Background/Question/Methods   The principal function of isoprene biosynthesis in plants remains unclear, but emission rates are positively correlated with temperature and light, supporting a role for isoprene in protecting photosynthesis from transient heat and light stress during sunflecks. Isoprene production is also inversely correlated with CO2 concentrations, implying that rising CO2 may reduce the functional importance of isoprene in the future. To understand the role of isoprene in protecting photosynthesis during sunflecks, we used RNAi technology to suppress isoprene production in poplar seedlings and compared the responses of these transgenic plants to wild-type and empty vector control plants. We grew isoprene-emitting and non-emitting trees at low and high CO2 concentrations and compared their photosynthetic responses to short, transient periods of high light and temperature, as well as their photosynthetic thermal response at constant light.

Results/Conclusions   Net photosynthesis at 42 °C was 50% lower in non-emitters than isoprene-emitting trees at low CO2, but only 22% lower at high CO2. When temperature stress was exacerbated by concurrent light stress at low CO2, isoprene-emitting leaves lost less and recovered more of their pre-stress photosynthetic capacity than non-emitting poplar leaves; there was little difference between emitting and non-emitting plants in their photosynthetic responses to light- and heatflecks at high CO2. We propose that isoprene may have evolved at low CO2 concentrations, where its physiological effect is greatest, and that rising CO2 will reduce the functional benefit of isoprene in the near future.