Reduced Growth, Competitiveness, and Photosynthetic Efficiency of Triazine-Resistant Senecio Vulgaris from California

(1) Triazine-susceptible and resistant biotypes of Senecio vulgaris, collected from the same field in Salinas, California, were compared to determine whether reduced productivity and competitiveness accompany the trait of triazine resistance in biotypes with identical environmental histories. Characteristics of growth, resource allocation, development and photosynthetic efficiency were measured on plants in non-competitive plantings; competitiveness was evaluated using the replacement series design. (2) In both non-competitive and competitive plantings, susceptible plants produced more total dry weight and reproductive output than did resistant plants. No differences were detected in number of leaves, root-shoot ratio, total leaf area, relative growth rate (RGR) or net assimilation rate (NAR). (3) Under non-competitive and competitive conditions, resistant plants produced relatively more leaf and less reproductive tissue than susceptible ones; leaf area ratio (LAR) was greater in resistant plants, as well. (4) The initial rise in leaf chlorophyll fluorescence was higher in resistant than susceptible plants, suggesting that photosystem II is less efficient, as has been reported for triazine-resistant biotypes of other species.