A seed is a part of a plant that contains an embryo and food store. This embryo develops into a new plant when conditions are favourable.
Seeds are found in a variety of plants, including gymnosperms (naked seeds) and angiosperms. Unlike their naked-seeded cousins, angiosperms have seeds enclosed in sacs called ovaries.
Germination
Germination is the process by which a seed transforms itself into a new plant. This requires a number of factors, including the right temperature, water, oxygen or air and sometimes light or darkness.
A seed has a small amount of stored energy, allowing it to ‘wait’ for the perfect conditions to start growing. This energy helps the seeds stay alive and is used to carry out some basic metabolic processes like cellular respiration.
Many seeds require a period of ‘breaking’ dormancy before they can germinate, which may depend on changes in the seed coat or the state of the embryo itself. Some seeds need to be exposed to cold temperatures (about 5degC) or have their seed coats damaged sufficiently to allow water to enter.
The water uptake of the seed causes a rapid swelling and softening of its outer coating, which leads to rupture of the covering layers and emergence of the radicle and root. Hydrolytic enzymes activated during this ‘imbibition’ break down food reserves in the seed to release their energy for growth.
Endosperm
The endosperm of the seed is a nutritive tissue that surrounds the developing embryo and stores food materials for the growing seed. It is a triploid structure and contains carbohydrates, proteins, and fats that both plants and animals rely on for their health.
In most flowering plants, the endosperm develops from nuclei in the embryo sac. These nuclei are triploid, which means they contain a complete set of three chromosomes.
Development of the endosperm is characterized by three major phases: cell division and differentiation (cellularization), syncytial endosperm development, and maturation. In each of these stages, the endosperm reorganizes its chromatin, reprogramming its expression patterns and gene function.
In early endosperm development, a cytokinin signaling pathway is activated. The cytokinin receptor FIS-PRC2 and AGL62 are expressed exclusively in endosperm cells. These genes presumably control the transition between mitotic domains in early endosperm development.
Cotyledons
Cotyledons are an important part of the embryo within a seed. They are the first parts that appear from a germinating seed and are essential for germination of all flowering plants (angiosperms).
When seeds germinate, the cotyledon/s may remain below the soil surface (hypogeous germination) or emerge above it (epigeous germination), depending on the type of plant. For example, daylily seeds usually germinate with hypogeous germination, where the single cotyledon remains enclosed within the seed coat below ground.
In contrast, the emergence of cotyledons plays an important role in early seedling growth for small-seeded species, such as Hakea, that rely on nutrient sources from the soil much earlier than large-seeded ones. In fact, in nutrient-impoverished habitats where Hakea grows, it is the presence of cotyledons that ensures early survival and growth of seedlings even with partial damage to the cotyledons.
To investigate the direct and indirect effects of cotyledon removal on components of plant fitness, the impact of cotyledon removal was measured in six species of Hakea growing in south-western Australia with seed sizes ranging from 2-500 mg. A structural equation model linking cotyledon damage (number of cotyledons removed) to plant biomass, days to flowering, flower number and the fitness components of seed number and seed mass was developed.
Seed Coat
The seed coat is the protective layer of tissue covering the embryo and endosperm within the seed. It protects the seed from a variety of environmental stresses and promotes the development of the embryo and endosperm.
It also serves as a source of nutrients for the growing embryo and endosperm. The architecture and chemical composition of the seed coat work together to ensure that nutrient flow is coordinated between the embryo and endosperm, promoting optimal growth and development.
In addition to regulating the size of seeds, the seed coat is responsible for controlling germination. For example, some seeds cannot germinate if the seed coat is too hard and cannot break down to allow water to penetrate.