GROWTH AND PHOTOSYNTHETIC CHARACTERISTICS OF 2 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 ingr edient ha(-1)) of chlorotoluron required to reduce fresh mass by 50% u nder standard conditions, were determined for a resistant biotype (39. 3 kg a.i. ha(-1)) collected from Peldon, Essex, UK and a susceptible b iotype (0.93 kg a.i. ha(-1)) obtained commercially, giving a resistanc e factor of 42. The resistance factor was calculated as the ratio of E D50 values and describes the increase in amount of herbicide needed to reduce fresh mass by 50% in the resistant, compared to the susceptibl e, biotype. Resistance was further characterized by measurements of wh ole plant growth and photosynthesis. Relative growth rate, number of t illers, leaf area and mean fresh mass were the same in untreated plant s of both biotypes, and rates of photosynthesis at both high and low p hoton flux were similar, with no difference in apparent quantum yield. Photosynthesis by whole plants was studied over a 24h period followin g chlorotoluron treatment. Resistant plants showed no reduction in pho tosynthesis over this period, whereas photosynthesis by susceptible pl ants ceased 10h after treatment and did not recover. (C) 1997 Annals o f Botany Company.