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The net selection effect of herbicides on herbicide-resistance traits in weeds is conditioned by the fitness benefits and costs associated with resistance alleles. Fitness costs play an important evolutionary role preventing the fixation of adaptive alleles and contributing to the maintenance of genetic polymorphisms within populations. Glyphosate is widely used in world agriculture, which has led to the evolution of widespread glyphosate resistance in many weed species. The fitness of glyphosate-resistant and -susceptible perennial ryegrass plants selected from within a single population were studied in two field experiments conducted during 2011 and 2012 under different soil water availability. Glyphosate-resistant plants showed a reduction in height of 12 and 16%, leaf blade area of 16 and 33%, shoot biomass of 45 and 55%, seed number of 33 and 53%, and total seed mass of 16 and 5% compared to glyphosate-susceptible plants in 2011 and 2012, respectively. The reduction in seed number per plant resulted in a 40% fitness cost associated with the glyphosate-resistance trait in perennial ryegrass. Fitness costs of glyphosate-resistant plants were expressed under both conditions of water availability. These results could be useful for designing management strategies and exploiting the reduced glyphosate-resistant perennial ryegrass fitness in the absence of glyphosate selection. Nomenclature: Glyphosate; perennial ryegrass, Lolium perenne L., LOLPE.

Acciaresi HA & Guiamet JJ (2010). Below‐ and above‐ground growth and biomass allocation in maize and Sorghum halepense in response to soil water competition. Weed Research50, 481–492. Summary The aim of this study was to establish the morphological or physiological nature of the changes in growth parameters of maize and Sorghum halepense plants growing under competition for water. Plant growth analysis studies in semi‐controlled conditions in plastic containers were conducted in two consecutive years. The competition treatments were established 4 weeks after emergence of both species and lasted for 4 weeks. Treatments of low (75% of field capacity) and very low water availability (irrigation withheld) were established. The variables obtained were: relative growth rate (RGR), leaf area ratio (LAR), net assimilation rate (NAR), specific leaf area (SLA), leaf mass fraction (LMF), rhizome mass fraction (rizMF), root length ratio (RLR), root mass fraction (RMF), specific root length (SRL) and length of fine roots (VfRL). The RGR of S. halepense was less affected by the two levels of competition than that of either of two maize hybrids. The weed was able to maintain its ability to grow, due to the maintenance of NAR during competition. A similar contribution of biomass from leaves and rhizomes to roots was observed in S. halepense, favouring the formation of very fine roots. In contrast, in maize, the decrease in RGR was due to a decline in NAR and there was no formation of fine roots to maintain water absorption during competition. The greater increase both in biomass partitioned to roots and root length by S. halepense, might negatively impact maize ability to compete for water during the critical competition period.

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