Isoprene leaf emission under CO2 free atmosphere: why and how?

Abstract:

Isoprene (C₅H₈) is a reactive hydrocarbon gas emitted at high rates by tropical vegetation, which affects atmospheric chemistry and climate and, in the leaf level, is a very important agent against environmental stress. Under optimal conditions for photosynthesis, the majority of carbon used for isoprene biosynthesis is a direct product from recently assimilated atmospheric CO₂. However, the contribution of ‘alternate’ carbon sources, that increase with leaf temperature, have been demonstrated and emissions of isoprene from ‘alternate’ carbon sources under ambient CO₂ below the compensation point for photosynthesis have been observed. In this study, we investigated the response of leaf isoprene emissions under 450 ppm CO₂ and CO₂ free atmosphere as a function of light and leaf temperature. At constant leaf temperature (30 °C) and CO₂ free atmospheres, leaves of the tropical species Inga edulis showed net emissions of CO₂ and light-dependent isoprene emissions which stagnated at low light levels (75 µmol m^-2 s^-1 PAR) and account for 25% of that observed with 450 ppm CO₂. Under constant light (1000 µmol m^-2 s^-1 PAR) and CO₂ free atmospheres, a increase of leaf temperatures from 25 to 40 °C resulted in net emissions of CO₂ and temperature-dependent isoprene emissions which reached values up to 17% of those under 450 ppm CO₂. Our observations suggest that, under environmental stress, as high light/temperature and drought (when the stomata close and the amount of internal CO₂ decreases), the ‘alternate’ carbon can maintain photosynthesis rates resulting in the production of isoprene, independent of atmospheric CO₂, through the re-assimilation of internal released CO₂ as an ‘alternate’ carbon sources for isoprene.