The effect of ozone fumigation on the biogenic volatile organic compounds (BVOCs) emitted from Brassica napus above- and below-ground

Abstract

The emissions of BVOCs from oilseed rape (Brassica napus), both when the plant is exposed to clean air and when it is fumigated with ozone at environmentally-relevant mixing ratios (ca. 135 ppbv), were measured under controlled laboratory conditions. Emissions of BVOCs were recorded from combined leaf and root chambers using a recently developed Selective Reagent Ionisation-Time of Flight-Mass Spectrometer (SRI-ToF-MS) enabling BVOC detection with high time and mass resolution, together with the ability to identify certain molecular functionality. Emissions of BVOCs from below-ground were found to be dominated by sulfur compounds including methanethiol, dimethyl disulfide and dimethyl sulfide, and these emissions did not change following fumigation of the plant with ozone. Emissions from above-ground plant organs exposed to clean air were dominated by methanol, monoterpenes, 4-oxopentanal and methanethiol. Ozone fumigation of the plants caused a rapid decrease in monoterpene and sesquiterpene concentrations in the leaf chamber and increased concentrations of ca. 20 oxygenated species, almost doubling the total carbon lost by the plant leaves as volatiles. The drop in sesquiterpenes concentrations was attributed to ozonolysis occurring to a major extent on the leaf surface. The drop in monoterpene concentrations was attributed to gas phase reactions with OH radicals deriving from ozonolysis reactions. As plant-emitted terpenoids have been shown to play a role in plant-plant and plant-insect signalling, the rapid loss of these species in the air surrounding the plants during photochemical pollution episodes may have a significant impact on plant-plant and plant-insect communications.

Fig 1

Schematic diagram of experimental setup (top) and plant chamber (bottom).

Fig 2. The principle leaf emitted BVOCs from Brassica napus.

Blue bars represent BVOC emission prior to ozone fumigation and red bars represent BVOC emission during the first 2 h of ozone fumigation detected using SRI-ToF-MS and plotted on a log scale. BVOC species quantified from measurements carried out in H₃O⁺ mode. Error bars represent the standard error in emission across 10 plants. * P < 0.05, ** P < 0.01 derived from a paired t-test comparing BVOC emission prior to ozone fumigation against BVOC emission during the first 2 h of ozone fumigation.

Fig 3. Sesquiterpene ozonolysis product formation following ozone fumigation.

Black circles represent sesquiterpene emission from the leaf chamber (protonated m/z 205.198) following ozone exposure. Emission of the ozonolysis products acetone (protonated m/z 59.050), methyl vinyl ketone (protonated m/z, 71.050), oxopentanal (protonated m/z 101.061) and 6-methyl-5-hepten-2-one (protonated m/z 127.111), from the leaf chamber is represented by blue diamonds, purples crosses, green squares and turquoise crosses respectively. Error bars represent the standard error across 10 plants.