Abstract: After spraying different concentrations of two brands pesticide omethoate on cole (Brassica campestris L.) leaves, the leaf chlorophyll a fluorescence transients were measured by a Plant Efficiency Analyzer (PEA), and the toxicological effects and rudimental dynamic courses of omethoate on the leaf photosystem Ⅱ (PSⅡ) were investigated by JIP-test. The results showed that after spraying omethoate except at the concentration of 0.50%, the maximal efficiency of photochemistry (F_v/F_m) did not have a remarkable change. However, with increasing omethoate concentration, the minimal fluorescence F_o, maximal fluorescence F_m, relative variable fluorescence at the J-step (V_J), and electron transport flux perreactive center in PSⅡ(ET_o/RC) increased remarkably, but ψ_o, the efficiency that a trapped exciton in PS Ⅱ moved an electron into the electron transport chain beyond Q_A^-, decreased remarkably. The test two brands of pesticide omethoate had almost alike effects on the PSⅡ of cole, and the residual effect of the pesticide was the strongest at the third day after spraying and petered out from the ninth to twelfth day. The main targets of omethoate on the PSⅡ of cole could be listed as promoting the reduction from Q_A to Q_A^- (increasing of V_J) and the electron transmission from Q_A^- to Q_B (increasing of ET_o/RC).

Key words: diversity index, nematode community, cultivation duration, microbial community