Fitness Costs Associated to an Ile2041Asn Mutation in the Geophyte Sorghum halepense Resistant to ACCase-Inhibiting Herbicides

Sorghum halepense (L.) Pers. is a troublesome weed of maize which reproducesthrough seeds and rhizomes. Resistance to ACCase-inhibiting herbicides due to atarget-site mutation Ile-Asn in position 2041 of the ACCase gene was confirmed in severalpopulations in Italy. The presence of fitness costs related to the allelic variant Ile2041Asnhas never been investigated in ACCase-resistant S. halepense. Three growth analysespot experiments were performed starting from rhizome buds (Exp1 and Exp3) or seed(Exp2). Plant development was monitored throughout the life cycle through destructivesamplings. The mutation responsible for resistance to ACCase inhibitors was detected inall plants included in the experiments using a Cleaved Amplified Polymorphic Sequencemethod. Classical growth analysis was performed for the multiple sampling experiments(Exp1 and Exp2) and allometric relationships were calculated for the different dry weightplant parts in all experiments. Allometric relationships showed that susceptible (S) andACCase resistant (R) biotypes allocated biomass differently in the reproductive structures:S populations allocated on average 30% more biomass in the panicles, whereas Rpopulations allocated 12% more biomass to rhizomes than S populations. This wasmore evident when the experiments started from rhizomes buds. Reproductive allocationand reproductive effort were higher in S biotypes and R biotypes, respectively. Thehigher production of rhizomes and the lower production of seeds of the R biotype cansignificantly impact the evolution of herbicide resistance under continuous application ofherbicides with the same mode of action. A medium-long term resistance managementstrategy which implements appropriate soil tillage and crop rotation taking into accountthe different reproductive attitude of biotypes S and R should be devised.