Carotenoid Pigment Inhibitors
Symptomology
Development Summary
Site of Action Summary
Injury Symptoms
Time-lapsed Video
Herbicide families
Herbicides
Symptomology Development Summary
These herbicides inhibit
the formation of carotenoid pigments in cells of susceptible species. Carotenoid
pigments act as “energy reducing agents” for capturing excess
energy during the photosynthesis process. Without carotenoid pigments, unstable,
high-energy light generated compounds destroy membranes of cells and
organelles, such as chloroplast. Without carotenoids, chlorophyll biosynthesis
is inhibited; consequently, there is no green pigmentation in the new growth and the plant
turns white.
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Carotenoids are pigments
that play several important roles in photosynthesis. Located inside chloroplasts
of plant cells, carotenoids are critical during photosynthesis as they first
increase the spectrum of light from which energy is captured. Also during
photosynthesis chlorophyll absorbs energy from light to split water (H2O).
High-energy free radicals are formed during this process when energy is adsorbed
and energy acceptors are not readily available. Unrestrained radicals will
destroy cell membranes through a process called photooxidation. Carotenoids
protect chlorophyll molecules and other components from these radicals by
reducing the free energy to a harmless level.
Herbicides such as clomazone,
isoxaflutole, amitrole,
norflurazon, and fluidone inhibit enzymes in the pathway for synthesis of carotenoid
pigments. By inhibiting cartenoid biosynthesis, new cartenoids cannot be formed
to protect the chlorophyll. Since formation of new cartenoids is only affected,
older plant tissues are not immediately affected by the herbicides. New growth shows the initial
whitening or “bleaching.” As cartenoids
naturally breakdown in older growth, the entire plant later develops the whitening symptomology. The
plant will progress through chlorosis and necrosis as a result of photooxidation.
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