Competitive interactions in mixtures of broccoli and Chenopodium album grown at two UV-B radiation levels under glasshouse conditions

Ultraviolet-B radiation effects on intra- and interspecific competition in broccoli (Brassica oleracea) and Chenopodium album were studied using bivariate factorial experiments. A randomized block design was used in which three monoculture densities for each species [144 (low), 256 (medium), and 400 (high) plants m⁻²] and all binary combinations were grown in a glasshouse at two (4 and 7 kJ m⁻² day⁻¹) UV-B_BE radiation levels for 4 weeks in 1999 and 5 weeks in 2000. Inverse yield–density relationships were more discernible at 4, compared with 7 kJ m⁻² day⁻¹ UV-B_BE radiation. Substitution rates, indicating the balance of intra- to interspecific competitive effects, declined for broccoli at 7, compared with 4 kJ m⁻² day⁻¹ UV-B_BE radiation, largely because of reduced interspecific competitive influences. Conversely, substitution rates increased for C. album grown at 7 kJ m⁻² day⁻¹ UV-B_BE radiation, as a result of both decreased intraspecific and increased interspecific competition. Interspecific competitive effects were influenced more than intraspecific competitive effects by UV-B radiation. Based on relative magnitude of substitution rates, C. album was a stronger competitor than broccoli at 4 kJ m⁻² day⁻¹ UV-B_BE radiation in both years, and at 7 kJ m⁻² day⁻¹ UV-B_BE radiation in 1999. In 2000, broccoli was the stronger competitor at 7 kJ m⁻² day⁻¹ UV-B_BE radiation. Overall, the relative competitiveness of broccoli was enhanced, while that of C. album diminished at 7, compared with 4 kJ m⁻² day⁻¹ UV-B_BE radiation. These findings indicate that above-ambient UV-B radiation has the potential to alter crop–weed competitive interactions, which could change acceptable weed threshold levels.