Seed Germination and Emergence - Comprehensive Notes SC 213 -Lecture 7

Seed parts: monocots vs dicots

• Monocots (e.g., corn) have three fundamental seed parts: the seed coat, the endosperm, and the embryo. The embryo has one cotyledon. In corn, that cotyledon is referred to as a scutellum (spelled here as scotella in the lecture).
• All seed parts collectively support germination and seedling development in monocots.
• Dicots (e.g., soybeans, peanuts, cotton) typically have two seed parts: the seed coat and the embryo, with the embryo consisting of two cotyledons.
• In many dicots, the endosperm is consumed during seed maturation as the cotyledonary tissue develops; a few dicots retain endosperm up to physiological seed maturity.
• Most dicots have only the two seed parts (seed coat + embryo), though some retain endosperm for a time.
• Legumes: soybeans and peanuts are dicots that are also legumes; cotton is a dicot that is not a legume. All legumes are dicots, but not all dicots are legumes.

Seed composition: oil and protein in dicot seeds (soybean as example)

• Oil content in soybean ≈ 20%.
• Protein content in soybean ≈ 40%.
• These constituents play important roles in germination and seedling vigor. Oil and protein provide energy and building blocks during early germination; the balance of nutrients influences the rate and success of germination.
• In monocots, similar principles apply: seeds with higher oil/protein content can show robust hydrolysis and energy supply during imbibition.
• Relevant numerical reference:
  $ext{Oil} <br /> ightarrow 0.20\quad \text{and} \quad \text{Protein} \rightarrow 0.40\; \text{(for soybean)}.$

Seed imbibition and hydrolysis: sequence and chemical changes

• Initial, critical step: imbibition — water entry into the seed.
• Wet seed coat becomes more permeable to oxygen, enabling cellular activities.
• Hydrolysis processes begin once water is absorbed:
  • Starch ( carbohydrate ) hydrolysis to sugars (monosaccharides):
    $\text{(C}<em>6\text{H}</em>{10}\text{O}<em>5)</em>n + n\,\mathrm{H_2O} \rightarrow \text{glucose (and other sugars)}.$
  • Proteins hydrolyze to constituent amino acids:
    $\text{Proteins} + \mathrm{H_2O} \rightarrow \text{amino acids}.$
  • Oils (lipids) hydrolyze to glycerol and fatty acids:
    $\text{Lipids} + \mathrm{H_2O} \rightarrow \text{glycerol} + \text{fatty acids}.$
• The hydrolysis of starch, proteins, and lipids occurs when seeds imbibe water, with monocots (especially oil/protein-rich seeds like soybean) showing strong activity due to reserves.
• Accumulation of hydrolysis products (sugars, amino acids, glycerol, fatty acids) leads to visible seed changes and initiates energy generation for growth.

Early seedling development in dicots: anatomy and emergence dynamics

• Seed coat remains surrounding the seed and protects the interior as germination begins.
• In dicots, the two cotyledons (cotyledonary halves) are attached to the embryonic axis at the cotyledonary node.
• Hypocaudal region (the tissue below the cotyledons) grows and elongates; a distinctive arch forms at the base of the cotyledons, called the hypocaudal arch (also described as shepherd’s crook).
• The primary root emerges from the embryonic axis region between the cotyledon halves.
• The region between the point of cotyledon attachment to the embryonic axis and the cotyledonary node expands as energy and respiration increase.
• The cotyledonary node is the attachment point where the cotyledons interface with the embryonic axis.
• The tissue between the cotyledons and the embryonic axis includes the plumule, a meristematic tissue that will give rise to all above-ground parts.
• The two cotyledons protect the delicate plumule during emergence.
• Analogy: the protective function of cotyledons in dicots is similar to the protective role of the coleoptile in monocots during emergence.
• Seed reserves are consumed as the seedling grows heterotrophically (before photosynthesis starts) and later autotrophically once photosynthesis can occur.
  • Heterotrophic growth dominates before the shoot is above the soil line.
  • Autotrophic growth begins after the seedling becomes photosynthetically capable.
• Monocots (e.g., corn) have a different protective structure (coleoptile) that shields the shoot apex during emergence; the seed remains and reserves are consumed similarly, but the protective anatomy differs.

