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

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 plantswere 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 comparedwith glyphosate-susceptible (S) plants.An increase in SIwas always evidentwith 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 fromthe 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.