MICRONUTRIENTS - THEIR FUNCTION IN METABOLISM - MAKING AUXINS, THE GROWTH REGULATORS - THE ROLE OF ZINC
Let's look now at how photosynthesis, respiration, construction and minerals channel into actual growth.
Zinc is the micronutrient key that activates the enzyme system responsible for the production of auxin. Auxin, when properly combined in the plant, becomes a growth regulator. This chemical agent, active in very small amounts, is carried to the growing points of the plant where it directs the growth effects. Such a chemical agent is known as a "growth hormone".
Because of its relationship to the production of the growth regulator, a deficiency of zinc is characterized by a lack of growth in terminal locations where these regulators should be active. When adequate zinc is applied as a soil amendment or as a foliar spray, the immediate result is an increase in auxin and a correction of the stunning effect first noted.
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Chemical Nature of Auxins
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The growth of a plant is regulated in an orderly way by many growth hormones, each produced in a particular part of the plant and translocated to other regions. One such hormone auxin has been identified in the apical bud and leaves. It is produced by an enzyme system that refines the amino acid tryptophane into raw auxin or indoleacetic acid. Zinc has been directly associated with the formation of Tryptophane.
Since we start with an amino acid as a base material, we immediately realize the need for nitrogen, copper and molybdenum.
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Production of Growth Regulations
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When indoleacetic acid is modified slightly, the result is an active growth regulator. The regulators are chemically different from each other, and each is specific for its own role at a specific location, such as the growing tips of the roots, branches, stems or leaves.
Here we find another job for manganese and discover a new enzyme system: indoleacetic acid oxidase. Manganese activates this enzyme which controls the distribution of the growth regulators produced from auxin. The task of this special enzyme is to limit the amount of active auxin in any growing area, and to do away with any over-supply. Also, it has the important job of deactivating auxin in the non-growing regions of the plant.
We can see the close relationship here between manganese and zinc; that is, zinc builds up auxin hormone and manganese regulates and controls the supply. Abnormal growth may result if manganese or zinc is in short supply.
Not much is known about the specific ways in which plant hormones are carried to their working areas. It is thought that iron activates an enzyme- transport system that controls the direction and movement of the regulators.
It should be clear now that zinc, manganese and iron are closely related in the metabolic processes of the plant that lead to growth. The integration of the growth rates of different parts of the plant is controlled by growth hormones.
All six micronutrients are necessary to assure ideal plant growth.
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