Key structures and terminology (dicot-focused)

• Cotyledons: the two seed halves that provide stored nutrients; not true leaves.
• Cotyledonary node: point of attachment of the cotyledons to the embryonic axis.
• Embryonic axis: the central axis from which the seedling will develop.
• Hypocaudal (hypocotyl): tissue below the cotyledons; elongation pulls cotyledons upward.
• Epicotyl: tissue above the cotyledons that will form the above-ground stem and leaves.
• Plumule: the shoot meristem within the embryo that will give rise to the above-ground plant tissues.
• Epicotyl vs hypocotyl growth: determines whether cotyledons end up above or below the soil line at emergence.
• Mesocotyl: in monocots, the segment between cotyledons and the shoot apex; elongation helps push the coleoptile out of the soil.
• Coleoptile: a hollow protective sheath covering the monocot shoot apex during emergence.
• Coleorhiza (not detailed in this talk, but related): protective sheath for the root tip in some monocots.

Emergence types in dicots and their examples

• Epigeal emergence (epigeal): hypocaudal (hypocotyl) lengthens faster than the epicotyl; cotyledons are pulled above the soil surface; cotyledons become green and photosynthesize. Example: soybean.
• Hypogeal emergence (hypogeal): epicotyl lengthens faster than the hypocaudal; cotyledons stay below the soil line for the life of the plant. Example: garden pea; cotyledons do not photosynthesize.
• Hypogeal-epigeal emergence (hyposgeal-epigeal): both hypocaudal and epicotyl lengthen at about the same rate so cotyledons end up near the soil line. Example: peanut.

Peanut-specific emergence and development (A focus on the Virginia peanut)

• Peanut biology: botanically, the fruit forms as a peg from the flower and grows into a pod that develops underground; embryos form inside the fruit.
• Emergence pattern: hypogeal-epigeal emergence; the rate of lengthening of hypocaudal and epicotyl are about the same, so cotyledons sit near the soil line as the peanut plant develops.
• Cotyledonary lateral branches: two cotyledonary lateral branches develop from the cotyledonary node; about 50% of final yield in Virginia-type peanuts comes from flowers/fruits formed on these cotyledonary laterals.
• Flowering and peg development: a flower forms at a cotyledonary node, produces a peg that grows downward into the soil; the peg carries the embryo and develops into a peanut fruit underground.
• Physiological seed maturity: a stage when the fruit has accumulated all the dry matter it can during development.
• Calcium needs and land plaster application: to support proper cell wall formation and calcium pectates in the developing fruit, gypsum/land plaster (calcium sulfate) is applied at first flowering; common rates are roughly 800–1200 pounds per acre.
  • Rationale: the mother plant can transport limited calcium down the peg to the developing fruit; external calcium via land plaster ensures adequate calcium for proper cell wall formation.
  • Regional variation: some peanut varieties/regions require less, and some contexts may not require land plaster at all.
• Harvest timing: a critical management decision due to a broad fall maturation window; overly late digging can result in the loss of older, mature fruit pods that drop from the plant.
• Cotyledons and photosynthesis: in epigeal emergence (e.g., soybean), cotyledons can photosynthesize after emerging; in hypogeal cases like peanut, cotyledons may stay near soil line and contribute stored reserves without photosynthesizing.
• Agronomic caution for cultivation: avoid placing excessive soil near cotyledonary lateral regions, as this can bury and damage developing structures; cultivation should be careful around the cotyledonary regions to prevent yield loss.
• Yield contribution from cotyledonary structure emphasis: cotyledonary lobes and their lateral branches are crucial for yield in peanuts, particularly the Virginia type.

