Free Hormonal Control of Apical Dominance revision notes for OCR A Level Biology – covering specification points 5.1.5 (b), 5.1.5 (c) and 5.1.5 (e).
Control of Apical Dominance
Apical dominance is the process by which the shoot tip (apical bud) suppresses the growth of lateral buds lower down the stem.
Apical dominance is controlled primarily by auxin, which influences the concentration of cytokinins and ABA in the lateral buds.
How Apical Dominance Works
The interaction between auxin and cytokinins regulates whether lateral buds remain dormant or begin to grow.
The process of apical dominance can be summarised as:
- Auxin is synthesised in the apical bud and transported down the stem and accumulates in the lateral buds.
- High auxin levels suppress their growth by reducing the movement of cytokinins into the buds and promoting ABA accumulation.
- Lateral buds remain dormant.
If the apical bud is removed (decapitation):
- Auxin levels drop rapidly in the stem below.
- Cytokinin levels increase in lateral buds.
- ABA levels fall.
- Lateral buds become activated, divide, and grow into side shoots.
Experimental Evidence for Apical Dominance
In OCR A Level Biology you need to be able to use your general understanding of the mechanisms of auxin, cytokinin and ABA interactions to interpret experimental investigations. You are not required to learn or recall any specific experiments (although variations of apical bud decapitation are the most common).
The table below outlines several examples of experiments or natural observations of apical dominance, and what we can infer from them:
| Experiment / Manipulation | Observation | Inference |
|---|---|---|
| Shoot tip removed | Lateral buds grow and form branches. | Apical bud provides a signal that inhibits lateral buds. |
| Auxin applied to cut surface after tip removal | Lateral buds remain inhibited. | Auxin from the apex maintains apical dominance. |
