Corn Seed Germination and Seedling Development Notes CS 213 - Lecture 6

Dry Matter and Moisture Content in Seeds

  • Seeds have a composition that includes protein, oil, carbohydrate, and mineral salts.
  • Drying to constant moisture content: place tissue in an oven at a set temperature until weight no longer changes on successive measurements (constant weight).
  • Constant weight implies a stable dry matter content; small amounts of moisture may remain, but the weight is effectively fixed.
  • Dry matter basis is preferred in many agronomy studies because it is a non-changing quantity, whereas wet weight varies with environmental conditions.
  • Typical drying temperatures:
    • Seeds: about 70∘C70^{\circ}\mathrm{C} (can be 75−80∘C75-80^{\circ}\mathrm{C} in some cases)
    • Soil samples: about 100−105∘C100-105^{\circ}\mathrm{C}
  • Process overview:
    • Seed or plant part is dried to constant weight, yielding its dry matter content.
    • Dry matter basis is widely used in research data and in course materials.

Seed Germination, Seed Quality, and Planting Considerations

  • Seed vigor and germination: for corn seed, commercial bags typically guarantee ≥95% germination; for grass/cereal seeds, germination is usually in the 90s; for dicots (e.g., soybean, peanut) germination is often in the 80s to mid-80s.
  • Germination definition: the percentage of seeds that germinate and produce a viable seedling from a uniform lot; e.g., 95/100 seeds germinating implies 95% germination.
  • Seed depth for germination: seeds are planted at depths appropriate to maintain moisture; too deep can prevent emergence, but planting deeper can sometimes be necessary to reach moisture.
  • Seed-to-soil contact: press wheels or other equipment help firm soil around the seed to ensure good contact.
  • The three key factors for good germination:
    • Adequate soil temperature
    • Adequate soil moisture
    • Adequate oxygen supply
  • Imbibition and inhibition:
    • Initial water uptake by the seed coat is called imbibition; as moisture enters, the seed coat becomes more permeable to oxygen.
    • The early uptake of water (inhibition) sets the stage for subsequent respiration and growth.
  • Early seed metabolism (no oxygen in initial hydrolysis):
    • Hydrolysis of stored starch to sugars is the first major metabolic step after imbibition and when oxygen begins to diffuse in later.

Seed Anatomy in Monocots: Corn Seed Structure

  • Three fundamental parts of a monocot seed (corn):
    • Seed coat (protective outer layer)
    • Endosperm (bulk of the seed’s interior storage; major starch reservoir)
    • Embryo (the developing plant)
  • The scutellum:
    • A tissue of the embryo adjacent to the endosperm
    • Contains enzymes that digest starch in the endosperm to sugars
    • Provides sugars to the growing embryo during early germination
  • Storage starch in endosperm:
    • Predominantly starch, with some sugar; starch is stored as two main forms:
    • Amylose (linear polymer of glucose)
    • Amylopectin (branched polymer of glucose)
    • The two major forms constitute the bulk of endosperm starch:
    • Amylose and amylopectin together form the endosperm starch.
  • Starch hydrolysis to sugar:
    • Enzymes (primarily in the scutellum) catalyze the hydrolysis of starch using water:
    • Overall reaction (illustrative):

    • (\text{C}6\text{H}{10}\text{O}5)n + n\,\text{H}2\text{O} \rightarrow n\,\text{C}6\text{H}{12}\text{O}6
  • Sugar as the starting point for aerobic respiration:
    • The produced glucose is then metabolized via aerobic respiration to energy (ATP), water, and carbon dioxide.
  • Hydrolysis vs. respiration:
    • Hydrolysis uses water and occurs before oxygen becomes a limiting factor.
    • Aerobic respiration requires oxygen and is a multistep process that produces ATP, water, and CO₂ as products.

Aerobic Respiration in Germinating Seed: Energy for Growth

  • Key concept: after hydrolysis, glucose undergoes aerobic respiration in the presence of oxygen and enzymes to yield energy and byproducts.
  • General aerobic respiration equation (illustrative):
    • C<em>6H</em>12O<em>6+6 O</em>2→6 CO<em>2+6 H</em>2O+ATP\text{C}<em>6\text{H}</em>{12}\text{O}<em>6 + 6\,\text{O}</em>2 \rightarrow 6\,\text{CO}<em>2 + 6\,\text{H}</em>2\text{O} + \text{ATP}
  • Major products and role:
    • ATP: primary energy currency driving growth and development
    • CO₂: can contribute to soil and air dynamics; relevant in discussions of soil respiration
    • H₂O: water produced but also consumed as the seed imbibes water and continues metabolism
  • Physiological sequence during germination:
    • Water continues to move into the seed; the seed swells as respiration proceeds
    • Oxygen diffuses into interior through the loosened seed coat
    • Hydrolysis and respiration continue as the embryo develops
  • Notable historical example:
    • Egyptians reportedly split granite by introducing wet seeds into cracks, leveraging swelling pressure from imbibition to generate forces capable of cracking rock

