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Rhizosphere Conference

Effect of Corn Root Exudates on the Degradation of Atrazine and Its Chlorinated Metabolites in Soils

^a Department of Microbiology, Cornell University, Wing Hall, Ithaca, NY 14853
^b University of Zurich, Institute of Organic Chemistry, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
^c Swiss Federal Institute for Environmental Science and Technology, Ueberlandstrasse 133, CH-8600 Dubendorf, Switzerland
^d FAL) Reckenholz, Swiss Federal Research Station for Agroecology and Agriculture, Reckenholzstrasse 191, CH-8046 Zurich, Switzerland
^e Institute of Terrestrial Ecology (ITO), ETH Zurich, Grabenstrasse 3, CH-8952 Schlieren, Switzerland

^* Corresponding author (kmw44{at}cornell.edu)

Received for publication November 4, 2004. DIMBOA (3,4-dihydro-2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one), a major benzoxazinone of Poaceae plants, was isolated and purified from corn seedlings. The effect of isolated and purified DIMBOA on the degradation of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], and its toxic breakdown products, desethylatrazine [2-chloro-4-amino-6-(isopropylamino)-s-triazine; DEA] and desisopropylatrazine [2-chloro-4-(ethylamino)-6-amino-s-triazine; DIA], was studied in the absence of plants using batch experiments, while the effect of corn root exudates on these compounds was determined in hydroponic experiments. Degradation experiments were performed in the presence and absence of 50 µM, 1 mM, or 5 mM DIMBOA resulting in ratios of DIMBOA to pesticide of 1:1, 20:1, and 100:1. We observed a 100% degradation of atrazine to hydroxyatrazine within 48 h at a ratio of DIMBOA to atrazine of 100:1. DIMBOA had the largest effect on atrazine, while it was about three times less effective on DEA and DIA. Corn (Zea mays L. cv. LG 2185) was exposed to 10 mg L^–1 of either atrazine, DEA, or DIA for 11 d in a growth chamber experiment. Up to 4.3 µmol L^–1 d^–1 of hydroxyatrazine were formed in the nutrient solutions by plants exposed to atrazine, while the formation of hydroxylated metabolites from plants exposed to DEA and DIA was smaller and also delayed. The formation of hydroxylated metabolites increased in the solution with plant age in all atrazine, DEA, and DIA treatments. HMBOA (3,4-dihydro-2-hydroxy-7-methoxy-2H-1,4-benzoxazin-3-one), the lactam precursor of DIMBOA, and a tentatively identified derivative of MBOA (2,3-dihydro-6-methoxy-benzoxazol-2-one) were detected in the corn root exudates. Mass balance calculations revealed that up to 30% of the disappearance of atrazine and DEA, and up to 10% of DIA removal from the solution medium in our study could be explained by the formation of hydroxylated metabolites in the solution itself. Our results show that higher plants such as corn have the potential to promote the hydrolysis of triazine residues in soils by exudation of benzoxazinones.

Abbreviations: BPC, base peak chromatogram • ESI, electrospray ionization • HPLC, high performance liquid chromatography • MS, mass spectrometry • t_r, retention time

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