Seed Biology
Part of Seed Saving
A seed is not simply a dormant plant waiting for water. It is a complex biological package with its own anatomy, energy reserves, protective structures, and chemical signaling systems. Understanding seed biology allows you to harvest at the right time, store correctly, break dormancy when needed, and diagnose problems when germination fails.
Seed Anatomy
Every seed contains three fundamental components: the embryo, the seed coat (testa), and the endosperm (or cotyledons, in dicots).
The Embryo
The embryo is the miniature plant. It contains:
- Radicle: The embryonic root, the first structure to emerge during germination
- Plumule: The embryonic shoot tip, including the first true leaves
- Hypocotyl: The stem region between radicle and cotyledons
- Cotyledons: Seed leaves, 1 in monocots (grasses, grains) and 2 in most dicots (tomatoes, beans, brassicas)
The embryo is metabolically dormant but alive. It requires a continuous supply of stored energy during storage and reactivation of metabolism during germination.
The Endosperm
The endosperm is a nutritive tissue that supplies the embryo with energy and nutrients during germination. In cereals (wheat, corn, rice), the endosperm is large and starchy — this is the bulk of what we eat as grain.
In legumes (beans, peas), the cotyledons absorb most endosperm nutrients during seed development and become the primary energy store. Bean cotyledons emerge above ground during germination, shrink as their reserves are consumed, then fall off.
The Seed Coat (Testa)
The seed coat is the outer protective layer. It:
- Prevents premature water uptake
- Protects the embryo from physical damage, pathogens, and UV radiation
- Regulates gas exchange (oxygen, carbon dioxide)
- May contain chemical inhibitors that enforce dormancy
- Contributes to longevity in storage by limiting moisture exchange
Seed coat integrity is critical for long-term storage. Cracked or broken seed coats allow water and pathogens direct access to the embryo.
Seed Formation and Maturation
Seeds develop in stages after fertilization. Understanding these stages helps determine optimal harvest time.
| Stage | Approximate Days Post-Pollination | Seed Characteristics |
|---|---|---|
| Cell division | 0–7 days | Seed barely visible; no viable embryo |
| Cell elongation | 7–14 days | Embryo forming; seed soft and green |
| Reserve accumulation | 14–40 days | Starch, oils, proteins accumulate; seed swelling |
| Maturation drying | Last 20–40% of seed-fill period | Moisture drops; seed hardens; seed coat forms |
| Physiological maturity | Moisture ~30–50% | Maximum dry weight reached; seed fully viable |
| Harvest maturity | Moisture ~12–20% | Seed visibly dried; optimal harvest window |
Physiological maturity is the moment a seed reaches maximum viability. After this point, the seed continues to dry on the plant but does not gain additional germination capacity. Harvesting after physiological maturity (when seeds have dried somewhat) is acceptable; harvesting before it risks immature seed with low germination.
Germination Biology
Germination is the transition from dormant seed to active seedling. It requires three conditions: adequate moisture, appropriate temperature, and (in some species) light.
Imbibition: Taking Up Water
Germination begins with imbibition — the seed absorbs water rapidly through the seed coat. Dry weight may increase 50–200% within the first 12–24 hours. This activates metabolic enzymes that begin breaking down stored reserves.
Imbibition Damage
Seeds imbibed in very cold water (below 5°C) can suffer chilling injury, particularly in warm-season crops (beans, corn, squash). This causes abnormal germination or seedling death. Always sow warm-season crops into soil that has warmed adequately.
Mobilization of Reserves
Once water is absorbed, the embryo secretes hormones (particularly gibberellins) that activate enzymes in the endosperm. These enzymes break down:
- Starch → sugars (energy for embryo growth)
- Storage proteins → amino acids (for new protein synthesis)
- Fats/oils → glycerol and fatty acids (energy, particularly in oil-rich seeds)
This mobilization continues until the seedling has established photosynthesis and can produce its own energy.
