Live-oak plants (Quercus virginiana Mill.) were subjected to various levels of CO2, water stress or photosynthetic photon flux density to test the hypothesis that isoprene biosynthesis occurred only under conditions of restricted CO2 availability. Isoprene emission increases as the ambient CO2 concentration decreased, independent of the amount of time that plants had photosynthesized at ambient CO2 levels. When plants were water-stressed over a 4-d period photosynthesis and leaf conductance decreased 98 and 94%, respectively, while isoprene emissions remained constant. Significant isoprene emissions occurred when plants were saturated with CO2, i.e., below the light compensation level for net photosynthesis (100 μmol m(-2) s(-1)). Isoprene emission rates increased with photosynthetic photon flux density and at 25 and 50 μmol m(-2) s(-1) were 7 and 18 times greater than emissions in the dark. These data indicate that isoprene is a normal plant metabolite and not - as has been suggested - formed exclusively in response to restricted CO2 or various stresses.