Resistance profile study of SU‐resistant <i>Sagittaria trifolia</i> in a comparison of seedlings and tuber‐derived plants

Abstract Sagittaria trifolia , a perennial weed of Alismataceae in Japanese rice paddies, has a single ALS gene, and both target and non‐target‐site resistance are known in the SU resistance of this species. In this study, the target‐site resistance profile at the whole‐plant and enzyme levels was investigated using self‐pollinated S 1 seedlings from a parent plant heterozygous for a Pro 197 Ser ALS mutation and their clonal tubers. Our objectives were to reveal differences and similarities between whole‐plant resistance of seedlings to imazosulfuron and that of tuber‐derived plants and to correlate the whole‐plant responses to enzymatic inhibition, as well as to quantify the effects of mutant ALS allele stacking. Genotyping confirmed a Mendelian segregation ratio (1:2:1) among S 1 plants for wild‐type, heterozygous, and homozygous ALS mutations. Enzymatic assays showed that resistance correlated with the number of mutant alleles, and heterozygous plants exhibited a double‐sigmoid curve plateauing at 50%, indicating the function of a 50:50 wild‐type to mutant enzyme ratio. Studies of tuber‐derived plants showed that resistance increased with more mutant alleles, but with a disproportionate jump over 3000‐fold from wild‐type to heterozygous, and only 5‐fold from heterozygous to homozygous, indicating a saturating tendency in the stacking effect of mutations. The dose response of S 1 seedlings was consistent with Mendelian segregation of genotypes and suggested a similar saturation of resistance increase due to allele stacking as observed in tubers, with comparable dose–response patterns across identical genotypes.