A population of shepherd's-purse suspected to be resistant to the triazinone herbicide hexazinone, a photosystem II (PS II) inhibitor, was collected from an alfalfa field in 2007 in Oregon. A whole-plant, dose–response assay confirmed that the putative-resistant population was highly resistant to hexazinone. The resistant population was 22-fold more resistant to hexazinone than the susceptible population. However, the hexazinone-resistant population was susceptible to other PS II-inhibiting herbicides, including atrazine, diuron, and terbacil. DNA sequence analysis of the chloroplast psbA gene encoding the D1 protein of PS II, the target site of PS II inhibitors, identified a point mutation from Phe to Ile at position 255 in the hexazinone-resistant population. Single- and double-point mutations at position 255, which is located in the Q_B binding niche of the D1 protein, were previously reported in Chlamydomonas reinhardtii, Synechococcus species, and Synechocystis species after site-directed mutagenesis and were associated with decreased binding of PS II inhibitors. To our knowledge, this is the first report of a mutation of the psbA gene at Phe₂₅₅ in a field-selected, herbicide-resistant plant.

Nomenclature: Atrazine; diuron; hexazinone; metribuzin; terbacil; alfalfa, Medicago sativa L. MEDSA; shepherd's-purse, Capsella bursa-pastoris (L.) Medik CAPBP.