Abstract
Sorghum halepense (L.) Pers. is a troublesome weed of maize which reproduces through seeds and rhizomes. Resistance to ACCase-inhibiting herbicides due to a target-site mutation Ile-Asn in position 2041 of the ACCase gene was confirmed in several populations in Italy. The presence of fitness costs related to the allelic variant Ile2041Asn has never been investigated in ACCase-resistant S. halepense. Three growth analyses pot experiments were performed starting from rhizome buds (Exp1 and Exp3) or seed (Exp2). Plant development was monitored throughout the life cycle through destructive samplings. The mutation responsible for resistance to ACCase inhibitors was detected in all plants included in the experiments using a Cleaved Amplified Polymorphic Sequence method. Classical growth analysis was performed for the multiple sampling experiments (Exp1 and Exp2) and allometric relationships were calculated for the different dry weight plant parts in all experiments. Allometric relationships showed that susceptible (S) and ACCase resistant (R) biotypes allocated biomass differently in the reproductive structures: S populations allocated on average 30% more biomass in the panicles, whereas R populations allocated 12% more biomass to rhizomes than S populations. This was more evident when the experiments started from rhizomes buds. Reproductive allocation and reproductive effort were higher in S biotypes and R biotypes, respectively. The higher production of rhizomes and the lower production of seeds of the R biotype can significantly impact the evolution of herbicide resistance under continuous application of herbicides with the same mode of action. A medium-long term resistance management strategy which implements appropriate soil tillage and crop rotation taking into account the different reproductive attitude of biotypes S and R should be devised.
Highlights
Fitness is a phenotypic response resulting from the combination of evolved life-history traits and is significantly influenced by environmental (Frenkel et al, 2017) and genetic variations (Vila-Aiub et al, 2015; Cousens and Fournier-Level, 2018)
Previous research revealed that S populations were totally controlled by the recommended field rate of both aryloxyphenoxypropionate (FOP) and cyclohexanedione (DIM) families of the Acetyl-CoA Carboxylase (ACCase) inhibitors, whereas the R populations survived the application of high rates of FOP herbicides, but showed a slight loss of susceptibility to DIMs herbicides (Scarabel et al, 2014)
It was enough to detect the presence of the mutation 2041 and CAPS analysis confirmed its presence in all plants of R populations: all 16 plants analyzed in Exp1, 24 in Exp2 and 16 in Exp3 had the undigested band at 360 bp indicating that at least a mutated allele 2041-Asn was present
Summary
Fitness is a phenotypic response resulting from the combination of evolved life-history traits and is significantly influenced by environmental (Frenkel et al, 2017) and genetic variations (Vila-Aiub et al, 2015; Cousens and Fournier-Level, 2018). A plant fitness cost associated with evolved herbicide resistance is seen as an “adaptation cost” to the herbicide (Vila-Aiub et al, 2011). The generalization of this concept has been challenged by studies demonstrating that herbicide resistance is not consistently associated with reduced fitness. The phenotypic expression of “resistance costs” depends on weed species, type of mutation, genetic background, dominance of the fitness cost and environmental conditions (Roux et al, 2004; Vila-Aiub, 2019)
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