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How do red light and far-red light affect 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.

In many plant species, seed germination is regulated by the phytochrome system, which acts as a light-sensitive switch. Red light (approximately 660 nanometres) typically stimulates germination, while far-red light (approximately 730 nanometres) inhibits it.1

This process is reversible: if seeds are exposed to alternating treatments of red and far-red light, the final exposure determines whether the seed will germinate. For example, if a red light treatment is followed by far-red light, the stimulatory effect of the red light is cancelled out.1

This mechanism serves an important ecological function. Because green leaves absorb red light, seeds beneath a dense canopy receive a higher proportion of far-red light, which inhibits their germination and prevents them from attempting to grow in light-limited environments where they would struggle to survive. Conversely, when seeds are exposed to direct sunlight—which is rich in red light—the phytochrome system signals that conditions are favorable for growth, thereby promoting germination.21