Influence of ultraviolet-B radiation on crop-weed competitive interactions

A greenhouse study showed that ultraviolet-B (UV-B; 290--315 nm) radiation effects on seedling growth and morphology differed widely among agricultural weeds, vegetable crops, and broccoli cultivars. Relative UV-B-sensitivity of plants within these groups also differed. These differential responses to UV-B radiation may alter competition among associated species for limited resources. Broccoli (Brassica oleracea L. var. italica cv. Purple Sprouting) and lamb's-quarters (Chenopodium album L.) were chosen for competition experiments based on their growth and morphological sensitivity to UV-B radiation. Greenhouse competition experiments were conducted in 1999 (summer) and 2000 (fall) at ambient (4 kJ m -2 d-1) and above-ambient (7 kJ m -2 d-1) biologically effective UV-B (UV-BBE) radiation levels. Broccoli and lamb's-quarters were grown in monocultures (144, 256, and 400 plants m-2) and all binary mixtures in a randomized block design with four replications. Yield per plant declined with increasing species densities; UV-B-effects and treatment interactions were often significant. Inverse yield-density relationships using biomass indicated that in both years broccoli gained in competitiveness relative to lamb's-quarters at above-ambient UV-B radiation. UV-B effects were greater on inter- compared with intraspecific competition. Growth indices (specific leaf weight, leaf weight ratio, leaf area ratio, leaf area index, and shoot:root ratio) indicated that morphology and biomass partitioning were influenced by experimental treatments. Total biomass growth was proportional to growth of biomass components and leaf area. Plant densities had greater influence compared with UV-B-effects on allometric adjustments. Direct UV-B radiation and species densities effects were more often detected in broccoli compared with lamb's-quarters. Broccoli height and leaf greenness increased at above-ambient UV-B radiation, while those of lamb's-quarters declined. Over time, inverse yield-density relationships indicated that the contribution of broccoli to interception of photosynthetically active radiation (PAR) decreased at ambient and increased at above-ambient UV-B radiation. Inverse yield-density relationships using overhead canopy coverage showed that lamb's-quarters was always the stronger competitor. Broccoli however, gained in competitiveness relative to lamb's-quarters at above-ambient UV-B radiation in both years. Overall, this study indicated that architectural and physiological plasticity of broccoli and lamb's-quarters, in response to UV-B radiation, influenced ability of each species within this association to compete for PAR.