Photosynthetic productivity parameters were determined for mayweed chamomile, a troublesome annual weed of the cropping systems in the Pacific Northwest. At a photosynthetic photon flux density of 1800 μE m−2 s–1, maximum net photosynthetic rate of greenhouse-grown plants was 35 mg CO2 dm−2 h–1 and maximum transpiration rate was 6.7 μg H2O cm−2 s–1. Dark respiration rate was 1.4 mg CO2 dm−2 h–1 and the light compensation point was 17.5 μE m−2 s–1. Carbon dioxide compensation point increased from 25 ppm at 15 C to 43 ppm at 30 C. At saturating photosynthetic photon flux densities, optimum leaf temperature for net photosynthesis was about 25 C. Maximum net photosynthesis of leaves of field-grown plants averaged 15.8 mg CO2 dm−2 h–1. After a 24-h exposure to 0.075 kg ha–1 metribuzin, maximum net photosynthesis and transpiration were reduced 85 and 40%, respectively. Soil water deficits reduced maximum net photosynthesis about 50%.
