Contribution of seed coat properties to seed performance and quality

Introduction

What is a seed?

A seed is the mature ovule that develops after fertilization. It typically consists of three primary components: the embryo, endosperm (present or absent depending on the species), and seed coat. Seed development is a defining feature of higher plants (gymnosperms and angiosperms), distinguishing them from lower plants such as pteridophytes, which do not produce seeds. In gymnosperms, seeds are formed without an ovary and are therefore referred to as naked seeds. In contrast, angiosperms produce seeds enclosed within fruits, formed from the ovary, and are typically equipped with protective structures including the seed coat, food reserves (e.g., endosperm or perisperm), and a well-developed embryo.

Seed coat: structure and formation

The seed coat is the outermost protective layer that surrounds the seed and develops from the integuments of the ovule after fertilization. It plays a crucial role in safeguarding the embryo and regulating its interaction with the environment. A small opening called the micropyle is often retained in the seed coat, allowing water and gases to pass through during germination. The seed coat generally comprises two main layers: Testa: The tough, thick outer layer derived from the outer integument, providing mechanical strength and protection.

Tegmen:

The thin, delicate inner layer derived from the inner integument, assisting in internal protection and permeability control (Matilla et al. 2019) [8] .

Multifunctional roles of the seed coat in seed quality and survival

The seed coat performs a variety of critical functions, contributing to seed quality, germination success, and seedling establishment. Its roles can be categorized as follows:

Dispersal mechanism

Seed coats often exhibit structural adaptations such as wings, hooks, hairs, or buoyant tissues, facilitating seed dispersal by wind, water, animals, or mechanical forces. These adaptations enhance the spatial distribution and survival chances of the species (e.g., tomato, lupins, maple, milkweed seed).

Hydration protection

The seed coat acts as a regulatory barrier, controlling the rate and extent of water uptake. This prevents osmotic shock or mechanical damage to the embryo during imbibition, particularly under fluctuating moisture conditions (e.g., lotus plant).

Physical support

As the seed germinates, the seed coat offers mechanical support to the emerging radicle and plumule, shielding them from soil particles, water pressure, and mechanical disturbance (e.g., beans).

Environmental sensing

The seed coat functions as a sensor, detecting cues such as light, temperature, moisture, and chemical signals, thereby determining the most favorable conditions for germination and dormancy release (e.g., oak trees).

Allelopathy

Certain seed coats release allelopathic compoundsbioactive chemicals that suppress the germination or growth of nearby competing plants. This chemical interference reduces interspecific competition and enhances seedling survival (e.g., black mustard).

Reserve mobilization control

During germination, the seed coat modulates the movement of enzymes and nutrients from storage tissues (like endosperm or cotyledons) to the growing embryo. This selective permeability ensures a controlled, efficient nutrient supply (e.g., quinoa).

Symbiotic relationships

In some species, seed coats facilitate associations with beneficial microbes, such as nitrogen-fixing bacteria or mycorrhizal fungi, aiding early nutrient acquisition and promoting healthy seedling growth (e.g., soybean) (Naflath et al. 2023) [9] .

Physical dormancy

In species like lodgepole pine, the seed coat imposes physical dormancy, requiring specific environmental triggers such as freezing temperatures or fire to break dormancy. This ensures germination occurs under optimal ecological conditions (e.g., lodgepole pine).

Chemical defense

Seed coats are rich in secondary metabolites such as phenolics, flavonoids, and alkaloids that provide chemical defense against herbivores, pathogens, and seed predators (e.g., walnut).

Water regulation

Specialized structures like micropyles or impermeable layers help control water absorption during seed imbibition, minimizing premature germination and improving seed longevity during storage (e.g., Acacia victoriae) (Julia et al. 2020) [5] .

Nutrient storage

In some species, the seed coat serves as a nutrient reservoir, storing carbohydrates, lipids, proteins and minerals which supplement the embryo's requirements during germination (e.g., Pisum sativum).

Physical protection

The seed coat offers mechanical protection against external threats such as abrasion, microbial attack, insect predation, and environmental stress, acting as the first line of defense for the developing embryo (e.g., maize).

Regulation of germination

By acting as a barrier to water, oxygen, and light, and through the presence of chemical inhibitors, the seed coat regulates timing and conditions of germination, preventing premature germination under suboptimal conditions (e.g., chick pea).

