Auxin Growth Regulators
Symptomology
Development Summary
Site of Action Summary
Injury Symptoms
Time-lapsed Video
Herbicide
Families
Herbicides
Symptomology Development Summary
The growth regulator herbicide
are composed of several herbicide families including the phenoxy aliphatic
acids, benzoic acids, picolinic acids, and quinolinecarboxylic acids. Their
activity mimics that of auxins (indoleacetic acid), a family of naturally
occurring plant hormones known to be involved in several aspects of growth
regulation ranging from cell elongation to root initiation. While auxins
occur naturally in plants and are important for normal growth, early studies
proved that increased doses of auxins, or synthesized chemicals similar to
auxins (i.e. the growth regulator herbicides), were effective herbicides.
Growth regulator herbicides are believed to have multiple sites of action
and therefore cause a wide variety of injury symptoms. These chemicals
are transported primarily in the symplast and move to the new growth. Young
tissues are usually first to be affected Broadleaf injury symptoms include
bending and twisting of stems, cupping of leaves, callus
formation, and malformation
of other plant parts. Grass injury symptoms include fused braced roots
and tightly rolled leaves.
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Growth regulator herbicides
closely mimic functions of auxin plant hormones, most notably IAA (indoleacetic
acid). Auxins are known for their roles in cell elongation, controlling lateral
growth, and cell wall formation. Susceptible plants have abnormal cell wall
development when treated. Characteristic symptomology of bent and twisted
stems results from increased cell wall plasticity. The increased plasticity
takes place due to induced proton movement out of the cell. This significant
movement of protons changes the pH around the cell, increasing the activity
of certain enzymes around the cell wall, which causes the cell to abnormally
elongate. Growth regulator herbicides also increase the production of RNA
polymerase, which stimulates the production of RNA, DNA, and many proteins.
This in turn leads to increased cell division. Together, uncontrollable cell
division and cell elongation leads to destruction of the vascular tissues
of the plant. Decreased ability to translocate through these tissues leads
to starvation for nutrients and water and the inability to move sugars away
from the source of production.
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Injury symptoms on broadleaf
plants include immediate bending and downward twisting that can lead to splitting
of the stem and immediate stoppage terminal leaf growth. Delayed symptoms
include cupping of the
leaves, formation of adventitious
roots and callus tissue, malformation of flowers, and thickened, stunted
roots. Injury symptoms on grass include fused brace roots, malformed
grain or missing kernels; buggy-whipped (tightly rolled) leaves, and
lodging.
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