Phytotoxicity of Branched Cyclohexanes Found in the Volatile Fraction of Diesel Fuel on Germination of Selected Grass Species

During a larger study to screen candidate plant species for phytoremediation of diesel fuel-contaminated soils, it was observed that at relatively low levels of diesel fuel contamination, delayed shoot/root emergence and reduced germination was observed for the majority of plant species investigated. It was theorised that these effects were the result of acute phytotoxicity, caused by the volatile fraction of diesel fuel, with results supporting this theory. This finding was investigated further in the current study. Headspace analysis of diesel fuel showed that between 5% and 10% of diesel fuel consisted of compounds that would volatilise at 20°C, with the most predominant compounds identified being the isomers of xylene (m-, o- and p-), n-alkanes (C9-C12) and alkylbenzenes. There were also low levels of toluene, branched cyclohexanes (methyl-, to butylcyclohexane) and alkenes. Of particular interest were branched cyclohexanes as little work has previously been reported on these compounds. To explain the phytotoxic effect of the volatile fraction of diesel fuel and attribute the effect to a specific compound or group of compounds within diesel fuel, seeds were germinated in petri dishes contaminated with a number of pure branched cyclohexanes. An unusual pattern of germination was apparent, with results varying depending on grass species and the length of cyclohexane branching. Results showed ethyl- and butyl-cyclohexane had a significant effect on the germination rate of selected grass species whereas methyl- and propyl-cyclohexane had little effect.