Positive Phototropism Directs Plant Shoots Toward Light Sources
- Imagine a seedling on a windowsill, bending toward the light.
- This curvature is not random it’s a precise response called positive phototropism.
- Positive phototropism is the growth of plant shoots toward a light source.
Definition
Phototropism
Phototropism is the growth response of a plant towards or away from light.
How Positive Phototropism Works
- Detection of Light
- The tip of a plant shoot contains light-sensitive pigments that detect the direction of light.
- When light shines from one side, these pigments sense the imbalance.
- Redistribution of Auxin
- Auxin, a plant hormone, is produced in the shoot tip and redistributed to the shadier side of the shoot.
- This uneven distribution is facilitated by auxin efflux carriers, which actively transport auxin across cells.
Definition
Auxin
Auxin is a phytohormone that regulates growth by promoting cell elongation.
Auxin concentration is higher on the shadier side of the shoot, causing differential growth.
Differential Growth
- Auxin promotes cell elongation by secretion of hydrogen ions (H⁺) into the apoplast (the extracellular space), resulting in lowering (acidifying) of pH in the cell wall, activating enzymes that loosen the cellulose fibers.
- As a result, it causes cell walls to soften and become more flexible.
- The cells on the shadier side elongate more than those on the brighter side, causing the shoot to bend toward the light.
Remember: Higher auxin concentration = more cell elongation.
