Auxin and Auxinic Herbicide Mechanism(s) of Action - Part 1 - Introduction

The growth and anatomy symptoms induced in plant species sensitive to auxinic herbicides are similar to those induced by high exogenous doses of the natural hormone, IAA. After exposure to a high dose, symptoms include cupping and stunting of leaves, brittleness, stunting and twisting of stems, and general abnormal growth of susceptible species (Figure 3). Terminal leaf growth ceases. Tissue proliferation along the stem takes place first at the stem tip, then the nodes and finally along the length of the stem resulting in disintegration or blockage of phloem and xylem tissue. Meanwhile, epinasty and bending and splitting of the stem occur. (See video clip of 2,4-D effects on tomato)

Video Clip: Effects on tomato sprayed with 2,4-D (Oregon State University)

The roots become thickened and stunted. Adventitious roots or callus tissues develop which crush the phloem and cortex, eventually resulting in rupturing of the epidermis of stem tissues. Symptoms normally appear within a few hours or days although death may not occur for several weeks or months. Click on the picture below to view an animation that demonstrates how auxins may cause some of these symptoms.

Animation 1: Overall Picture of Auxinic Herbicide Action

Fig. 3a: Activity on dicots - swollen and bent stems in soybean	Fig. 3b: Activity on dicots - leaf crinkling and cupping; new leaves are malformed in soybean	Fig. 3c: Activity on dicots - stem twisting and bending (epinasty) in soybean
Fig. 3d: Activity on monocots - corn leaf rolling or wrapping	Fig. 3e: Activity on monocots - severe leaf rolling in corn; ear shoot may not emerge	Fig. 3f: 2,4-D on pigweed - stem twisting and bending (epinasty)
Fig. 3g: 2, 4-D on corn - goosenecking of stem and fused brace roots	Fig. 3h: Dicamba on corn - poor development of brace roots	Fig. 3i: Dicamba on soybeans - leaf crinkling and cupping
*Images obtained from Cyanamid Interactive CD Rom. American Cyanamid Company, 1996

Grass crops are tolerant to auxinic herbicides but may be injured if these herbicides are applied during rapid cell division (tillering, flowering) or during rapid growth (high temperatures and high soil moisture) (Figure 3). Corn and sorghum stems may become brittle after auxinic herbicide application. Wheat and rice may exhibit buggy-whipped, malformed seed heads after 2,4-D application.

Broadleaf crops such as cotton and grapes are especially sensitive to drift from 2,4-D, 2,4-DB, and triclopyr when used in nearby fields. Cotton leaves will be narrow and crinkled (okra-leafed) and bolls may have a beaked appearance. Cotton will grow out of the injury if the concentration in drift is low. If the drift concentration is high, cotton maturity will be delayed and yield will be reduced. Soybeans, peanuts, and alfalfa are tolerant to 2,4-DB because they do not convert it to 2,4-D as susceptible plants do. However, soybeans may display crinkled, wavy leaves (strapped) after 2,4-DB applications, but will grow out of this condition.