Physiological Fitness Cost Associated with Glyphosate Resistance in Echinochloa colona: Seed Germination Ecology

Abstract

The expression of fitness cost associated with herbicide resistance in weeds is not universal and may vary during different life stages. Hence, an analysis of the fitness cost of a glyphosate-resistant Echinochloa colona at the seed stage was conducted. Seed germination ecology of glyphosate-susceptible and - resistant E. colona was studied to examine the physiological fitness cost between the S (susceptible) and R (resistant) phenotypes. The plant material was selected from within one segregating glyphosateresistant E. colona population to minimise allelic interference from other fitness-related loci. Experiments were conducted in a growth incubator at alternating temperatures of 30/20C day/night with a 12-h photoperiod. Seed dormancy and germination were also tested under constant darkness at the same temperature regime. Seeds of both the S and R phenotypes kept at warmer temperatures (after-ripening dry storage at 15 to 35C) or on the ground surface outdoors for 14 days, germinated better (with more than 95% of seeds had germinated) than those seeds (about 20% germination) that were kept at lower temperature (8C) for the same period of storage. Light and longer dry after-ripening times increased the rate of seed dormancy release and germination (almost 100% seed germination) for both the S and R phenotypes. Equal decline in seedling emergence in both the S and R phenotypes was evident with increasing soil burial depth (P > 0.05). Thus, the S and R phenotypes exhibited similar characteristics of seed dormancy release, germination and seedling emergence. As there is lack of physiological fitness cost in the R phenotype of E. colona at the seed and early seedling stage, it remains a challenge to develop specific control measures for the R phenotype. An alternative herbicide with different mode of action should be integrated with other weed management approach such as cultural, mechanical and biological control to reduce sole reliance of glyphosate in controlling the R phenotype of this E. colona population.