Ultraviolet-B (UVB, 280 to 320 nm) radiation levels reaching the earth's surface are increasing because of depletion of the stratospheric ozone layer. Adverse biological effects of this harmful radiation are a serious concern. The effects of UVB radiation on spotted chickweed, dandelion, downy brome, green foxtail, redstem filaree, and common catsear seed germination, shoot and radicle elongation, and seedling growth and development in a UVB-free environment after exposure to UVB were investigated in a greenhouse study. UVB radiation (4, 7, or 11 kJ m⁻² d⁻¹) did not influence seed germination of any species except downy brome. Postgermination UVB exposure (8 h d⁻¹, for 10 d), however, inhibited radicle elongation in all species, except in spotted catsear. Shoot elongation was also inhibited in some species but to a lesser extent than radicle elongation. A significant inhibition by UVB treatment (11 kJ m⁻² d⁻¹) of root and shoot biomass, leaf area, and leaf number was observed in some species after transfer of UVB-treated seedlings to a UVB-free environment for 3 wk. The inhibition of postgermination shoot and radicle elongation by UVB radiation, the continuation of growth inhibition after transfer of the treated seedlings to a UVB-free environment, and the differential responses of weedy species could have significant implications for their ability to compete with each other and with associated crops in a UVB-enriched environment that is likely as the stratospheric ozone layer gets depleted.

Nomenclature: Common chickweed, Stellaria media (L.) Vill. STEME; dandelion, Taraxacum officinale Weber in Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; green foxtail, Setaria viridis (L.) Beauv. SETVI; redstem filaree (stork's-bill in Canada), Erodium cicutarium (L.) L'Her. ex Ait. EROCI; spotted catsear, Hypochoeris radicata L. HRYRA.