[Effect of enhanced UV-B radiation on competition among several plant species]

The effect of enhanced UV-B radiation (280-315 nm, approximating a 15% ozone layer reduction) on competitive interaction between spring wheat (Triticum aestivum) and wild oat (Avena fatua) was examined in the field. The density-dependent mortality of both wheat and wild oat did not exhibit a significant difference between control and UV-B treatments. A relatively high degree of competitive stress enhanced the effect of UV-B stress on biomass reduction. Under UV-B enhancement, the relative competitive status of wheat in terms of total biomass and aboveground biomass increased, but decreased when based upon grain production. Shifts in competitive balance occurred with significant changes in total biomass, especially when plants grew at higher densities in monocultures and mixtures. There were different effects in direction and intensity of UV-B radiation on competition balance of the other three species pairs, but in general, the effect of UV-B was in favor of monocotyledonous. The results implied that total degree of competitive stress, especially interspecific competition, might be crucial to assess the effect of UV-B enhancement on agriculture ecosystem properly.