Roles of seed coat during storage condition

During storage, the seed coat plays a crucial role in maintaining seed viability and quality by providing multiple protective functions (Durgbanshi et al. 2005) [3] . In groundnut, the seed coat prevents desiccation, preserving internal moisture and reducing the risk of loss in seed vigour. In sunflower, it offers a physical barrier against microbial invasion. The maize seed coat resists mechanical damage that may occur during handling or storage, safeguarding the embryo (Kozaki et al. 2022) [6] . In apple seeds, it helps regulate dormancy, ensuring seeds do not germinate prematurely under fluctuating storage conditions. The seed coat of black sesame contributes to aging resistance by slowing oxidative deterioration and preserving cellular integrity over time. Additionally, in almond, the seed coat provides thermal insulation, protecting the seed from temperature extremes and maintaining stable internal conditions, which are essential for long-term storage (Radchuk et al. 2011) [10] . [4] Chemical factors

Factors influencing the functions of seed coat

 Lignin: Increases rigidity and impenetrability of the seed coat.  Tannins: Provide antimicrobial activity and deter herbivory.  Pro-anthocyanidins: Contribute to pigmentation and protect against oxidative stress.  Flavonoids: Act as antioxidants and chemical barriers against pathogens.  Chitinases: Enhancing defense against microbial attack

Roles of seed coat in human health

 Promotes digestion: Rich in dietary fiber, aiding bowel movement and preventing constipation (e.g., Pumpkin).  Regulates blood sugar: Soluble fiber slows glucose absorption, supporting glycemic control (e.g., Barley).  Lowers cholesterol: Contains phytosterols that reduce LDL cholesterol and heart disease risk (e.g., Sunflower, Almonds) (Zoe et al. 2024) [11] .  Provides antioxidants: Flavonoids and phenolic compounds help reduce inflammation and oxidative stress (e.g., Cacao beans) (Ali et al. 2022) [1] .  Supplies essential minerals: High in zinc, magnesium, and others crucial for metabolic functions (e.g., Sesame).  Supports heart health: Rich in omega-3 fatty acids and healthy fats that improve lipid profile and reduce inflammation (e.g., Walnut).

Dark side of seed coat: hindrances and hazards

The seed coat essential for protecting the embryo. It can also present several hindrances and hazards that affect seed performance and plant development. In certain crops like maize, the seed coat colour plays a role in attracting insects, making the seeds more vulnerable to pest damage. In walnuts, the seed coat contains chemical inhibitors that prevent premature germination, which, although protective, can delay desired sprouting (Dave et al. 2011) [2] . Avocado seeds often face oxygen restriction due to the thick seed coat, hindering respiration and germination. In legumes like rajma (kidney beans), the seed coat may increase susceptibility to pathogens by trapping moisture or creating entry points for fungal infections (Leslie et al. 2021) [7] . Furthermore, in crops such as wheat, a hard or uneven seed coat contributes to seed dormancy and results in uneven emergence in the field. These examples illustrate the "dark side" of the seed coat, while it plays protective and regulatory roles, it can also hinder seedling vigour, uniformity, and health under certain conditions.

Conclusions

The seed coat is essential for seed protection, germination control and environmental interaction. Its many uses include microbial relationships, nutrition storage, physical defense and dormancy management. The seed coat can be advantageous in many ways but it can also present problems including oxygen limitation, delayed germination and heightened vulnerability to diseases or pests. While optimizing its protective advantages, targeted breeding and seed treatment techniques can help minimize its drawbacks.

Future Thrust

 Research should focus on understanding and utilizing seed-coat-mediated disease resistance and toxin reduction  Utilizing seed coat's protective role may help to develop pest and disease resistant crop varieties which could be a way for sustainable farming  Investigating the seed coat's impact on seed nutrition may aid in breeding crops with enhanced nutritional value to tackle global malnutrition  Enhancing seed longevity by incorporating genetic insights through targeted breeding with identified QTLs  Scrutinizes seed coat genetics and breeders optimize traits like thickness, permeability and composition using advanced techniques helps in enhancing crop performance, food quality and meet the evolving needs of farmers and consumers

References

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