Glucosinolate content and isothiocyanate evolution--two measures of the biofumigation potential of plants.

A total of 570 lyophilised Brassica root and shoot tissue samples were hydrolyzed, and the liberated isothiocyanates (ITCs) were analyzed by gas chromatography-flame photometric detection (GC-FPD). Glucosinolates (GSLs) were extracted from samples of the same tissues and analyzed by high-performance liquid chromatography (HPLC). The concentrations of six GSLs/ITCs (2-propenyl, 3-butyl, 4-pentenyl, benzyl, 4-methylthiobutyl, and 2-phenylethyl) as determined by the two techniques were compared. In 79% of the samples, the concentration of GSLs in the tissues was greater than that of the ITCs released on hydrolysis. Several possible reasons for the difference are proposed, including the effect of tissue storage time, hydrolysis of GSLs may be less efficient than the GSL extraction procedure, or some of the ITCs formed reacted with plant proteins and amino acids in the sample and were therefore not detected in the extract. GSL concentration in plant tissues is used to estimate the biofumigation potential of the plant tissue, whereas the actual biofumigation effect is thought to be due to the ITCs formed by hydrolysis of the plant-based GSLs. The variation between ITC and GSL values therefore has implications for the assessment of the biofumigation potential of the plant tissue.

Advantageous attributes of larval whitefringed weevil, Naupactus leucoloma (Coleoptera: Curculionidae) for bioassaying soil fumigants, and responses to pure and plant-derived isothiocyanates.

First instars of the soil-inhabiting whitefringed weevil, Naupactus leucoloma (Boheman), are a particularly good bioassay model for assessing volatile soil fumigants and biofumigants. Eggs are readily obtained and can be stored for long periods with larvae hatched on demand and the first instar is non-feeding, surviving without food or shelter. Longevity varies with temperature, but readily accommodates the period required to conduct bioassays without appreciable mortality of untreated controls. In vitro bioassays of pure methyl isothiocyanate, the active ingredient from metham sodium soil fumigant, and the less volatile 2-phenylethyl isothiocyanate, sensitively detected differences in toxicity and effects of temperature. Bioassay of volatiles emitted from hydrolysed tissue of various isothiocyanate-producing Brassica plants revealed widely varying toxicity effects, indicating that bioassays with N. leucoloma are a sensitive and relevant indicator of the potential of different plants for biofumigation of soil-borne pest organisms.