Mode of action, crop selectivity, and soil relations of the sulfonylurea herbicides

Abstract

The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2-75 g ha-'), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results @om rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range fiom 1-5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives >20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative 0-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combinatiori of bridge hydrolysis and microbial degradation. Hydrolysis is signijicantly faster under acidic (PH 5 ) than alkaline (PH 8 ) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of I d weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensuljiuron-methyl) have been developed.

Eficácia de controle de plantas daninhas e toxicidade ao milho da mistura de foramsulfuron e Iodosulfuron isoladamente ou em associação com atrazine e/ou clorpirifós

“…Eficácia de controle de plantas daninhas e toxicidade ... É comum observar injúria acentuada causada pela adição de inseticida fosforado a inibidores da ALS mesmo em híbridos de milho que lhe são tolerantes. Esse fenômeno é atribuído à redução da deposição de cera epicuticular pelo inseticida, o que resulta em aumento da absorção desses herbicidas (Diehl et al, 1995), ou à interferência do clorpirifós na atividade da enzima citocromo P450, responsável pela metabolização desses herbicidas em milho (Brown, 1990). Por isso, recomenda-se intervalo mínimo de sete dias entre a aplicação da mistura formulada foramsulfuron+iodosulfuron e a de inseticida fosforado.…”

“…A utilização de herbicidas inibidores da ALS, como o caso do nicosulfuron e da mistura foramsulfuron + iodosulfuron, pode causar toxicidade diferencial em híbridos de milho (Damião Filho et al, 1996), devido à diferente capacidade destes para metabolizá-los a compostos não-tóxicos (Brown, 1990;Bunting et al, 2004a).…”

“…Esse fato é atribuído à redução da deposição de cera epicuticular pelo inseticida fosforado (Diehl et al, 1995). Esse tipo de inseticida poderia, também, reduzir a atividade da enzima citocromo P450, responsável pela metabolização dos herbicidas inibidores da ALS (Brown, 1990). …”

Effects of Metsulfuron-Methyl on Soil Microbial Acitvity

“…Sulfonylureas have high selectivity and very low acute and chronic animal toxicity. These compounds are readily degraded in soil and are not environmentally persistent [5,6,7,8].…”

Rice seedling and plant development as affected by increasing rates of penoxsulam under controlled environments

“…ALS is the first common enzyme responsible for the synthesis of branched chain amino acids valine, leucine and isoleucine (Shaner, 1991;Eberlein et al, 1997) and it is also present in chloroplasts. ALS-inhibiting herbicides are widely used because they control many weeds at low rates, provide excellent crop safety, and have low mammalian toxicity (Beyer et al, 1988;Brown, 1990;Newhouse et al, 1991). Organisms other than plants are not strongly affected because they do not possess this enzyme (Leite et al, 1998).…”