The G-protein activator mastoparan (MP) was found to elicit the hypersensitive response (HR) in isolated Asparagus sprengeri mesophyll cells at micromolar concentrations. The HR was characterized by cell death, extracellular alkalinization, and an oxidative burst, indicated by the reduction of molecular O 2 to O 2 ⅐؊ . To our knowledge, this study was the first to monitor photosynthesis during the HR. MP had rapid and dramatic effects on photosynthetic electron transport and excitation energy transfer as determined by variable chlorophyll a fluorescence measurements. A large increase in nonphotochemical quenching of chlorophyll a fluorescence accompanied the initial stages of the oxidative burst. The minimal level of fluorescence was also quenched, which suggests the origin of this nonphotochemical quenching to be a decrease in the antenna size of photosystem II. In contrast, photochemical quenching of fluorescence decreased dramatically during the latter stages of the oxidative burst, indicating a somewhat slower inhibition of photosystem II electron transport. The net consumption of O 2 and the initial rate of O 2 uptake, elicited by MP, were higher in the light than in the dark. These data indicate that light enhances the oxidative burst and suggest a complex relationship between photosynthesis and the HR.Plants employ a wide array of defense mechanisms against pathogenic microbes. Although plants have protective structures that are always in place (including cuticle layers and thick cell walls) and constitutive biochemical defenses (such as fungitoxic exudates), many resistance strategies are induced after an encounter with a pathogen (Johal et al., 1995). The HR is an induced plant-resistance mechanism. The HR has been defined as localized cell death in an area of pathogen invasion, a phenomenon that may function to isolate and thus limit colonization by biotrophic pathogens (Mehdy, 1994). Certain characteristics have consistently been observed in conjunction with hypersensitive cell death. These include ion fluxes across the plasma membrane (namely Ca 2ϩ influx and K ϩ and Cl Ϫ effluxes), external pH changes (usually alkalinization; Atkinson et al., 1985;Levine et al., 1994), plasma membrane depolarization (Keppler and Novacky, 1986), and the oxidative burst, which features the consumption of molecular O 2 and its reduction to O 2 ⅐ Ϫ at the plasma membrane (Bolwell et al., 1995). Cell death accompanied by these phenomena provides substantial evidence for the HR. Micromolar concentrations of MP have been used to evoke cellular responses, similar to those evoked by fungal elicitors, in suspension cultures of parsley (Kauss and Jeblick, 1995) and soybean cells (Legendre et al., 1992(Legendre et al., , 1993 Chandra et al., 1996), as well as in etiolated cucumber hypocotyls (Kauss and Jeblick, 1996). First isolated from wasp venom, MP is an amphipathic tetradecapeptide. Although MP possesses numerous biological activities, at micromolar concentrations it is best known for its ability to activate G proteins (H...