C3.1.18 Positive phototropism (HL) Notes

Positive Phototropism Directs Plant Shoots Toward Light Sources

1. Imagine a seedling on a windowsill, bending toward the light.
2. This curvature is not random it’s a precise response called positive phototropism.
3. Positive phototropism is the growth of plant shoots toward a light source.

How Positive Phototropism Works

1. Detection of Light
   1. The tip of a plant shoot contains light-sensitive pigments that detect the direction of light.
   2. When light shines from one side, these pigments sense the imbalance.
2. Redistribution of Auxin
   1. Auxin, a plant hormone, is produced in the shoot tip and redistributed to the shadier side of the shoot.
   2. This uneven distribution is facilitated by auxin efflux carriers, which actively transport auxin across cells.

Differential Growth

1. 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.
2. As a result, it causes cell walls to soften and become more flexible.
3. The cells on the shadier side elongate more than those on the brighter side, causing the shoot to bend toward the light.

Reaching the Light

1. As the shoot curves toward the light, the auxin distribution evens out, and growth stabilizes.
2. The plant continues growing in the new direction, maximizing light exposure for photosynthesis.

Investigating Phototropism

Designing an Experiment

1. Hypothesis: Shoots will bend toward a light source due to auxin redistribution.
2. Materials: Seedlings, a unidirectional light source, opaque covers, and a ruler.
3. Procedure:
   1. Place seedlings under a light source.
   2. Cover some shoot tips with opaque caps to block light detection.
   3. Measure the angle of curvature over time.

Expected Results

1. Uncovered Shoots: Bend toward the light due to auxin redistribution.
2. Covered Shoots: Grow straight because the light stimulus is blocked.