Seedling growth and leaf surface morphological responses of three rangeland weeds to ultraviolet-B radiation.
The influence of ultraviolet-B (UV-B) radiation on seedling growth and leaf surface characteristics of the important rangeland weeds Cynoglossum officinale (hounds-tongue), Centaurea diffusa (diffuse knapweed), and Tragopogon pratensis (meadow salsify) was investigated in a greenhouse by exposing seedlings to 0, 7, or 11 kJ m-2 day-1 of biologically effective UV-B radiation. UV-B radiation reduced leaf, stem, and root fresh weights, leaf area, and leaf:shoot ratio and increased shoot dry matter content, specific leaf weight, and leaf greenness of Cynoglossum officinale seedlings. Leaf, stem, and root fresh weight, and leaf area of Centaurea diffusa seedlings decreased, whereas shoot dry matter content, specific leaf weight, leaf:shoot ratio, and leaf greenness were unaffected. T. pratensis seedlings exposed to UV-B radiation had diminished leaf greenness, but no change was observed in leaf or stem fresh weight, leaf area, or leaf:shoot ratio. Uniseriate trichome abundance in Cynoglossum officinale seedlings increased and trichome orientation changed in response to UV-B exposure. UV-B treatments decreased uniseriate trichome abundance in Centaurea diffusa; glandular trichome abundance and the ratio of glandular:uniseriate trichomes were not affected. T. pratensis seedlings showed no change in the amount of epicuticular wax on the adaxial leaf surface in response to UV-B irradiation. Shoot dry weight was not influenced by removal of epicuticular wax prior to UV-B exposure. Results suggested that epicuticular wax is not an important factor in the acclimation of T. pratensis seedlings to UV-B radiation. Susceptibility to UV-B-induced damage was greatest for Cynoglossum officinale seedlings and least for T. pratensis seedlings. Modification of plant morphology or reduction in growth induced by enhanced UV-B radiation may influence competitive relationships between rangeland weeds and their associated forage species.