Acetolactate synthase (ALS) –inhibitor resistance has been recently documented in a yellow nutsedge biotype in Arkansas rice production, with a target-site mutation resulting in an amino acid substitution from Trp574 to Leu. Preliminary observations have indicated that the resistant biotype showed distinct phenotypic characteristics. Two greenhouse experiments were conducted on the resistant biotype in comparison with three susceptible standards (1) to understand differential growth habit and spatial distribution, and (2) to characterize shoot emergence pattern and seedling vigor. The resistant biotype exhibited a drastically different growth habit with secondary and tertiary basal bulbs emerging away from the parent shoot, resulting in a wider spatial distribution and ground coverage compared to the very compact growth habit of susceptible biotypes. Unlike the susceptible biotypes, the rhizomes developing into tubers were not often connected to the primary basal bulb, but were originating randomly from daughter shoots. The resistant biotype produced an extensive subterranean network of rhizomes and basal bulbs, with wider root spread and distribution compared to the susceptible biotypes. The growth habit of the resistant biotype appeared to be intermediate between yellow and purple nutsedges. Further, the resistant biotype showed a considerably delayed emergence pattern with relatively high levels of tuber dormancy. Although the resistant plants exhibited low early-growth seedling vigor and biomass production compared to the susceptible biotypes (perhaps because of smaller tubers), final aboveground biomass production was greater than that of susceptible biotypes. The overall growth habit and phenotype of the resistant biotype may provide a competitive advantage over adjacent species through the ability to occupy niches and gain improved access to critical resources. The distinct growth pattern may also mean that tillage should not be relied upon for control because it can assist further spread by disconnecting and displacing the chains of rhizomes.
Nomenclature: Yellow nutsedge, Cyperus esculentus L.; purple nutsedge, Cyperus rotundus L.; rice, Oryza sativa L.