Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem

The emission of isoprene from the leaves of forest trees is a fundamental component of biosphere-atmosphere interactions, controlling many aspects of photochemistry in the lower atmosphere. As almost all commercial agriforest species emit high levels of isoprene, proliferation of agriforest plantations has significant potential to increase regional ozone pollution and enhance the lifetime of methane, an important determinant of global climate. Here we show that growth of an intact Populus deltoides plantation under increased CO2 (800 micromol x mol(-1) and 1,200 micromol x mol(-1)) reduced ecosystem isoprene production by 21% and 41%, while above-ground biomass accumulation was enhanced by 60% and 82%, respectively. Exposure to increased CO2 significantly reduced the cellular content of dimethylallyl diphosphate, the substrate for isoprene synthesis, in both leaves and leaf protoplasts. We identify intracellular metabolic competition for phosphoenolpyruvate as a possible control point in explaining the suppression of isoprene emission under increased CO2. Our results highlight the potential for uncoupling isoprene emission from biomass accumulation in an agriforest species, and show that negative air-quality effects of proliferating agriforests may be offset by increases in CO2.

Differential accumulation of dimethylallyl diphosphate in leaves and needles of isoprene- and methylbutenol-emitting and nonemitting species

The biosynthesis and emission of volatile plant terpenoids, such as isoprene and methylbutenol (MBO), depend on the chloroplastic production of dimethylallyl diphosphate (DMAPP). To date, it has been difficult to study the relationship of cellular DMAPP levels to emission of these volatiles because of the lack of a sensitive assay for DMAPP in plant tissues. Using a recent DMAPP assay developed in our laboratories, we report that species with the highest potential for isoprene and MBO production also exhibit elevated light-dependent DMAPP production, ranging from 110% to 1,063%. Even species that do not produce significant amounts of volatile terpenoids, however, exhibit some potential for light-dependent production of DMAPP. We used a nonaqueous fractionation technique to determine the intracellular distribution of DMAPP in isoprene-emitting cottonwood (Populus deltoides) leaves; approximately 65% to 70% of the DMAPP recovered at midday occurred in the chloroplasts, indicating that most of the light-dependent production of DMAPP was chloroplastic in origin. The midday concentration of chloroplastic DMAPP in cottonwood leaves is estimated to be 0.13 to 3.0 mM, which is consistent with the relatively high K(m)s that have been reported for isoprene synthases (0.5-8 mM). The results provide support for the hypothesis that the light dependence of isoprene and MBO emissions is in part due to controls over DMAPP production.