Abstract: The experiment was conducted in the green house in Hebei province in order to determine the Minimum Lethal Herbicide Dosage (MLHD) of photosynthesis inhibit ing herbicide, isoproturon on five weed species and safety to the winter wheat with Plant Photosynthetic Meter (PPM). The results showed that the diagnostics of weed caused by isoproturon which the young leaves turned yellow could be observed after treatment 6d, and the PPM value decreased sharply after treatment 2d. Weed growth was inhibited later on. Both PPM values of weed leaves and weed biomass (shoot fresh weight ) decreased as herbicide dosage increasing. However, the susceptibility of weed species to the isoproturon was different among weed species. Descurainia Sophia and Chenopodium album were very susceptible to the herb-i cide, and ED90 for themwere 301.5 and 286.02 respectively. Alopecurus aequalis was relatively less susceptible to the herbicide which the ED90was 654.83, and Capsella burse pastors and Bromus japonicus were not suscept ible to it. MLHD could be calculated by weed biomass 14 d after applicat ion of the herbicide. There were close relationship between weed biomass reduction rate and PPM values of weed leaves 2- 4d after treatment. This indicated that the PPM value being measured in early days after treatment could give reasonable predict ion on weed control efficacy and on winter wheat safety. The isoproturon usually provided 90% control by weed biomass when PPM value was about 20 and relatively poor efficacy was obtained when PPM value was below 40. The isoproturon usually could not injure wheat by plant height and biomass when PPM value was over 60. The MLHD of isoproturon should be the dosage when PPM value was below 20 measured for weed and over 60 for wheat after treatment 2- 4d.

Key words: MLHD, Plant photosythesis meter, Isoproturon, Weed control, Wheat safety

CLC Number: 

• S451.1