Seed dormancy describes a natural state where a viable seed will not germinate, even when environmental conditions like water, temperature, and oxygen are suitable for growth. This temporary suspension of germination is a survival mechanism that helps plants delay sprouting until conditions are optimal for seedling establishment. It ensures that seeds do not sprout prematurely, for instance, during a brief warm spell in winter, which would likely lead to the death of the young plant. This inherent characteristic allows seeds to persist for varying periods, from days to months or even years, until specific cues trigger their emergence.
Types of Seed Dormancy
Physical dormancy occurs when the seed coat is impermeable to water or gases, preventing the embryo from imbibing water and initiating growth. This hard outer layer acts as a barrier, common in many legume species, where scarification, a process of weakening the seed coat, is often required for germination.
Physiological dormancy results from internal chemical inhibitors within the embryo or a lack of necessary growth promoters. This type of dormancy can vary in depth, from non-deep, which might break with short periods of after-ripening, to deep dormancy requiring prolonged cold stratification. For instance, many tree seeds exhibit embryo dormancy, where the embryo needs a period of maturation before it can germinate.
Morphological dormancy is observed in seeds where the embryo is underdeveloped at the time of seed dispersal. These embryos require a period of growth and differentiation before they are capable of germinating.
Morphophysiological dormancy combines morphological and physiological dormancy. It involves both an underdeveloped embryo and internal physiological inhibitors, often requiring a sequence of environmental cues, such as a warm period followed by a cold period, to break dormancy.
Factors Influencing Dormancy
Environmental conditions during seed development can influence the induction of dormancy, with factors like drought, high temperatures, or specific light exposure contributing to its severity. Once dormant, specific environmental cues are then required to break this state and allow germination.
Stratification is a process involving exposure to cold, moist conditions, typically between 0-4°C (32-40°F) for several weeks to months. This mimics the natural winter period, allowing internal chemical changes to occur that promote germination.
Scarification involves physically or chemically weakening the seed coat, allowing water and oxygen to penetrate the seed. Natural methods include weathering or passage through animal digestive tracts, while artificial methods can involve mechanical abrasion, hot water treatment, or acid treatment.
Light
Light plays a role for certain seeds, known as photoblastic seeds, where specific wavelengths of light can either promote or inhibit germination. For example, red light can break dormancy in some species, while far-red light can reverse this effect, indicating the involvement of phytochrome pigments.
Water
Water is always necessary for imbibition, the initial uptake of water by the seed, but it is not sufficient to trigger germination if dormancy is present.
Hormones
Endogenous plant hormones, such as abscisic acid (ABA), maintain dormancy by suppressing germination, while gibberellins (GA) promote it.
Ecological Role of Seed Dormancy
Seed dormancy serves as an evolutionary adaptation, ensuring that seeds germinate only when environmental conditions are suitable for seedling survival. This strategy increases the likelihood of a plant successfully establishing itself and thriving. Dormancy allows species to endure unfavorable periods, such as harsh winters, prolonged droughts, or fire events, by delaying germination until conditions improve.
The phenomenon also facilitates seed dispersal over both time and geographical areas, contributing to the formation of a “soil seed bank”. This bank acts as a reservoir of viable seeds, ensuring that if mature plants are lost due to disturbances like fires or floods, new plants can emerge from the dormant seeds in the soil. Spreading germination over time can also reduce competition among seedlings from the same parent plant. Seed dormancy contributes to the long-term resilience and stability of plant populations within their natural habitats.
Seed Dormancy in Agriculture
Seed dormancy presents challenges in agriculture, leading to uneven germination and making it difficult to achieve a uniform crop stand. This inconsistency can result in reduced yields and increased management costs. For instance, pre-harvest sprouting in crops like mung beans, where seeds germinate prematurely on the parent plant, can lower seed quality and yield.
To overcome dormancy in agriculture, various pre-sowing treatments are employed. These include cold stratification or scarification. Chemical treatments, such as soaking seeds in dilute solutions of gibberellic acid, can also stimulate germination.
Understanding seed dormancy is also important for long-term seed storage in gene banks. Controlling dormancy ensures seeds remain viable for extended periods, allowing for future use in breeding programs or ecological restoration. However, dormancy in weed seeds complicates their control, as they can persist in the soil for many years, emerging sporadically and competing with cultivated crops.