Growth and photosynthetic characteristics of two biotypes of the weed black-grass (Alopecurus myosuroides Huds) resistant and susceptible to the herbicide chlorotoluron

Response of two biotypes of black-grass (Alopecurus myosuroides Huds.) to the herbicide, chlorotoluron, was characterized in glasshouse and laboratory studies. ED50 values, defined as the amount (kg active ingredient ha(-1)) of chlorotoluron required to reduce fresh mass by 50% under standard conditions, were determined for a resistant biotype (39.3 kg a.i. ha(-1)) collected from Peldon, Essex, UK and a susceptible biotype (0.93 kg a.i. ha(-1)) obtained commercially, giving a resistance factor of 42. The resistance factor was calculated as the ratio of ED50 values and describes the increase in amount of herbicide needed to reduce fresh mass by 50% in the resistant, compared to the susceptible, biotype. Resistance was further characterized by measurements of whole plant growth and photosynthesis. Relative growth rate, number of tillers, leaf area and mean fresh mass were the same in untreated plants of both biotypes, and rates of photosynthesis at both high and low photon flux were similar, with no difference in apparent quantum yield. Photosynthesis by whole plants was studied over a 24h period following chlorotoluron treatment. Resistant plants showed no reduction in photosynthesis over this period, whereas photosynthesis by susceptible plants ceased 10h after treatment and did not recover. (C) 1997 Annals of Botany Company.