Variation among different genotypes of hybrid poplar with regard to leaf volatile organic compound emissions

Plantations of hybrid poplar are used in temperate regions to produce woody biomass for forestry-related industries and are likely to become more prevalent if they are used as a source of cellulose for second-generation biofuels. Species in the genus Populus are known to emit great quantities of the volatile organic compounds (VOCs) isoprene and methanol, and lesser quantities of terpene VOCs, giving poplar plantations the potential to significantly influence regional atmospheric chemistry. The goals of this study were to quantify the differences in isoprene, methanol, and monoterpene emissions from 30 hybrid poplar genotypes, determine how well VOC emissions could be explained by growth, photosynthesis, and stomatal conductance, determine whether the parental crosses that created a genotype could be used to predict its emissions, and determine whether VOC emissions from different genotypes exhibit different responses to elevated CO₂. We found that 40–50% of the variation in isoprene emissions across genotypes could be explained by a combination of instantaneous photosynthesis rate and seasonal aboveground growth and 30–35% of methanol emissions could be explained by stomatal conductance. We observed a threefold range in isoprene emissions across all 30 genotypes. Both genotype and parental cross were significant predictors of isoprene and monoterpene emissions. Genotypes from P. tricocarpa × P. deltoides (T × D) crosses generally had higher isoprene emissions and lower monoterpene emissions than those from P. deltoides × P. nigra (D × N) crosses. While isoprene and monoterpene emissions generally decreased under elevated CO₂ and methanol emissions generally increased, the responses varied among genotypes. Our findings suggest that genotypes with greater productivity tend to have higher isoprene emissions. Additionally, the genotypes with the lowest isoprene emissions under current CO₂ are not necessarily the ones with the lowest emissions under elevated CO₂.