A seed is a mature ovule that contains an embryo or a miniature undeveloped plant and food reserves, all enclosed within a protective coat. It is a characteristic reproductive body of both angiosperms (flowering plants) and gymnosperms (conifers, cycads).
Seeds are an essential part of the plant’s life cycle. They play an important role in reproduction, multiplication, perennation, dormancy, and dispersal.
Embryonic Development
The development of an embryo from a single-celled zygote is a highly complex and well-regulated process. It is a crucial step in the fitness of an organism because it determines its future state (Figure 1).
In planta, a single-celled zygote becomes a multi-cellular organism through the formation of two germ layers — the epiblast and the endoderm. The endoderm is composed of a sheet of cells that develops into the digestive tract, organs, and other tissue.
During the first week of development, the cell layers begin to move together and differentiate into tissues and organs. Embryonic tissue includes the central and peripheral nervous systems, sensory organs, skin, hair, nails, muscles, blood vessels, skeleton, and kidneys.
The cytokinins CKs play a critical role in early embryo and endosperm development, promoting cotyledon expansion, seed filling, desiccation tolerance during maturation, and dormancy induction. In addition, ABA has important influences during the germination and storage stages, affecting the synthesis of seed storage proteins, acquisition of dormancy, and induction of dormancy in some species.
Germination
Germination is the process by which a seed grows into a plant. It can take a few weeks for the whole process to finish.
A seed needs air, water and warmth to start the process of germination. These factors affect a seed’s ability to grow and can also vary from one species to another.
When environmental conditions are optimal, a seed is stimulated to “wake up.” This occurs when the embryo breaks through a thick layer of the seed coat.
Then, the radicle (primary root) and plumule (shoot) can emerge from the seed. This growth is initiated by specific enzymes that become activated when the seed is exposed to water.
The enzymes hydrolyze the endosperm’s food reserves that have been stored in the seed. This produces free amino acids that support protein synthesis in the seed’s endosperm and embryo. It also initiates a signaling cascade that leads to the synthesis of certain phytohormones.
Dormancy
Many animals and plants experience a period of dormancy. Animals such as arctic lemmings, bears, and certain types of trees and grasses go dormant during periods of extreme cold or dry weather to save energy.
Seeds also have periods of dormancy. These can be genetic or environmental, like the dormancy of some seeds during a dry season.
In laboratory experiments, a variety of factors have been used to influence the level of dormancy in developing seeds. These include day length, light quality, temperature, soil moisture, and mineral nutrition.
Exposure to elevated temperatures during the development phase results in seeds with less dormancy, while water stress during seed formation reduces seed dormancy in most species. In addition, dormancy is induced in seeds that are still on the mother plant (referred to as primary dormancy). The level of dormancy released during dispersal, following dispersal, can be influenced by factors that have been used in the laboratory, such as after-ripening in a relatively dry state or dormancy-release treatments in an imbibed state.
Dispersal
Dispersal of seeds is an important life-history stage that determines the future range of organisms. It allows plants to reduce competition among kin and to colonize new, suitable habitats.
Seeds are transported away from their parent plants by a range of agents such as wind, animals, birds and water. This varies according to plant and seed shape and characteristics.
Animals are often the most reliable and effective dispersers of plant seeds, especially in tropical rainforests. This is because fruit-eating animals (frugivores) have relatively large digestive tracts that allow them to digest and regurgitate seeds.
However, this also means that the effectiveness of internal seed dispersal depends on a number of factors, including the physiology of animal guts. For example, thinner seed coats may have a higher risk of abrasion through the digestive tract than thicker coatings. This, in turn, affects the time it takes for seeds to germinate. This, in turn, varies between different species of animal.
