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

Abstract

SummaryUltraviolet‐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‐BBE 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‐BBE 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‐BBE radiation, largely because of reduced interspecific competitive influences. Conversely, substitution rates increased for C. album grown at 7 kJ m−2 day−1 UV‐BBE 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‐BBE radiation in both years, and at 7 kJ m−2 day−1 UV‐BBE radiation in 1999. In 2000, broccoli was the stronger competitor at 7 kJ m−2 day−1 UV‐BBE 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‐BBE 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.