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How does water absorption start seed germination?

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Cluster of bright red seeds emerging from the cone-shaped fruit of a magnolia grandiflora, surrounded by large, glossy green leaves. — seed Fruit and seeds of the southern magnolia (*Magnolia grandiflora*). The fruit is an aggregate of follicles, and each follicle bears a single red seed.

Water absorption, or imbibition, is the critical first step in seed germination. When a seed takes in water, it triggers several essential physiological changes:

Cellular Rehydration and Expansion: Water is absorbed by the embryo, causing its cells to rehydrate and expand.12
Resumption of Metabolism: Shortly after imbibition begins, the rate of respiration increases. This allows various metabolic processes that were suspended or significantly reduced during dormancy to resume.12
Structural Changes: These metabolic shifts are accompanied by structural changes within the organelles of the embryonic cells, preparing the seed for active growth.12

In many cases, water also plays a role in breaking dormancy by leaching away chemical inhibitors that prevent the seed from sprouting. Once these processes are underway, the embryo can begin the active growth phase, typically marked by the emergence of the primary root.12