Growth habit in soybeans: determinate vs indeterminate (relevant to seedling development and yield)

• Determinate growth habit: topmost main-stem node ends in reproductive growth only (flowers and seeds) with limited vegetative growth beyond the terminal flowering stage; typical of many Southeastern U.S. cultivars.
• Indeterminate growth habit: topmost main-stem node maintains both vegetative and reproductive growth; common in Midwest regions; results in continued vegetative growth and potential additional nodes/leaves/flowers beyond terminal flowering.
• Implications for yield: indeterminate systems can produce more branches and yield from multiple nodes; determinate systems may have more synchronized flowering and possibly earlier cessation of vegetative growth.
• Summary statement from lecture: In indeterminate types, the topmost node on the main stem always has potential for both vegetative and reproductive development; in determinate types, the topmost node ultimately transitions to reproductive growth only.

Post-emergence: seedling architecture and transition to autotrophy

• Once the seedling fully emerges, the arrangement typically includes:
  • Primary root (developed from the embryonic axis)
  • Hypocaudal (below cotyledons) tissue
  • Epicaudal (above cotyledons) tissue
  • Plumule (shoot apex) in position to form the shoot
  • Cotyledons remain as stored-food providing energy; they are not true leaves.
• Transition: seedling shifts from heterotrophic growth (relying on stored seed reserves) to autotrophic growth (photosynthesis) once the shoot system can be sustained above the soil line.

Quick recap of key structures and relationships (glossary-type prompts)

• Cotyledons: the embedded storage organs; in epigeal germination, they can photosynthesize; in hypogeal germination, they stay below ground and do not photosynthesize.
• Cotyledonary node: attachment point of cotyledons to the embryonic axis.
• Embryonic axis: central axis giving rise to plumule and root tissues.
• Plumule: shoot meristem that grows into the above-ground plant parts; protected during emergence by cotyledons or by coleoptile in monocots.
• Hypocotyl (hypocaudal in lecture): below cotyledons; elongation drives cotyledons toward the surface in many cases; arching appearance during emergence described as shepherd’s crook.
• Epicotyl: region above the cotyledons; gives rise to the first true leaves above cotyledons.
• Mesocotyl: in monocots, the segment between cotyledons and the shoot apex; elongation helps raise the coleoptile and emerging shoot through the soil.
• Coleoptile: a hollow protective sheath around the shoot apex in monocots; protects the meristem as it emerges and opens to reveal the first true leaf.
• League of terms for emergence patterns: epigeal, hypogeal, hypogeal-epigeal (hyposgeal-epigeal).

Exam-oriented takeaways

• Understand the fundamental seed parts for monocots and dicots and how these parts contribute to germination and seedling establishment.
• Be able to distinguish epigeal vs hypogeal vs hypogeal-epigeal emergence and give plant examples (soybean, garden pea, peanut).
• Recognize the key structures in dicot germination: cotyledonary node, hypocaudal arch, primary root, plumule, epicotyl, and cotyledons’ changing roles from stored food to potential photosynthetic tissue (in the right emergence type).
• For peanuts, know the special roles of the cotyledonary lateral branches in yield, the peg mechanism, the underground fruit development, and the importance of calcium supplementation via land plaster at first flowering.
• Distinguish determinate vs indeterminate soybean growth habits and relate to yield potential and management in different U.S. regions.
• Remember the protective roles of seed coats, coleoptile/coleorhiza (in monocots) and cotyledons in shielding delicate embryonic tissues during emergence.
• Key numbers to recall:
  $ext{Soybean oil} \approx 20\%,$ $\text{Soybean protein} \approx 40\%,$ $\text{Land plaster/calcium sulfate rate} \approx 800\text{–}1200\,\text{lb/acre}.$
• Practical cultivation cautions: avoid burying cotyledonary lateral regions too deeply; cultivation should be careful around emerging peanut tissues to prevent yield loss.