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 ) ¼ 0.83] than by the lipophilic terbuthylazine [log(K ow ) ¼ 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. Keywords: fluorescence induction curve, photochemical quenching, chlorophyll fluorescence, OJIP steps, metamitron, terbuthylazine, dose-response, herbicide injury. ABBASPOOR M, TEICHER HB & STREIBIG JC (2006) The effect of root-absorbed PSII inhibitors on Kautsky curve parameters in sugar beet. Weed Research 46, 226-235. / / Fig. 5 Relationship between dry matter and three selected fluorescence parameters (F V /F m , F Vj and area) for metamitron in the first experiment, and for terbuthylazine in the second experiment at 2 DAT.Photosystem II inhibitors in sugar beet 233
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 − F 0)/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.
Models proposed for risk assessment of chemical mixtures assume no interactions between the chemicals. There are, however, studies indicating that some organophosporous insecticides can inhibit the detoxification of other chemicals in plants thereby enhancing their effect. The present study investigates whether interactions between selected organophosporous insecticides and herbicides can take place in the aquatic algae Pseudokirchneriella subcapitata and the aquatic macrophyte Lemna minor. For both species binary mixtures of the organophosphate insecticides: malathion, endosulfan and chlorpyrifos were tested together with the herbicides metsulfuron-methyl, terbutylazine and bentazone. For mixtures with malathion on algae, dose-response surfaces were made and the results tested against the model of concentration addition (CA) and independent action (IA). The Lemna minor tests showed no indication of synergy for any of the combinations, on the contrary, significant antagonism was found for several of the mixtures. The response surface analysis showed antagonism in relation to both concentration addition and independent action for mixtures between malathion and metsulfuron-methyl and terbuthylazine, while the mixtures with bentazone could be explained with CA. The study shows no indications of synergistic interactions between the tested pesticides, confirming the applicability of CA as a reference model predicting mixture effects of pesticides for aquatic plants and algae.
Desmedipham, phenmedipham and a 50% mixture of the two decreased the maximum quantum efficiency of photosystem II (F(v)/F(m)) and the relative changes at the J step (F(vj)) immediately after spraying in both sugar beet and black nightshade grown in the greenhouse. Sugar beet recovered more rapidly from phenmedipham and the mixture than from desmedipham. Desmedipham and the mixture irreversibly affected F(v)/F(m) and F(vj) in black nightshade at much lower doses than in sugar beet. Black nightshade recovered from phenmedipham injury at the highest dose in the first experiment (120 g AI ha(-1)) but not in the second experiment (500 g AI ha(-1)). The dry matter dose-response relationships and the energy pipeline presentation confirmed the same trend. There was a relatively good correlation between F(vj) taken 1 day after spraying and dry matter taken 2 or 3 weeks after spraying. The differential speed of herbicide metabolism between weed and crop plays an important role in herbicide selectivity and can be studied by using appropriate chlorophyll a fluorescence parameters.
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