Senescing grass crops as regional sources of reactive volatile organic compounds

Abstract

Grass crop species, rice and sorghum, that are widely grown in the southeastern Texas region were analyzed for release of biogenic volatile organic compounds (VOCs) in simulated leaf‐drying/senescence experiments. VOC release was measured by both online proton transfer reaction mass spectrometry (PTR‐MS) and proton transfer ion trap mass spectrometry (PIT‐MS) methods, and it was demonstrated that these two grass crops release a large variety of oxygenated VOCs upon drying under laboratory conditions primarily from leaves and not from stems. VOC release from paddy rice varieties was much greater than from sorghum, and major VOCs identified by gas chromatography PTR‐MS included methanol, acetaldehyde, acetone, n‐pentanal, methyl propanal, hexenol, hexanal, cis‐3‐hexenal, and trans‐2‐hexenal. The latter four VOCs, all C6 compounds known to be formed in wounded leaves, were the major volatiles released from drying rice leaves; smaller but substantial amounts of acetaldehyde were observed in all drying experiments. Online detection of VOCs using PIT‐MS gave results comparable to those obtained with PTR‐MS, and use of PIT‐MS with collision‐induced dissociation of trapped ions allowed unambiguous determination of the ratios of cis‐ and trans‐hexenals during different phases of drying. As rice is one of the largest harvested crops on a global scale, it is conceivable that during rice senescence releases of biogenic VOCs, especially the reactive C6 wound VOCs, may contribute to an imbalance in regional atmospheric oxidant formation during peak summer/fall ozone formation periods. A county‐by‐county estimate of the integrated emissions of reactive biogenic VOCs from sorghum and rice production in Texas suggests that these releases are orders of magnitude lower than anthropogenic VOCs in urban areas but also that VOC emissions from rice in southeastern coastal Texas may need to be included in regional air quality assessments during periods of extensive harvesting.