Gibberellin breaks bud dormancy of potato tuber.

Explanation:
Potato tubers are underground structures that store food for the plants. These tubers develop buds that can grow into new plants. However, these buds remain dormant until the appropriate environmental conditions are met. Bud dormancy is the state of arrested growth in which the buds do not grow even when the conditions for growth are available.

Gibberellin is a plant hormone that is involved in various processes such as stem elongation, seed germination, and flowering. It also plays a crucial role in breaking bud dormancy in potato tubers. When gibberellin is applied to the potato tubers, it promotes the synthesis of enzymes that break down the starch stored in the tuber. This breakdown of starch provides the energy required for the bud to grow.

Gibberellin also promotes the elongation of the stem and the development of leaves, which are essential for the growth of the new plant. It also stimulates the synthesis of other hormones like cytokinins, which further promote the growth of the bud.

In conclusion, Gibberellin is a plant hormone that plays a vital role in breaking bud dormancy in potato tubers. It stimulates the breakdown of starch stored in the tuber and promotes the growth of the bud by promoting the synthesis of other hormones like cytokinins.