Germination is the biological process by which a seed wakes from dormancy and begins to grow. It is also sometimes colloquially referred to as “sprouting” and results in the emergence of a seedling. It is a quintessential step in a plant’s life, as its future prosperity is entirely dependent on successful seed germination.
While germination is most commonly used in the context of plant seeds, the term is also relevant in the context of spores produced by fungi and non-seed-bearing plants.
The Steps of Seed Germination
The process of germination involves a series of biochemical processes in which a seed utilizes its stored energy reserves to produce the roots, leaves, and other plant tissues necessary for photosynthesis and plant growth. For germination to occur, seeds often require specific environmental conditions, such as distinct temperatures and access to water. Often seeds disperse far and wide in the wild before they meet the perfect conditions to sprout.
- Absorption of Water: Imbibition
Seed germination begins with a process known botanically as “imbibition”. This is essentially the absorption of water into the seed. Some plants have semi-permeable seed coats that allow the entrance of water, while others have a specialized pore known as a micropyle,” which facilitates the entry of moisture. Plants with thick seed coats may require physical damage or some mild decomposition before water can be absorbed. Eventually, the seed coat becomes soft and bursts open due to the pressure applied to it by the expanded embryo. - Activation:
This is the turning point at which a seed begins ramping up its metabolism and waking up from dormancy. During activation, there are several processes that simultaneously occur to drive growth. - Enzyme Activation: Once hydrated, metabolic processes that have been dormant begin occurring. This includes the activation of enzymes such as amylase, which break down complex carbohydrates into simple sugars. These simple sugars can then be used by the plant cells as an energy source. Other enzymes, such as lipases and proteases, also play a crucial role in making stored nutrients available for plant growth.
- Respiration: To turn simple sugars into energy, the seed begins the process of respiration. This entails a cellular process that consumes oxygen, creates energy in the form of ATP, and produces carbon dioxide as a byproduct. This is the quintessential process in which a plant begins “breathing”, signifying a wake from dormancy. By utilizing the stored energy within the seed, the plant can begin conducting cell division and other processes important to growth.
- Production of Hormones: Once activated, the plant begins producing hormones such as gibberellins. These play a pivotal role in further promoting the production of new enzymes and stimulating cell elongation.
- Emergence:
This is the final stage in the germination process and is characterized by the emergence of the Root Radicle (also known as the embryonic root) and the Embryonic Shoot. These are tissues that are already partially developed within a dormant seed but only become functional and fully developed during the stage of emergence. - Growth of Root Radicle: Once the embryo has swollen, the embryonic root can easily pierce through the water-softened shell. This embryonic root, known as a radicle, is guided by gravity down into the soil, where it anchors the seed. Following this, the radicle sprouts root hairs, which allow for the absorption of water and essential nutrients. In certain species, this root radicle becomes an elongated “tap root” which grows deep into the soil.
- Growth Of Embryonic Shoot: The embryonic shoot consists of the tissues that will become the future stems and leaves of the plant. They typically emerge after the seed coat has been bursted by the sweeling embryo and is guided towards light using a process called phototropism. This ensures the growing plant finds the light resource it needs for photosynthesis. In some plant species (such as beans), the first leaves come in the form of cotyledons, while in others (such as corn), the cotyledons stay below ground, where they provide stored energy for shoot growth. In some plant species, such as most palms, shoots emerge before the embryonic root.
- Photosynthesis Begins: Before the first leaves have emerged, the plant quickly begins producing the chlorophyll needed for photosynthesis. This marks the end of the germination process and the beginning of the seedling stage. At this stage, the plant gains its independence from stored energy within the seed and begins producing its own energy by utilizing water, carbon dioxide, and solar energy to make sugars. This energy is used for all future metabolic processes required for plant growth.
Breaking Seed Dormancy
To understand seed germination and the importance of dormancy it is advised that we look at the different parts of a seed too. Dormancy is an important adaptation that allows seeds to improve their chances of germination and establishment. In many cases, it allows seeds to “wait out” unfavorable conditions, which could impede growth. This includes drought, excessive competition, and unfavorable climatic conditions. Seeds can stay dormant for months, years, or even decades if the conditions are not suitable.
Some seeds must undergo proper environmental conditions to “break dormancy”. This process, otherwise known as “scarification”, is often conducted manually in horticulture to properly germinate seeds of certain plant species.
- Some seeds require near-freezing temperatures before they can germinate. In the wild, this ensures that seeds germinate in the spring after the passing of winter instead of in the fall, when conditions may otherwise be favorable for growth.
- Other plant species require extreme temperatures, such as those of a forest fire, before they germinate. This allows them to grow with low competition in the aftermath of a devastating fire. In horticulture, this can often be simulated by the application of “liquid smoke”. Plants adapted to fire are called “pyrophytes” and include certain types of eucalyptus, manzanita, and a wide array of conifers.
- Some plants require light for proper germination. These seeds will remain dormant within the soil until disturbance brings them to the surface. Other species inhibit their germination in the presence of light. These “photosensative” seeds often have thin seed coats that can be penetrated by light.
- Other triggers that can help in the germination of seeds are physical abrasion, moisture, decomposition, and passing through the digestive system of an animal. In horticulture, a common strategy for scarification is to carefully cut the seed coat. This allows for the quick absorption of water and can greatly speed up the time needed for germination.
Lastly, certain species stay in dormancy for very short periods. This includes many tropical species and short-lived annuals. Certain plant species that undergo the process of “vivipary”, germinate before they are even detached from the parent plant.
Germination of Spores in Plants and Fungi
Spores are reproductive cells produced by fungi and many different types of non-seed-bearing plants. Plants that produce spores include ferns, mosses, liverworts, and hornworts. Unlike seeds, spores are reproductive cells known as gametes, which only contain 50% of the genetic information of an individual. This is equivalent to the pollen, or ovules, found in the flowers of plants. Like seeds, spores also undergo the process of germination, although it is biologically distinct compared to seed-bearing plants.
