Seed Structure and Dormancy
Built to wait โ seed anatomy and dormancy
A seed consists of three parts: the embryo (the next sporophyte generation, with radicle/embryonic root, plumule/embryonic shoot, and cotyledons/seed leaves), the endosperm (nutritive tissue, 3n in angiosperms, provides energy for germination), and the seed coat/testa (protective outer layer derived from the ovule integuments).
Seed dormancy is the state of metabolic quiescence in which a viable seed will not germinate even under apparently favorable conditions. Dormancy prevents germination during temporarily favorable conditions that would not sustain the seedling long-term โ for example, preventing germination during a brief warm spell in winter.
๐ก Seedling Establishment โ The Critical Phase
After germination, seedling establishment is the most vulnerable phase of the plant's life cycle:
Hypogeal vs epigeal germination: Hypogeal โ cotyledons remain below ground (peas, oaks, corn). The hypocotyl (embryonic stem below cotyledons) does not elongate dramatically. Epigeal โ cotyledons are pulled above ground by hypocotyl elongation (beans, squash, sunflower). Cotyledons photosynthesize initially, supplementing endosperm reserves.
The hook: In epigeal germinants, the apical hook (curved shoot tip) protects the delicate shoot apical meristem as it pushes through the soil. Ethylene maintains the hooked form. Light triggers hook straightening (phytochrome response) and initiates de-etiolation (transition from dark-grown elongated etiolated form to light-grown compact green form).
Etiolation: Seeds germinating in the dark produce etiolated seedlings โ long, pale, fragile, with unexpanded leaves and no chlorophyll. This maximizes the chance of reaching light before endosperm reserves are exhausted. Once light is detected, rapid de-etiolation occurs: stem elongation stops, leaves expand, chlorophyll is synthesized. Phytochrome (red/far-red light receptor) mediates this transition.
Dorm
Types of dormancy and how to break them
Three main types of seed dormancy:
Physical dormancy (seed coat dormancy): Impermeable seed coat prevents water uptake or oxygen exchange. Seeds of legumes (beans, peas), morning glory, and many hard-coated seeds have physical dormancy. Broken by: scarification โ mechanical abrasion (sandpaper, file), acid treatment (passage through animal digestive tract), or fire (heat cracks the coat). The hard outer coat must be breached before water can enter.
Physiological dormancy: The embryo itself is in a hormonally regulated dormant state โ usually maintained by high ABA levels relative to GA. Most common form of dormancy. Broken by: cold stratification (exposure to cold, moist conditions for weeks to months โ mimics winter, gradually reduces ABA and increases GA sensitivity). Many temperate tree seeds (apple, cherry, oak, maple) require stratification.
Morphological dormancy: The embryo is underdeveloped at seed dispersal and needs time to complete development before germination. Found in some orchid and carrot family seeds.
Memory trick: Physical dormancy = hard coat = scarify (scratch). Physiological dormancy = hormone imbalance = stratify (cold + moist). ABA keeps seed dormant. GA breaks dormancy. Cold stratification = simulated winter.
Cond
Conditions required for germination
Once dormancy is broken, germination requires three essential conditions:
Water (imbibition): The first event in germination is imbibition โ absorption of water by the dry seed. Water rehydrates the embryo and activates metabolism. Water uptake causes the seed to swell dramatically (sometimes 2ร volume) โ generating enough pressure to crack the seed coat. No germination is possible without adequate water.
Oxygen: Germinating seeds have very high metabolic rates and require aerobic respiration for ATP production. Waterlogged soils with low oxygen prevent germination (and can kill germinating seeds by anaerobic conditions).
Appropriate temperature: Each species has a specific optimal temperature range for germination (and minimum and maximum temperatures outside of which germination fails). Cool-season crops (lettuce, spinach, peas): 10โ20ยฐC. Warm-season crops (tomatoes, corn, cucumbers): 20โ30ยฐC. Some seeds require alternating day/night temperatures for optimal germination.
Light (for some species): Photoblastic seeds require specific light conditions for germination. Positive photoblastic seeds (lettuce, tobacco, many grasses) require red light to germinate. Negative photoblastic seeds (phacelia, some onions) are inhibited by light. Most large-seeded crop plants are light-indifferent.