Temperature Requirements
Each crop has an optimal temperature range for germination. Below the minimum, germination stalls or fails. Above the maximum, enzymes are damaged and germination fails.
| Crop | Minimum (°C) | Optimal (°C) | Maximum (°C) |
|---|---|---|---|
| Lettuce | 2 | 18–22 | 30 |
| Peas | 4 | 15–20 | 29 |
| Wheat, barley, oats | 2 | 15–25 | 35 |
| Brassicas | 4 | 15–25 | 35 |
| Tomato | 10 | 20–27 | 35 |
| Beans (common) | 10 | 25–30 | 35 |
| Corn | 10 | 25–30 | 40 |
| Squash, cucumber | 15 | 25–35 | 40 |
| Melon | 18 | 30–35 | 40 |
Light Requirements
Most vegetable and grain seeds are indifferent to light — they germinate equally in light or darkness. A minority require light (lettuce, some wild species) or are inhibited by light (some root vegetables). For practical purposes, most agricultural seeds sown at the correct depth (soil cover blocks light) germinate without issue.
Seed Respiration During Storage
Even dormant seeds respire — they consume oxygen and release carbon dioxide at a very low rate. This is why seeds stored in completely sealed containers do not suffer oxygen depletion (the rate is so low that equilibrium with atmospheric oxygen is maintained through minor exchanges).
However, respiration rate is highly sensitive to temperature and moisture:
- Every 1% increase in seed moisture roughly doubles the respiration rate
- Every 5°C increase in temperature roughly doubles the respiration rate
- High respiration = faster consumption of reserves = faster loss of viability
This is why cool, dry storage dramatically extends seed longevity. It is not arbitrary preference — it directly reduces the metabolic drain on stored reserves.
Seed Vigor vs. Germination Percentage
Germination percentage measures how many seeds germinate under ideal conditions. Vigor measures how fast and uniformly seeds germinate, and how well they perform under stress.
A seed lot may show 90% germination in a warm laboratory test but only 60% germination when sown in cold spring soil, because the seeds lack vigor to overcome suboptimal conditions.
Vigor declines before germination percentage drops. A high-germination lot may still have declining vigor — seedlings emerge slowly, unevenly, and are more susceptible to damping-off fungi.
Practical vigor test: Germinate seeds on wet paper towel at 10–12°C (rather than optimal temperature). The percentage that germinate successfully at this sub-optimal temperature gives a better estimate of field performance than a warm-conditions test.
Seed Oil Chemistry and Long-term Storage
Seeds rich in unsaturated fats (flax, sunflower, sesame) are more vulnerable to oxidative degradation during storage than starchy seeds (cereals) or protein-rich seeds (legumes). Oxidation of seed oils degrades cell membranes, eventually killing the embryo.
| Seed Type | Primary Reserve | Storage Vulnerability | Relative Longevity |
|---|---|---|---|
| Cereal grains | Starch | Low | High (5–10+ years) |
| Legumes | Protein | Medium | Medium (4–8 years) |
| Oilseeds (sunflower, flax) | Fat | High | Lower (2–4 years) |
| Alliums (onion, leek) | Mixed | High | Low (1–2 years) |
For oilseeds and alliums, refresh seed every 1–2 years and store in the coolest possible conditions to delay oxidative degradation.
Seed Anatomy and Harvest Decisions
Understanding anatomy guides harvest timing:
- Cereal seeds: Harvest when starch reserves are fully deposited and moisture has dropped to 15–20%. Seeds that snap when bitten have reached target dryness.
- Legume seeds: Harvest when pods begin to yellow and rattle; embryo cotyledons should be firm and fully developed.
- Tomato seeds: Fermentation separates the gel coat (which contains germination inhibitors). Seeds are fully mature when the surrounding gel is clear rather than milky.
- Squash and melon seeds: Harvest only from fully ripe (eating-ripe) fruit; seeds continue maturing after the fruit appears ripe to the eye.
Immature Seeds Do Not Catch Up
A seed harvested before physiological maturity does not continue maturing after harvest. It remains partially developed and will germinate poorly or not at all. When in doubt, wait longer before harvesting.
Seed Biology Summary
Seeds consist of an embryo (the developing plant), endosperm or cotyledons (energy reserves), and a protective seed coat. Maturity proceeds from cell division through reserve accumulation to physiological maturity — the point of maximum viability. Germination requires moisture, appropriate temperature, and sometimes light; imbibition activates reserve mobilization. Storage longevity depends on seed type (cereal > legume > oilseed), temperature, and moisture, because all three affect the rate of metabolic drain and oxidative damage. Vigor — performance under stress — declines before germination percentage, making cool-condition germination tests more informative than standard tests.