The effect of root-absorbed PSII inhibitors on Kautsky curve parameters in sugar beet

The effects of the photosystem II inhibitors metamitron and terbuthylazine on the shape of the Kautsky (chlorophyll fluorescence induction) curve were investigated in sugar beet grown in hydroponic culture. The objective of the study was to trace recovery processes following herbicide injury using Kautsky curve parameters. Metamitron is used for selective weed control in sugar beet because it is metabolized in this crop. In contrast, terbuthylazine is toxic to sugar beet. Two hours after treatment, various fluorescence induction curve parameters, such as maximum quantum efficiency (F_V/F_m), the relative changes at the J step (F_vj) and area (the area between the Kautsky curve and maximum fluorescence, F_m), were affected by metamitron at concentration ranges of 70–280 mg active ingredient (a.i.) L^?1 in plants treated at the four‐true‐leaf stage. Shortly after herbicide application, F_v/F_m was more affected by the hydrophilic metamitron log(K_ow) = 0.83] than by the lipophilic terbuthylazine log(K_ow) = 3.21], but these differences between compounds were alleviated as metamitron was metabolized and terbuthylazine was not. Terbuthylazine at 1 mg a.i. L^?1 affected sugar beet at the four‐ and six‐true‐leaf stages to the same extent, whereas metamitron at a dose of 140 mg a.i. L^?1 affected much more at four‐ than at the six‐true‐leaf stage. Sugar beet recovered from metamitron injury even at high doses (140 and 280 mg a.i. L^?1). Fluorescence induction curve parameters were similarly affected by terbuthylazine and, although sugar beet recovered from terbuthylazine injury at low doses (<0.2 mg a.i. L^?1), the Kautsky curve was irreversibly affected at higher doses (1–10 mg a.i. L^?1), leading finally to plant death. Older plants were affected later, and recovered sooner, from both herbicides.