Water-Seeded Rice Seedling Response to Soil-Water Partitioning of Pendimethalin

Abstract

Weed management in California water-seeded rice (Oryza sativa L.) is challenging due to herbicide-resistant weeds. Research on additional herbicide options is necessary to control herbicide-resistant weeds. Pendimethalin is a dinitroaniline herbicide commonly used in dry-seeded rice; however, it is not registered in water-seeded rice. This study was conducted to determine the pendimethalin fate in water-seeded rice after application at 1-, 3-, and 5-leaf stage rice. Ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was utilized to quantify pendimethalin and degradants in the water, soil and rice seedling tissue at 1, 5, and 14 days after treatment. Over 50% of recovered pendimethalin was observed in the rice tissue and over 25% in the soil and least observed in the water at all application timings and sampling dates. Three pendimethalin degradants were observed at low concentrations: p36 [1-(1-ethylpropyl)-5,6-dimethyl-7-nitro-1H-benximidazole], p44 [4-[(1-ethylpropyl) amino]-2-methyl-3,5-ditrobenzoic acid] and p48 [4,5-dimethyl-3-nitro-N²-(pentan-3-yl) benzene-1,2-diamine]. The degradant p36 was observed in all samples and most abundant in the soil. The degradants p36 and p44 increased in concentration in the water by 14 days after treatment. The degradants p44 and p48 were at low concentrations or below the lowest level of quantification in water, soil, and tissue samples. The pendimethalin parent molecule remained intact and was not readily metabolized in rice tissue. The crown region and shoots of the rice seedlings demonstrated greater pendimethalin concentrations compared to the roots at all rice stages; however, pendimethalin concentrations remained similar across the three sample timings. Rice root and shoot reduction was 16 and 13%, respectively, after the 1-leaf stage application averaged over sample timings, and 6 and 4% after the 5-leaf stage application. The results suggest the rice stage at the application timing is an important factor for pendimethalin tolerance; therefore, encouraging early root development can be beneficial for pendimethalin tolerance in water-seeded rice.