Interaction of Glufosinate and Colletotrichum truncatum on Ammonia Levels and Glutamine Synthetase Activity in Hemp Sesbania

Hemp sesbania (Sesbania exaltata) is an erect, annual plant noted for prolific branching, large, yellow flowers, and distinctive curved seed pods and has high reproductive capacity. It is a vigorous, nodulating, epigeal, leguminous weed in soybean, cotton and rice and can attain a height of 3 m. It is also deemed one of the 10 most troublesome weeds in the southern U.S. and can reduce crop yield through shading and competition. Until now, no pre- or post-emergent herbicide can provide season-long control of hemp sesbania.

In fact, biological weed control using plant pathogens (bioherbicides) has received attention by academic and industrial researchers for quite some time and combinations of bioherbicides and herbicides have been examined to discover possible synergistic interactions to improve weed control efficacy. However, many herbicides exhibit toxicity towards bioherbicides, caused by either the active ingredient and/or the various adjuvants, surfactants, etc. (inert ingredients) formulated in the commercial products.

The enzyme glutamine synthetase (GS) plays a pivotal role in nitrogen metabolism and is responsible for the condensation of ammonium ion and glutamate to produce glutamine. This metabolic reaction is vital for glutamine synthesis and for ammonia re-assimilation and detoxification in plants. Glufosinate, originally isolated from Streptomyces species as phosphinothricin, inhibits GS and causes toxic levels of ammonia to accumulate in plants.

In this study, the authors aimed to test for these possible interactions by a series of growth, GS assay, and ammonia analysis experiments using hemp sesbania treated with Colletotrichum truncatum (CT), glufosinate, and their combination under controlled environmental conditions.

These experiments demonstrated that glufosinate inhibition of growth was expressed in the combination of herbicide and bioherbicide treatment, and the effect on collapsed stem tissue also appeared, but was generally weaker and appeared later in the time course. This could be explained since glufosinate exhibited some growth inhibition to C. truncatum when conidia were grown on PDA supplemented with the herbicide. Compared to glufosinate, CT treatment did not cause substantial growth reduction or GS inhibition and ammonia increases until after ~40 h.

Article by Robert E. Hoagland, et al, from USDA-ARS, Stoneville, MS, USA.

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