Altered leaf and root emissions from onion (Allium cepa L.) grown under elevated CO₂ conditions

Elevated atmospheric CO₂ concentrations ([CO₂]) have been hypothesized to increase photosynthesis rates and volatile organic compound (VOC) emissions; however, field measurements from a select group of conifer and angiosperm trees have shown that VOC emissions are in fact not affected or reduced by elevated CO₂ levels. To broaden the understanding of how different plant species respond to elevated atmospheric [CO₂], air-flow-through, glass chambers were designed and utilized to measure photosynthesis and emissions from onion (Allium cepa cv. 'Purplette') under controlled environmental conditions. Here we report on VOC release and root exudation while monitoring photosynthesis from whole plants grown under ambient (400 μmol mol^-1) and elevated (1000 μmol mol^-1) [CO₂]. A 22% increase in photosynthesis in the elevated CO ₂ plants and a 17-fold and 38-fold increase in the VOC hydrocarbons 2-undecanone and 2-tridecanone, respectively, were observed in 30-day-old onion seedlings compared to plants grown under ambient CO₂ conditions. In contrast TOC from root exudates decreased significantly with elevated CO ₂ conditions. Plants harvested at 30 days had on average over 40% greater biomass when grown at elevated CO₂ levels. The demonstration that VOC emissions increase in plants grown under elevated [CO₂] and higher photosynthesis rates points to a fundamental difference in how carbon partitioning alters in herbaceous species such as onion versus the previously studied tree species in response to elevated concentration of atmospheric CO₂.