSSG/2GSH) is part of the signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD. E(GSSG/2GSH) is therefore a universal marker of plant cell viability and allows us to predict whether a seed will live, germinate, and produce a new plant, or if it will die.