Step 1: Understanding the Concept:
Apical dominance is a physiological phenomenon in plants where the central stem grows more vigorously than the lateral (side) branches.
The terminal bud at the tip of the stem exerts control over the axillary buds located in the leaf axils, keeping them in a dormant state.
This process is vital for the plant to prioritize vertical growth to compete for sunlight in crowded environments.
The primary internal chemical signal that mediates this process is a class of phytohormones known as auxins.
Step 2: Detailed Explanation:
Auxins, most notably Indole-3-Acetic Acid (IAA), are synthesized in high concentrations within the shoot apical meristem and young developing leaves.
From the apex, auxins move downward through the plant via polar transport, a process involving specific influx and efflux carrier proteins (PIN proteins).
As auxin moves down the stem, it maintains a high concentration in the vicinity of the axillary buds.
High levels of auxin in the main stem indirectly inhibit the growth of these lateral buds by regulating other signaling molecules like strigolactones and cytokinins.
Specifically, auxin suppresses the synthesis of cytokinins in the nodes; since cytokinins are required for bud outgrowth, their absence keeps the buds dormant.
The experimental proof of this was demonstrated by early plant physiologists: when the apical bud is removed (decapitation), the source of auxin is eliminated.
This leads to a rapid drop in auxin levels and a corresponding increase in cytokinin levels in the axillary buds, triggering their growth and resulting in a bushier plant.
In contrast, other hormones play different roles:
(a) Cytokinins: These are considered the "antagonists" to auxin in this context as they promote lateral bud growth and cell division.
(c) Ethylene: A gaseous hormone mainly involved in senescence, abscission, and the ripening of fruits.
(d) Abscisic acid (ABA): Primarily a stress hormone that induces seed dormancy and regulates stomatal closure during water stress.
Step 3: Final Answer:
The hormone produced at the shoot tip that prevents the growth of lateral buds is Auxin.