Leaf isoprene emission rate as a function of atmospheric CO 2 concentration

There is considerable interest in modeling isoprene emissions from terrestrial vegetation, because these emissions exert a principal control over the oxidative capacity of the troposphere. We used a unique field experiment that employs a continuous gradient in CO 2 concentration from 240 to 520 ppmv to demonstrate that isoprene emissions in Eucalyptus globulus were enhanced at the lowest CO 2 concentration, which was similar to the estimated CO 2 concentrations during the last Glacial Maximum, compared with 380 ppmv, the current CO 2 concentration. Leaves of Liquidambar styraciflua did not show an increase in isoprene emission at the lowest CO 2 concentration. However, isoprene emission rates from both species were lower for trees grown at 520 ppmv CO 2 compared with trees grown at 380 ppmv CO 2 . When grown in environmentally controlled chambers, trees of Populus deltoides and Populus tremuloides exhibited a 30–40% reduction in isoprene emission rate when grown at 800 ppmv CO 2 , compared with 400 ppmv CO 2 . P. tremuloides exhibited a 33% reduction when grown at 1200 ppmv CO 2 , compared with 600 ppmv CO 2 . We used current models of leaf isoprene emission to demonstrate that significant errors occur if the CO 2 inhibition of isoprene is not taken into account. In order to alleviate these errors, we present a new model of isoprene emission that describes its response to changes in atmospheric CO 2 concentration. The model logic is based on assumed competition between cytosolic and chloroplastic processes for pyruvate, one of the principal substrates of isoprene biosynthesis.