High rates of absorption of triasulfuron in high pH soils are due to greater herbicide availability rather than a physiological response

Abstract

Chlorsulfuron and triasulfuron are two sulfonylurea herbicides used for selective control of weeds in wheat, rice and other grass crops. Rates of application should be lower if the soil pH is near or above neutral to avoid phytotoxicity to the present crop and injury to other crops in succession. Increased absorption and phytotoxicity at high pH could be due to exposure to higher concentrations over a longer time because of less adsorption and slower hydrolysis of these sulfonylureas at high pH. Plant factors may also contribute to phytotoxicity at high pH. The pH was manipulated in soil and solution culture to investigate the response to pH of triasulfuron absorption by wheat (Triticum aestivum cv. Kulin). Absorption at 18 C by seedlings (at the 4-leaf stage) in solution culture from a 5-h pulse of triasulfuron decreased from 0.4 to 0.2 μg g-1 plant fresh weights when pH was increased from 5.0 to 6.5 for the duration of the pulse; there was no influence on absorption when the seedlings were defoliated. Wheat was grown in acid (pH 4.9) and near-neutral (pH 6.8) sandy soils in which pH was adjusted to 7.0 and 5.0, respectively, 11 wk before sowing and to which 14C-triasulfuron was added at 0 and 50 μg kg-1 soil immediately prior to sowing. Mean rates of absorption/mean plant fresh weight (RA) were calculated for 30 d after sowing. Most of the absorbed 14C was in the roots with less than 5% in the shoots of any treatment. The RA in the acidic soil increased from 2.7 to 14.4 Bq d-1 when pH was adjusted from 4.9 to 7.0 and the RA for the neutral soil decreased from 11.7 to 3.1 Bq d-1 when the soil was acidified from pH 6.8 to 5.0. The RAs did not differ between the soils at the same pH. Significantly more triasulfuron was adsorbed in acidic than in near-neutral soils. Increased phytotoxicity of the sulfonylureas at high soil pH is due to the greater availability of higher proportions of the applied dose in the soil solution rather than an effect on the physiology of absorption. Consequently, the higher phytotoxicity at high soil pH can be ameliorated by reductions in the rates of application.

Source or Periodical Title

Philippine Agricultural Scientist

ISSN

317454

Page

408-416

Document Type

Article

Subject

Herbicide absorption, Herbicide availability, Triasulfuron

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