Does the dilution agent affect root lesion nematode mortality evaluation?

Abstract

Phytochemicals are known for their profuse and well documented biological and pharmaceutical capabilities. They have been extensively tested in nematode, insect or bacteria mortality bioassays. Since some of these compounds have a very low solubility in water, choosing the right dilution agent or technique is of the utmost importance. The selected agent should allow a dilution that does not significantly alter the properties of the tested compound, have no effect on the mortality of the organism to be tested, and finally, have low impact on the environment or on human health. The dilution effect should be, from the macroscopic point of view, complete. Additionally, if the goal is to apply a selected compound as a biological control agent, the dilution agent should itself be as much as possible neutral to the environment and easily applicable to soil, plants or trees. In this research, standards of naturally occurring phytochemicals were individually tested in five dilution agents (96% alcohol, pure acetone, 70% acetone, 5 mg ml-1 Triton-X and 10% DMSO). Thymol solutions prepared in Triton-X or DMSO required heating at 50oC for 15 min prior to testing for total solubilization. The effect of each dilution agent was evaluated on Pratylenchus penetrans, the root-lesion nematode (RLN) prior to dilution. Since 96% alcohol attained 49.5% mortality it was not further used in the assay. The RLN mortality attained with benzaldehyde, carvacrol, octanol and thymol, at 2 mg ml-1, was assessed according to the existing methodology. Overall, 100% mortality was achieved in dilutions with pure acetone, followed by dilutions with 10% DMSO. In face of the environmental impact that pure acetone may have, DMSO may provide the best relation between full dilution and mortality effectiveness, although unable to achieve 100% nematode mortality at 2 mg ml-1.