ALTERED VOC AND ROOT EMMISSIONS FROM ONION (ALLIUM CEPA L.) GROWN UNDER ELEVATED CO2 CONDITIONS

Authors: JASONI, RICHARD - TEXAS TECH UNIVERSITY; KANE, CHAD - TEXAS TECH UNIVERSITY; GREEN, CARY - TEXAS TECH UNIVERSITY; PEFFLEY, ELLEN - TEXAS TECH UNIVERSITY; TISSUE, DAVID - TEXAS TECH UNIVERSITY; THOMPSON, LESLIE - TEXAS TECH UNIVERSITY; Payton, Paxton; PARE, PAUL - TEXAS TECH UNIVERSITY

Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2003
Publication Date: 6/1/2003
Citation: Jasoni, R., Kane, C., Green, C., Peffley, E., Tissue, D., Thompson, L., Payton, P.R., Pare, P. 2003. Altered voc and root emmissions from onion (allium cepa l.) grown under elevated co2 conditions. Environmental and Experimental Botany. 51:273-280.

Interpretive Summary: A major goal in plant biology is to assess the responses to atmospheric increases in CO2 concentration that have been on a steady climb from 200 'mol mol-1 in the early 1920s and is predicted to double from the current level of 370 'mol mol-1 within the next century. One theory is that increases in carbon accumulation in excess of that required for growth will result in increases in carbon-based defense compounds. It is known that the emission of volatile organic compounds (VOCs) from forests has significant control over regional atmospheric conditions, as well as biotic interactions in terrestrial ecosystems. While a considerable amount of information has been gathered concerning VOC emissions from woody species, virtually no work has been performed on herbaceous plants. Owing to the varied mix of low-molecular-weight hydrocarbons and sulfur derivatives emitted from onion, this plant system was selected as a model system for assessing the role of CO2 in the induction of VOCs in a herbaceous plant species. Onion is an important agricultural crop high in antioxidant flavonoids, as well as sulfur volatiles enzymatically generated with tissue damage. Sulfur volatiles are the source of the distinctive pungency and flavor found in the allium family. Additionally, a number of other metabolites have been reported to be emitted from onion, although their role is not fully understood. Here we tested whether photosynthesis, VOC emissions from the leaves, and total organic carbon exudation from the roots increases in whole onion plants exposed to elevated levels of atmospheric [CO2]. The results of this study support the hypothesis that whole onion plant VOC emission is linked to onion leaf carbon balance. Furthermore, this data supports that theory that plants grown under elevated CO2 will accumulate excess carbon and that at least a portion of this excess carbon is used for the increased production of carbon-based defense compounds when compared to the control plants. One potential benefit to increased volatile emissions is an increase in insect tolerance. It is possible that plants grown in the field exposed to elevated levels of CO2 could benefit from the increased emissions of methyl ketones. It is conceivable that plants could have an increased level of resistance from certain insects, reduce the need for spraying costly and potentially dangerous pesticides.

Technical Abstract: A major goal in plant biology is to assess the responses to atmospheric increases in [CO2] that have been on a steady climb from 200 'mol mol-1 in the early 1920s and is predicted to double from the current level of 370 'mol mol-1 within the next century (Cure and Acock 1986; Conway et al., 1994). Because the emission of volatile organic compounds (VOCs) from conifer as well as mixed angiosperm forests exerts dominant control over regional oxidative chemistry of the atmosphere in many terrestrial ecosystems, interest has focused on the impact of VOC emissions, especially that of isoprenoids and monoterpenes as it relates to biosphere-atmosphere interactions (Wuebbles et al., 1989; Fehsenfel et al., 1992; Monson et al., 1995; Lerdau et al., 1995; Constable et al., 1999). Divergent to the resource allocation model that predicts that carbon in excess of that required for immediate growth is used in the production of carbon-based defense compounds, certain tree species maintain or lower VOC emission with increased CO2 availability. For example, with Ponderosa pine stored monoterpene concentrations and VOC emission rates in the needles did not change significantly when the trees were exposed to 700 'mol mol-1 CO2 for 2.5 years compared to trees grown at 350 'mol mol-1 (Constable et al. 1999). In the case of Douglas fir, elevated CO2 treatment resulted in greater than a 50% reduction in the monoterpene emissions (Snow et al., 2003). However, in both of these plant systems total carbon allocation could not in fact be determined since metabolites generated in roots and exuded into the soil was not monitored. To more closely examine the impact of elevated [CO2] on leaf VOCs and root TOCs a non-woody agricultural crop rich in volatile constituents was selected and tested under strictly controlled under controlled atmospheric conditions. Owing to the varied mix of low-molecular-weight hydrocarbons and sulfur derivatives emitted from onion, this plant system was selected as a model system for assessing the role of CO2 in the induction of VOCs in a herbaceous plant species. Onion is an important agricultural crop high in antioxidant flavonoids, as well as sulfur volatiles enzymatically generated with tissue damage. Sulfur volatiles are the source of the distinctive pungency, flavor and lachrymator factor found in the allium family (Block et al., 1992). The odd-chain ketones 2-undecanone and 2-tridecanone are also emitted from onion. Although the function of these methyl ketones is has not been reported in onion, these metabolites present in tomato trichomes have been shown to confer insect resistance against a major agricultural pest, the spider mite (Fery and Kennedy, 1987; Chatzivasileiadis and Sabelis, 1998; Chatzivasileiadis et al., 1999). Here we tested whether photosynthesis, VOC emissions from the leaves, and TOC exudation from the roots increases in whole onion plants exposed to elevated levels of atmospheric [CO2]. To broaden the understanding of how different plant species respond to elevate atmospheric [CO2], air-flow-through, glass chambers have been designed and built that allow the measurement of photosynthesis and VOC release from onion (Allium cepa cv. 'Purplette'). Here we report on photosyntheis, VOC emissions, and harvest biomass from whole plants grown under two [CO2] (400 and ''''''mol mol-1). There was a 15% increase in photosynthesis in the 1000 'mol mol-1 plants and a 94% and 96% increase in release of the volatile hydrocarbons 2-undecanone and 2-tridecanone, respectively, in 30-day-old onion seedlings when compared to the plants grown at 400 'mol mol-1. At harvest, plants exposed to 1000 'mol mol-1 of CO2 had 41% greater total biomass than plants grown at ambient (400 'mol mol-1) CO2.