Effects of pesticides on the germination of weed seeds: implications for manipulative experiments.
The effects of 2 insecticides (chlorpyrifos and dimethoate at 6.4 × 10-4 and 1.9 × 10-4 g/Petri dish, resp.) and a fungicide (iprodione at 6.4 × 10-3 g) on the seed germination of 20 weed species collected from a field at Ascot were examined under laboratory conditions. Dimethoate significantly reduced percentage germination and the germination rate in Poa annua and 6 of the 8 annual forb species tested (Conyza canadensis, Sonchus oleraceus, Vicia sativa, Spergula arvensis, Stellaria media and Veronica persica). It had no effect on 9 perennial forbs and 2 perennial grasses. Chlorpyrifos reduced germination in P. annua and S. arvensis, and iprodione did so in Plantago lanceolata. Combinations of pesticides were phytotoxic to the seeds of some species, even though the single compounds were not. It was suggested that the mode of dimethoate action is either by the inhibition of hydrolytic enzyme synthesis or by blocking of enzyme pathways in seeds during germination. The significance of these tests and their results are discussed in relation to manipulative field experiments in natural vegetation.