EARLY-SEASON EFFECTS OF SUPPLEMENTED SOLAR UV-B RADIATION ON SEEDLINGEMERGENCE, CANOPY STRUCTURE, SIMULATED STAND PHOTOSYNTHESIS AND COMPETITION FOR LIGHT

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (A vena fatua), a common weedy competitor of wheat, were exposed to enhan ced solar UV-B radiation simulating a 20% reduction in stratospheric o zone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of fiel d study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-s eason measurements showed significant UV-B inhibition of leaf insertio n height in wild oat in mixture and monoculture in the second year (ir rigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B tre atment had no detectable effect on monoculture or total (combined spec ies) mixture LAI but did significantly increase (5-7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reduct ions in wild oat LAI in lower canopy layers in both years while midsea son Year 2 mixtures showed significant reductions in wild oat LAI in u pper canopy layers. The changes in canopy structure were found to sign ificantly increase (6-7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findi ngs, and others, indicate that the effects of UV-B enhancement on comp etition are realized very early in canopy development and provide addi tional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering co mpetitive interactions for light.