Embryo, Embryonic Axis, and Early Seedling Development in Corn

  • Embryo components and initial emergence:
    • The first tissue to emerge from the seed coat is the radical (primary root)
    • The second tissue to emerge is the shoot tissue
    • Germination is defined as the emergence of the first root tissue from the seed coat
  • Embryonic axis terminology:
    • Embryonic axis: the portion of the embryo that develops into the root and shoot
    • In monocots, the term “embryo” is sometimes constrained to the seed’s tissue prior to its emergence; once the growing processes unfold, the term embryonic axis is used for the developing seedling after germination
  • Early root system: seminal roots
    • The first roots that form below the scutellar node are the seminal roots (seed-form roots)
    • There are typically about five to six initial seminal roots
  • Early shoot system and key shoot tissues:
    • Mesocotyl: tissue between the scutellar node and crown node; lengthens early in the life of the seedling
    • Scutellar node: the lowermost node on the shoot where the embryo was attached to the endosperm
    • Crown node: the next node above the scutellar node; site of adventitious root initiation later
    • Shoot apex (plumule): region above the crown node responsible for producing all above-ground tissues
    • Coleoptile: hollow sheath that protects the emerging shoot and through which the first leaf emerges
  • Emergence and transition to autotrophy:
    • The coleoptile tip emerges first, followed by the first true leaf whose growth will photosynthesize
    • Before coleoptile emergence, growth is heterotrophic (relying on stored reserves)
    • After coleoptile emergence and leaf greening, growth becomes autotrophic (photosynthesis supplies energy), although belowground tissues remain heterotrophic and rely on translocated resources
  • Hormonal changes associated with coleoptile emergence:
    • Mesocotyl lengthening ceases after coleoptile tip emerges from soil
    • Seminal root growth slows after coleoptile emergence; adventitious roots begin to form and replace seminal roots over time

Transition from Seminal Roots to Adventitious (Brace/Prop) Roots

  • Adventitious roots arise from crown node and subsequent nodes as the plant grows, replacing seminal roots in importance as development proceeds
  • Brace or prop roots:
    • Adventitious roots that emerge from stem nodes above or below ground
    • Can be numerous, particularly around tasseling and silking stages
  • Role of brace/prop roots in phosphorus uptake:
    • These roots form in the upper soil region where much phosphorus resides
    • They are important for phosphorus absorption and translocation to developing ears
    • About half of the total phosphorus accumulated in a corn plant ends up in the ear; continual phosphorus uptake during ear development is crucial for ear size and yield
  • Phosphorus availability in soil:
    • Phosphorus is the least soluble major plant nutrient; most of it is concentrated in the upper soil profile
    • This makes the upper soil phosphorus pool particularly important for the seedling and early growth stages

Nutrient Essentials and Early Growth: Role of Phosphorus and Beyond

  • Essential plant nutrient elements definition:
    • Elements required for a plant to complete its life cycle (seed to seed) in the right quantities
    • If an essential element is absent, the plant cannot complete its life cycle
  • First essential element required from outside the seed’s storage content:
    • Phosphorus (P): crucial for early root development and overall seedling vigor
  • Endogenous vs exogenous nutrient supply:
    • Seeds contain some essential elements within their dry storage, but once those reserves are exhausted, the developing seedling depends on soil and air for essential nutrients
  • Growth stage importance for nutrient needs:
    • The six-leaf stage marks a shift in nutrient demand, with nitrogen becoming notably critical at this stage

Growth Stages: How to Identify and Interpret Seedling Stages

  • Growth stage terminology in corn seedlings:
    • One-leaf stage: when the first leaf partially shows; the early stage after germination
    • Two-leaf stage: the second leaf is visible
    • Three-leaf stage: the third leaf is visible with the tip of the fourth leaf beginning to emerge
    • Six-leaf stage: defined when the tip of the seventh leaf is just visible; used as a common growth reference point
  • Practical example provided in lecture:
    • If the tip of the coleoptile is visible and the first leaf has emerged, you observe the two-leaf stage
    • When the third leaf is nearly fully developed and the tip of the fourth leaf begins to emerge, the plant is at the three-leaf stage

Additional Concepts and Practical Notes

  • Autotrophy vs heterotrophy summarized:
    • Heterotrophic growth: initial phase before photosynthesis begins; energy from stored reserves
    • Autotrophic growth: after the first leaf turns green and begins photosynthesizing; the plant starts making its own food
  • Interaction between roots and shoots:
    • Aboveground tissues become autotrophic and provide energy for growth, but rely on belowground tissues for water and nutrients via translocation
    • Belowground tissues remain heterotrophic for much of the plant's life, since roots lack chlorophyll for photosynthesis
  • Importance of timely nitrogen management around the six-leaf stage:
    • Nitrogen becomes critically important around this stage to support rapid growth and development of the shoot system

Quick Reference: Key Terms and Concepts

  • Seed coat, Endosperm, Embryo
  • Scutellum: embryo tissue with starch-digesting enzymes
  • Amylose and Amylopectin: two major forms of starch in endosperm
  • Hydrolysis: starch breakdown to sugars using water
  • Aerobic respiration: sugar + oxygen → CO₂ + H₂O + ATP
  • Radicle: first root emergent from seed
  • Seminal roots: early seedling roots formed from the seed
  • Mesocotyl: section between scutellar node and crown node
  • Crown node: base of the shoot system where adventitious roots form
  • Shoot apex / Plumule: growing point for aboveground tissues
  • Coleoptile: hollow sheath through which the first shoot emerges; not a true leaf but can photosynthesize when exposed to light
  • Heterotrophic vs Autotrophic growth: energy sources before/after the first photosynthetic leaf
  • Brace/Prop roots: adventitious roots from stem nodes, important for upper-soil phosphorus uptake
  • Growth stage determination by leaf tips (e.g., six-leaf stage when the tip of the seventh leaf is visible)
  • Important growth-stage nutrient implications: phosphorus early, nitrogen around six-leaf stage