Glufosinate-Ammonium: Discovery and Physicochemical Properties

General Description

Glufosinate-ammonium, discovered from the actinomycetes Streptomyces hygroscopicus and S. viridochromogenes, is derived from the tripeptide bialaphos and metabolized into the active herbicide L-phosphinothricin. Commercially introduced in the 1990s in the U.S. and Canada as a non-selective herbicide, Glufosinate-ammonium provides an alternative to glyphosate, especially amid rising glyphosate-resistant weeds. Glufosinate-ammonium is not volatile, with a logKow of −4.0, and presents unique physicochemical properties that influence its uptake and translocation in plants. Its continued relevance in agriculture is expected due to its effectiveness in managing resistant weed populations and the limitations of new herbicide developments.

Figure 1. Glufosinate-ammonium

Discovery

Origin and Initial Research

Glufosinate-ammonium was first discovered through the work of two distinct research teams exploring the actinomycetes Streptomyces hygroscopicus and S. viridochromogenes. These microorganisms were found to produce a tripeptide known as bialaphos, which is composed of L-homoalanyl, L-alanine, and a unique phosphinoyl group. The production of bialaphos involves a complex process that spans over 18 steps, starting from phosphoenolpyruvate. While bialaphos serves as an important herbicide in Eastern Asia, it is the metabolism of this compound within plants that leads to the generation of L-phosphinothricin, the active and phytotoxic element responsible for its herbicidal activity. As Glufosinate-ammonium, the active ingredient is typically utilized in forms that are racemic mixtures of D- and L-phosphinothricin, although the most utilized formulations are the ammonium-salt derivatives. ¹

Development and Commercialization

The commercialization of glufosinate-ammonium began in the United States and Canada during 1993-1994 as a broad-spectrum non-selective herbicide. As weed management evolved, so did the need for alternatives to glyphosate, which had become heavily relied upon but faced resistance issues in various weed populations. Glufosinate-ammonium emerged as a viable alternative and was particularly adopted in regions like the Midwest and Southern United States, where glufosinate-resistant crops, such as soybean and cotton, became prevalent. By 2014, approximately 12 million hectares globally were treated with glufosinate, reflecting its increasing importance in agricultural practices, especially in farming systems adopting no-till strategies. However, its effectiveness compared to glyphosate has been a matter of discussion, leading to varied adoption rates based on specific crop and weed challenges. ¹

Future in Agriculture

Looking ahead, the usage of glufosinate-ammonium is anticipated to rise further due to the prevalence of glyphosate-resistant weeds and the emergence of herbicide-resistant crop technologies. The distinct characteristics of glufosinate, which is not classified as volatile and can effectively combat various weed species that have developed resistance to other herbicides, contribute to its ongoing relevance. In regions like Brazil, where grass weeds are particularly problematic and alternatives to certain banned herbicides, such as paraquat, are limited, glufosinate-ammonium will play a crucial role in weed management strategies. Moreover, while research continues into developing new herbicides, the lack of new non-selective herbicides expected in the near term suggests that glufosinate-ammonium will remain a critical tool for farmers combating resistant weed populations in diverse cropping systems. ¹

Physicochemical Properties

Glufosinate-ammonium, a phosphinic acid, possesses unique physicochemical properties that distinguish it from other herbicides in the organophosphorus chemical family. With a molecular weight of 181.13 g mol−1, Glufosinate-ammonium is characterized by its highly hydrophilic nature, demonstrated by a logKow value of −4.0. This extreme hydrophilicity influences its behavior in agricultural applications and provides insight into its uptake mechanisms in plants. The ionization constants (pKa values of 2, 2.9, and 9.8) highlight the structural presence of amine and hydroxyl groups, which facilitate various interactions in the plant environment. Glufosinate-ammonium is classified as non-volatile with a vapor pressure of 10−4 Pa, indicating its stability in the field. Available formulations are primarily ammonium salts, which dissociate in aqueous solutions, thereby allowing efficient use in herbicide applications. ²

Uptake and Translocation Mechanisms

The uptake and translocation of glufosinate-ammonium in plants are influenced by several factors, including its concentration and the presence of competing metabolites like glutamine. At low concentrations, Glufosinate-ammonium uptake is facilitated through active transporters, but at typical field application rates, absorption occurs predominantly via cell-to-cell diffusion. This process relies heavily on the concentration gradient of Glufosinate-ammonium between cellular interiors and exterior environments. Furthermore, translocation mainly occurs via apoplastic movement in the xylem, which is contingent on transpiration rates. As glufosinate translocates, it tends to accumulate in older leaves with greater transpiration, potentially affecting its efficacy as a herbicide. Notably, the rapid action of Glufosinate-ammonium does not inhibit its translocation, yet it exhibits low movement across lipophilic membranes, a limitation not shared by some of its counterparts, like glyphosate. This complex interplay of physicochemical properties significantly impacts the functional dynamics of glufosinate-ammonium in agricultural practices. ²

Reference

1. Takano HK, Dayan FE. Glufosinate-ammonium: a review of the current state of knowledge. Pest Manag Sci. 2020; 76(12): 3911-3925.

2. Takano HK, Beffa RS, Preston C, Westra P and Dayan FE, Physiological factors affecting uptake and translocation of glufosinate. J Agric Food Chem. 2020; 68: 3026–3032.