Germination

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Seeds are usually shed from their parent plant in a mature dry state. The dry seed contains an embryo that is the next generation of the plant in miniature. Before the seed can grow, however, it must first emerge from the seed and establish itself as an independent, photosynthetic seedling. Germination, by definition, starts when the seed takes up water, a process known as imbibition, and is completed when the embryonic root, the radicle, penetrates the outer structures of the seed (usually the seed coat and, in some species, the surrounding storage tissues of the endosperm).

In the mature dry state, the seed is metabolically inactive (quiescent) and can withstand environmental extremes of temperature and drought. When water enters the seed during imbibition there is a leakage of solutes (ions, sugars, and amino acids) because cell membranes are temporarily unstable during hydration. Cellular metabolism recommences within minutes after imbibition begins, using cell components and enzymes that were present in the dry seed. Respiration to provide energy and protein synthesis to produce new enzymes that support metabolism are important early events in germination.

Following imbibition, there is a period when no further water is taken up (plateau phase) and during which metabolism proceeds to ready the seed to complete germination. Restitution of cellular damage resulting from drying and imbibition is completed (e.g., DNA and mitochondria are repaired), and new enzymes and other proteins are synthesized. Elongation of the cells of the radicle is responsible for its emergence from the seed. Their cell walls become more stretchable and the internal water pressure (turgor) of the cells causes them to expand. Cell division and deoxyribonucleic acid (DNA) synthesis occur after radicle emergence, and later the mobilization of food reserves occurs within the storage organs of the seed to provide nutrients for post-germinative growth.

In some seeds the embryo is surrounded by a storage tissue that is sufficiently rigid to prevent extension of the radicle and completion of germination. This tissue frequently has thickened hemicellulose-containing cell walls, and a reduction in their resistance is necessary to permit radicle penetration. This might be achieved by cell-wall hydrolases or cell-separating enzymes, perhaps induced in the storage tissue in response to hormones released from the embryo late during germination.

Seeds of many noncultivated species, such as weeds, are often dormant when mature. When imbibed, these seeds exhibit the same intense metabolic activity as non-dormant seeds but do not complete germination. Germination does not occur unless the seeds receive an external stimulus (e.g., low or fluctuating temperatures, or light) while in the imbibed state. Theplant hormone abscisic acid plays some role in inducing dormancy during seed development, and its application to many seeds can prevent radicle emergence. Conversely, the plant hormone gibberellic acid, when applied in low concentrations to dormant seeds, will promote the completion of germination. How abscisic acid and gibberellic acid control germination is not known.

Germination and Growth; Hormones; Seeds.

Bewley, J. D. "Seed Germination and Dormancy." The Plant Cell 9 (1997): 1055-66.

——, and M. Black. Seeds: Physiology of Development and Germination, 2nd ed. New York: Plenum Press, 1994.

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