Plant hormones and phototropism Introduction When a plant bends towards the light, we call that a positive phototropic response. The structural and physiological mechanisms involved in this response still pose questions for plant researchers. Our lab today will help us explore what is happening in this response. The hormone involved in this response is an auxin. This chemical messenger is found in the apical meristem regions of both root and stem tips. The hormone is associated with plant growth. Its chemical structure you see below is similar to the amino acid tryptophan.
What do we know about how this chemical messenger works? 1. Auxin is involved in the elongation of cells in plants. Auxin causes a drop in the pH in the region of the cell walls between plant cells. This drop in pH triggers an enzyme reaction that breaks the carbohydrate linkages of the cell wall. This permits the turgor pressure in the cell to increase, thus elongating the cell. 2. All plant cells must absorb minerals needed for metabolic processes such as magnesium for chlorophyll, phosphorus for ATP, etc. As a plant begins to decrease its metabolic processes for winter, auxins trigger a series of metabolic steps that reabsorb valuable minerals into storage spaces during the dormant months. One result of this auxin effect is the breakdown of chlorophyll and the increased visibility of the other pigments during the fall. 3. Auxin is produced in the growing tip of the plant. As a result, excessive growth
this apical dominance.
4. Phototropism - plants grow in response to light (nonreversible growth toward light stimulus. Light causes auxin to move laterally in the apical meristem. An
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of lateral branches is inhibited by