Memory trick: Germination requires WOOT: Water (imbibition), Oxygen (aerobic respiration), Optimal temperature, and sometimes light (photoblastic seeds). Water first โ imbibition is always step 1.
GA
Gibberellin โ the germination hormone
Gibberellin (GA) is the primary hormonal signal that triggers and drives germination after dormancy is broken. GA is synthesized by the embryo scutellum โ diffuses to the aleurone layer (protein-rich outer layer of endosperm) โ aleurone cells secrete ฮฑ-amylase and other hydrolytic enzymes โ enzymes digest stored starch and proteins in the endosperm โ glucose and amino acids released โ fuel embryo growth and radicle elongation.
This GA โ aleurone โ ฮฑ-amylase pathway is the classic demonstration that hormones act as chemical signals between tissues. Experiments: (1) Remove embryo from barley seed โ no ฮฑ-amylase produced in aleurone. (2) Add GA to embryo-less seed โ ฮฑ-amylase production restored. (3) Actinomycin D (transcription inhibitor) blocks GA-induced ฮฑ-amylase โ confirms GA acts at the transcriptional level.
ABA antagonizes GA: high ABA maintains dormancy by repressing GA signaling. Cold stratification gradually degrades ABA and increases GA sensitivity โ dormancy broken โ germination can proceed when water and temperature conditions are met.
Memory trick: GA = Germination Activator. GA from embryo โ aleurone makes ฮฑ-amylase โ starch digested โ food for embryo. ABA = dormancy maintainer (opposes GA). Cold stratification: ABA down, GA sensitivity up.
๐ฌ Applied Scenario โ Germination in Agriculture and Horticulture
Controlling germination is fundamental to agriculture and plant propagation:
A
Stratification for tree propagation. Many temperate tree seeds (apple, cherry, peach, maple, oak) require cold stratification to break physiological dormancy. Nurseries store seeds in moist sand or peat at 2โ5ยฐC for 4โ12 weeks (depending on species) before spring sowing. Without stratification, these seeds will not germinate or germinate very poorly. GA application can sometimes substitute for stratification โ bypassing the cold requirement by artificially raising GA levels.
B
Scarification for hard-coated seeds. Seeds of legumes (clover, alfalfa, vetch), morning glory, and other species with physical dormancy require scarification before planting. Commercial seed treatments: mechanical scarification (tumbling seeds in a drum with sandpaper), hot water treatment (brief immersion in near-boiling water), acid scarification (brief sulfuric acid treatment). Passage through an animal's digestive system is natural scarification โ explaining why many berry seeds germinate better after being eaten and excreted.
C
Malting โ GA in food production. Malting is the controlled germination of barley grains for beer and whiskey production. Barley is soaked (imbibition) โ allowed to germinate for 4โ6 days โ embryo releases GA โ aleurone produces ฮฑ-amylase โ starch in endosperm is partially hydrolyzed to maltose โ germination is halted by kilning (heating) before the seedling grows. The resulting malt contains activated enzymes and partially converted starches used in brewing. Malting is applied plant hormone biology โ exploiting the GA-amylase pathway.
D
Seed priming and uniform germination. Seed priming is controlled partial hydration of seeds (osmotic priming with polyethylene glycol solution, or hydropriming with water) before sowing โ allowing metabolic activation and repair to occur without radicle emergence. Primed seeds germinate faster, more uniformly, and at wider temperature ranges than unprimed seeds. Seed priming is widely used for vegetable seeds (carrot, onion, pepper) where uniform germination and rapid establishment are economically important.
๐ Exam Application
1. Seed structure: Embryo (radicle + plumule + cotyledons) + endosperm (food reserve, 3n) + seed coat (testa).
2. Dormancy types: Physical (hard coat โ scarify). Physiological (ABA maintained โ stratify = cold + moist). ABA keeps dormant, GA breaks dormancy.
3. Germination requirements: WOOT โ Water (imbibition first), Oxygen (aerobic respiration), Optimal temperature, sometimes light (photoblastic seeds = phytochrome mediated).
4. GA โ aleurone โ ฮฑ-amylase: GA from embryo signals aleurone to secrete ฮฑ-amylase โ starch digested โ energy for growth. Key hormone mechanism in germination.
