Clodionafop, an acetyl-coenzyme A carboxylase (ACCase) inhibitor, changed the shape of the chlorophyll fluorescence induction curve (Kautsky curve) in barley and oat in greenhouse experiments. Biomass ED₅₀, based on log-logistic dose–response curves, for barley was considerably higher than that for oat in all experiments. Biomass ED₅₀ and relative potency (ED₅₀ [barley]/ED₅₀ [oat]) were consistent among experiments when sprayed at the same phenological stage of plant development. Especially at high doses, clodinafop changed the shape of the Kautsky curve more for oat than for barley. From the numerous parameters that can be derived from the OJIP steps of the Kautsky curve, we found that (1) F_vj, the relative changes at the J step [F_vj = (F_m − F_j)/F_m], (2) area between Kautsky curve and maximum fluorescence (F_m), and (3) F_v/F_m, maximum quantum efficiency of Photosystem II [F_v/F_m = (F_m − F₀)/F_m], were closely linked to the biomass dose–response relationships for both species. The linkage between biomass and the fluorescence parameters may be used to shorten the screening period for ACCase inhibitors.

Nomenclature: Clodinafop; barley, Hordeum vulgare L.‘Oatira’; oat, Avena sativa L. ‘Revisor’.