The role of plant size in the selection of glyphosate resistance in Sorghum halepense

BACKGROUND: The effect of plant size (seedlings versus young plants versus adult plants) on the phenotypic level of glyphosate resistance and selection intensity (SI) in Sorghum halepense with and without a reduced glyphosate translocation resistance mechanism was evaluated. RESULTS: Resistance parameters [the 50% lethal dose (LD₅₀) and the dose required to cause a 50% reduction in plant growth (GR₅₀)] in adult plants were notably higher than in seedlings regardless of the resistance status. However, under similar plant size increases, populations comprised of glyphosate-resistant (R) individuals showed higher survival and growth when glyphosate treated compared with glyphosate-susceptible (S) plants. An increase in SI was always evident with increasing glyphosate doses. However, the rate of increase in SI was higher under glyphosate selection of young R and S plants, followed by seedlings and adult R and S plants. However, in conditions of R seedlings coexisting with adult S plants under glyphosate treatment (1000–4000 g ha⁻¹), selection against glyphosate resistance was observed. CONCLUSION: Any increase in size from the seedling stage of R plants translates into an amplification of resistance. Depending on the particular size combinations of spatially coexisting R and S plants, selection for glyphosate resistance may be faster, slower or even not evident.