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How can light promote germination in some seeds but inhibit it in others?

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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.

The sensitivity of seeds to light is a critical mechanism for regulating germination, ensuring that seeds sprout only under favorable environmental conditions.12

The Phytochrome System

The primary mechanism for light-sensitive germination involves phytochrome, a pigment that acts as a biological switch. In many species, such as lettuce, germination is stimulated by red light (approx. 660 nm) and inhibited by far-red light (approx. 730 nm). The final light exposure a seed receives determines its response, as the phytochrome system can be toggled between active and inactive states.1

Ecological Adaptations

Light sensitivity serves as an ecological filter:
* Stimulation: Species that require light often germinate only when near the soil surface, preventing them from exhausting their energy reserves if buried too deeply. For example, dense vegetation or deep water can inhibit germination by intercepting red light, preventing seeds from sprouting in environments where they would struggle to survive.2
* Inhibition: Conversely, some species, such as certain lily family members and desert plants like Citrullus colocynthis, are inhibited by light. This adaptation restricts germination to buried seeds, protecting them from the hostile, fluctuating microenvironments found at the soil surface.12