Abstract:

Seedlings of eight-leafed Chinese cabbage (Brassica rapa pekinensis) were cultured in germ-free conditions
supplemented with five dithiophosphate organophosphorus pesticides including malathion, dimethoate, phorate, disulfoton
and terbufos. The residues of the organophosphorus pesticides and O,O-dimethyl phosphorodithioate in the seedlings of
Chinese cabbage were determined by gas chromatography-liquid chromatography-mass spectrometry (GC-MS). As the
culture period progressed, the contents of all these dithiophosphate pesticides decreased and were degraded by 13.4%–65.0%
in 10 days. The dithiophosphate pesticides in Chinese cabbage seedlings were degraded significantly slower under germfree
culture conditions than under normal culture conditions. The metabolic kinetics of the pesticides followed a first-order
kinetic model. Under sterile conditions, malathion was degraded fastest with a rate constant of 0.034 67 and a half life of
19.9 d, Terbufos showed the smallest degradation rate constant of 0.007 17, and the degradation half life was 96.7 days.
Dimethoate, phorate and disulfoton were more stable, but the degradation kinetic parameters considerably varied among them.

Key words: Brassica rapa pekinensis, dithio organophosphorus pesticides, degradation kinetics