MYCORRHIZAE, BIOCIDES, AND BIOCONTROL, .1. HERBICIDE-MYCORRHIZA INTERACTIONS IN SOYBEAN AND COCKLEBUR TREATED WITH BENTAZON

Interactions between herbicides and vesicular-arbuscular mycorrhizal ( VAM) fungi are little-known but may differentially affect the developm ent of tolerant and susceptible plants. We conducted this greenhouse s tudy to determine if VAM fungi modify the effects of the herbicide ben tazon (3-isopropyl-1H-2,1,3-benzothidiazin-(4)3H-one 2,2-dioxide) on s oybean (Glycine mar (L.) Merr.) and common cocklebur (Xanthium strumar ium L.). The experiment was designed as an eight-treatment, 2 x 2 x 2 factorial, Individual potted soybean or cocklebur plants were grown in a high (28 mg kg(-1)) P-content soil. The plants were colonized by VA M fungi (a mixture of Glomus etunicatum Becker and Gerdemann, Glomus l eptotichum Schenck and Smith, and Glomus mosseae (Nicol. and Gerd.) Ge rd. and Trappe) and sprayed with bentazon. Alternatively, they were ex posed only to VAM fungi or to the herbicide, or to neither of these fa ctors. The effects of VAM fungi on plant dry mass were small, but larg er in cocklebur than in soybean, especially for the herbicide-treated plants. Root/shoot ratios in cocklebur were larger and shoot dry matte r content smaller than those in soybean. The VAM effect in the herbici de-treated plants was greater than in the nontreated ones for both par ameters. Colonized root length in cocklebur was drastically reduced by the herbicide (43%), as were leaf dry weight and N, P, and K concentr ations. Changes were small, and generally not significant in soybean. Gradual necrosis of treated cocklebur shoots was related to an acceler ated loss of shoot dry matter, especially in VAM plants. The data sugg est that changes in source-sink relations following herbicide treatmen t favor enhanced export of shoot dry matter to the roots in susceptibl e relative to tolerant plants. This biomass may be preferentially avai lable for export from susceptible-plant roots to the soil system throu gh the VAM mycelium.