5. Etiolation: Dark germination = elongated, pale, no chlorophyll (maximizes chance of reaching light). De-etiolation on light exposure (phytochrome). Hypogeal = cotyledons below ground. Epigeal = cotyledons emerge above ground.
โ ๏ธ Most Common Seed Germination Mistakes
Imbibition (water uptake) is always the FIRST event in germination โ before any hormonal or metabolic responses. Students sometimes describe GA signaling or metabolic reactivation as the first germination event. Imbibition (passive water absorption by the dry seed) is always first โ it rehydrates the seed, activates enzymes, and causes the seed to swell and crack the coat. Only after imbibition can GA be synthesized and released, the aleurone can respond, and radicle emergence can occur.
The endosperm is 3n (triploid) in angiosperms โ not 2n. The endosperm forms from the fusion of one sperm (n) with two polar nuclei (n + n) during double fertilization = 3n. The embryo is 2n. The seed coat is maternal tissue (2n of the mother plant). Students frequently say 'the endosperm is diploid' โ it is triploid in angiosperms.
Cold stratification breaks PHYSIOLOGICAL dormancy โ scarification breaks PHYSICAL dormancy. Students confuse which treatment breaks which type of dormancy. Physical dormancy = impermeable seed coat โ scarification (mechanical abrasion, acid, heat, animal digestion) to breach the coat โ water can enter. Physiological dormancy = hormone imbalance (high ABA) โ cold stratification (cold + moist conditions) to gradually reduce ABA and increase GA sensitivity. Wrong treatment = no germination.
โ Quick Self-Test
1. What are the three parts of a seed and what does each become?
2. What is seed dormancy and what are the two main types?
3. What are the four conditions required for germination?
4. How does gibberellin trigger the mobilization of food reserves in germinating seeds?
5. What is etiolation and what triggers de-etiolation?
Answers:
1. Three seed parts: (1) Embryo โ the miniature plant (radicle/embryonic root, plumule/embryonic shoot, cotyledons/seed leaves) that develops into the new sporophyte. (2) Endosperm โ nutritive tissue (3n in angiosperms, formed by fusion of sperm + two polar nuclei during double fertilization) that provides energy and nutrients during germination and early seedling growth. (3) Seed coat (testa) โ protective outer layer derived from the ovule integuments, prevents desiccation and physical damage.
2. Seed dormancy is the state in which a viable seed will not germinate even under apparently favorable conditions. Two main types: (1) Physical dormancy โ impermeable seed coat prevents water uptake or gas exchange; broken by scarification (mechanical abrasion, acid treatment, passage through digestive tract). (2) Physiological dormancy โ the embryo is in a hormonally regulated dormant state, usually maintained by high ABA relative to GA; broken by cold stratification (cold, moist conditions for weeks to months, mimicking winter, which gradually degrades ABA and increases GA sensitivity).
3. Four conditions for germination (WOOT): (1) Water โ imbibition (passive water absorption) is always the first event; rehydrates the embryo and activates metabolism. (2) Oxygen โ germinating seeds have high metabolic rates requiring aerobic respiration for ATP. (3) Optimal temperature โ each species has a specific temperature range for germination. (4) Light โ for photoblastic seeds (positive photoblastic = require red light, e.g., lettuce; negative photoblastic = inhibited by light).
4. After imbibition, the embryo scutellum synthesizes and releases gibberellin (GA) โ GA diffuses to the aleurone layer (protein-rich outer endosperm layer) โ GA binds receptors in aleurone cells โ triggers transcription of ฮฑ-amylase and other hydrolytic enzyme genes โ enzymes are secreted into the endosperm โ ฮฑ-amylase digests stored starch into maltose and glucose โ proteases digest stored proteins into amino acids โ these nutrients are absorbed by the embryo to fuel radicle elongation and seedling growth.
5. Etiolation is the morphological adaptation of seedlings germinating in the dark: extreme elongation (maximizing chance of reaching light), pale yellow/white color (no chlorophyll synthesis without light), unexpanded leaves, and an apical hook (protects the shoot tip). De-etiolation occurs when the seedling encounters light: phytochrome (a red/far-red light receptor) detects red light โ rapid signal transduction โ stem elongation halts, leaf expansion begins, chlorophyll synthesis starts, the apical hook straightens. De-etiolation transforms the seedling from a light-seeking etiolated form to a photosynthetically competent form.