Germination

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. The plant 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.

see also Germination and Growth; Hormones; Seeds.

J. Derek Bewley

Bibliography

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.

Germination

Germination is the earliest stages of growth when a seed begins to transform itself into a living plant that has roots, stems, and leaves. Although conditions vary according to species, all seeds require a certain amount of moisture and oxygen as well as a suitable temperature before they will germinate.

Some seeds are ready to germinate almost as soon as they are ripe and will sprout open wherever and whenever they land in a suitable environment. However, the seeds of most plants need to lie dormant (inactive or resting) for a period of time before they will germinate. This enforced dormancy can be caused by many factors within the seed itself. First, the seed coat may be so hard that it will not allow water or oxygen to penetrate until it has begun to soften or break down over time. Second, seeds may contain chemicals that prevent germination, and sprouting will not occur until these antigermination hormones have been washed away by rainwater. Other seeds need to be exposed to prolonged periods of cold, while others must pass through the gut of an animal or even be exposed to fire before germination will occur. During this dormancy period, the seed is inactive and no growth occurs. Seeds have remarkable properties, and some can remain dormant for extremely long periods of time. In fact, some seeds have been known to germinate after remaining dormant for centuries.

A mature or ripe seed is surrounded by a hard coat called a testa. Inside this coat is the beginning of a plant called an embryo. The embryo has one or more seed leaves called cotyledons. Also inside is all the food the embryo will need to fuel its early growth. Germination usually happens in the spring when the soil warms, and the seed breaks its dormancy. At the beginning of germination, the seed takes in water very quickly. This process is called imbibition, and as the dry seed takes in more water, it swells and finally bursts the seed coat. Once dormancy ends and the seed coat bursts, germination becomes irreversible. With its coat now open, the seed can take in even more water and oxygen, so that it soon doubles in size.

The next stage of germination begins when the food stored in the endosperm is converted into useable forms and sent to the seed's growing points. These points consist of the seeds beginning root system called the radicle and its early stem and leaf stage called the plumule. The radicle or young root is the first to emerge from the seed and begins to grow downward into the soil. Soon after, the young shoot or plumule appears and starts to grow upward. The plumule breaks through the soil with its tip bent over, protecting the young, tender tip and allowing the older, stronger part of the shoot to bear the brunt of pushing upwards. In some plants like garden beans, the cotyledon (the first leaf to appear from a sprouting seed) is also raised out of the ground, while others, like peas, the cotyledon stays buried. After the radicle becomes a root system and the plumule straightens out, the cotyledon begins to open and the first true leaves start to grow. By now, the seedling is well established and the life cycle of another generation has begun.

[See alsoBotany; Embryo; Fertilization; Seed ]

Germination

Germination is the process by which a seed begins its development into a mature plant. It begins with an increase of metabolic activity within the seed. The first visible sign of germination in angiosperms (flowering plants) is generally an enlargement of the seed, due to intake of water from the environment. The seed’s covering may wrinkle and crack at this time. Soon afterward, the embryonic root (called the radicle) emerges from the seed and begins to grow down into the soil. At about this time the shoot (plumule) also emerges, and grows upward out of the soil.

In most species, the food reserves that provide fuel for the seed’s development are contained in the fleshy part of the seed. In some seeds, this fleshy part is divided into two seed leaves, or cotyledons. Seeds having two seed leaves are said to be dicotyledonous; those having only one are monocotyledonous. In some plants, the growth of the shoot carries the cotyledons above the soil into the sunlight, where they become more leaflike in appearance while continuing to provide sustenance for the growing plant. Germination that follows this pattern is called epigeal germination. In other species the cotyledons remain underground; this is known as hypogeal germination.

Germination requires the presence of suitable environmental conditions, including sufficient water, oxygen, and an appropriate temperature. However, in many species the onset of germination is preceded by a period of metabolic inactivity, known as dormancy. While dormant, seeds will not germinate even under favorable conditions, but eventually they break their dormancy and begin to develop. The processes in the seed by which dormancy is broken are known as after-ripening. Dormancy gives seeds a better chance of surviving unfavorable conditions and developing successfully into plants. For example, seeds produced and dispersed just before the beginning of a cold season might not survive if they germinated at once. Dormancy enables them to wait out the cold season, and to begin growth when conditions are more favorable for the mature plant, in the springtime. Typical dormancy periods of seeds vary widely from species to species (and even within the seeds of a given species), as do the mechanisms by which dormancy is broken.

Germination

Germination is the process by which a seed begins its development into a mature plant . Germination begins with an increase of metabolic activity within the seed. The first visible sign of germination in angiosperms (flowering plants) is generally an enlargement of the seed, due to intake of water from the environment. The seed's covering may wrinkle and crack at this time. Soon afterward, the embryonic root (called the radicle) emerges from the seed and begins to grow down into the soil . At about this time the shoot (plumule) also emerges, and grows upward out of the soil.

In most species , the food reserves that provide fuel for the seed's development are contained in the fleshy part of the seed. In some seeds , this fleshy part is divided into two seed leaves, or cotyledons. Seeds having two seed leaves are said to be dicotyledonous; those having only one are monocotyledonous. In some plants, the growth of the shoot carries the cotyledons above the soil into the sunlight, where they become more leaf-like in appearance while continuing to provide sustenance for the growing plant. Germination that follows this pattern is called epigeal germination. In other species the cotyledons remain underground; this is known as hypogeal germination.

Germination requires the presence of suitable environmental conditions, including sufficient water, oxygen , and an appropriate temperature . However, in many species the onset of germination is preceded by a period of metabolic inactivity, known as dormancy. While dormant, seeds will not germinate even under favorable conditions, but eventually they break their dormancy and begin to develop. The processes in the seed by which dormancy is broken are known as after-ripening. Dormancy serves to give seeds a better chance of surviving unfavorable conditions and developing successfully into plants. For example, seeds produced and dispersed just before the beginning of a cold season might not survive if they germinated at once. Dormancy enables them to wait out the cold season, and to begin growth when conditions are more favorable for the mature plant, in the springtime. Typical dormancy periods of seeds vary widely from species to species (and even within the seeds of a given species), as do the mechanisms by which dormancy is broken.

germination The beginning of growth of a seed, spore, or other structure (e.g. pollen), usually following a period of dormancy, and generally in response to the return of favourable external conditions, most notably warmth, moisture, and oxygen. The internal biochemical status of the seed or spore must also be appropriate. In seeds, germination may be epigeal, with cotyledons emerging above the ground, or hypogeal, with the cotyledons staying below ground.