seed

SEED AND SEEDLING STRUCTURE

SEED DEFINITION

  • An embryonic plant enclosed in a protective covering.

  • Developed from a ripened ovule after fertilization by sperm from pollen.

  • Typically enclosed within fruit (ripened ovary).

SEED TYPES

  • Monocot Seed: Contains a single embryonic leaf (scutellum). Example: Maize.

  • Dicot Seed: Contains two embryonic leaves (cotyledons).

SEED STRUCTURE - EUDICOTS

  • Testa: Seed coat, often fused with pericarp (fruit wall derived from ovary wall).

  • Embryo includes:

    • Cotyledon: Embryonic leaf.

    • Epicotyl: Embryonic shoot above cotyledon.

    • Plumule: First true leaf on epicotyl.

    • Hypocotyl: Embryonic stem.

    • Radicle: Embryonic root.

  • Endosperm: Food reserve.

    • Endospermic Seed: Endosperm located outside of seed.

    • Non-Endospermic Seed: Cotyledon acts as endosperm or encloses perisperm (nutritive tissue of seed).

SEED STRUCTURE - MONOCOTS

  • Cereal grains: Single cotyledon modified to scutellum.

  • Scutellum: Elongated to form coleoptile, protecting first leaves while buried.

  • Coleorhiza: Base of hypocotyl elongated to protect radicle.

  • Mesocotyl: Found in some species, such as maize, helps raise leaves to soil surface.

SEED DORMANCY

  • Definition: Temporal delay in germination process, begins with rupture of testa by radicle.

  • Primary dormancy: Induced by abscisic acid (ABA) during maturation.

  • Secondary dormancy: Occurs under unfavorable environmental conditions.

EXOGENOUS DORMANCY

  • Also called coat-imposed dormancy.

  • Restrictions: Seed coat limits water penetration, surrounding tissues limit O2 to embryo.

  • Dormancy break methods:

    • Mechanical or chemical scarification.

    • Scarification: Weakening or opening the seed coat.

ENDOGENOUS DORMANCY

  • Also known as embryo dormancy.

  • Cotyledon can impose dormancy in some species (e.g., European hazel).

  • Embryo may not be fully matured, requiring time to develop.

NON-DORMANT SEEDS

  • Vivipary: Seeds germinate while attached to mother plants (e.g., red mangrove).

  • Preharvest sprouting: Germination of physiologically mature seeds on mother plant (e.g., wheat).

  • Precocious germination: Germination occurs directly on the cobs (e.g., maize).

RELEASE FROM DORMANCY

  • Factors inducing germination include:

    • Light

    • Stratification: Exposure to cold (1-10 °C).

    • Chemical compounds: Nitric oxide (NO), Karrikinolides present in smoke.

HORMONAL CONTROL FOR SEED GERMINATION

  • Abscisic Acid (ABA): Inhibits germination, associated with dormancy.

  • Gibberellin (GA): Promotes germination, initiates metabolic processes.

  • Hormone balance theory: Germination influenced by ABA:GA ratio.

GERMINATION PROCESS

  • Phase I: Imbibition – Dry seed takes up water.

  • Phase II:

    • Imbibition declines, metabolic processes resume (transcription, translation).

    • Emergence of radicle from seed coat.

  • Phase III: Resume water uptake and mobilize food reserves (e.g., starch in amyloplast).

GA SIGNAL FOR GERMINATION

  • GA signals aleurone layer to secrete hydrolyzing enzymes (e.g., α-amylase).

  • Breakdown of starch and other macromolecules, providing nutrients to the embryo.

SEEDLING ESTABLISHMENT

  • Critical for survival and growth, enabling:

    • Photosynthesis.

    • Water and nutrient assimilation.

    • Cellular differentiation and maturation.

SHOOT EMERGENCE

  • Transition from skotomorphogenesis (growth in the dark) to photomorphogenesis (growth in light):

    • Cease etiolation (elongation of hypocotyl under darkness).

    • Apical hook opening, inhibition of ethylene synthesis, symmetry growth via auxin.

VASCULAR DIFFERENTIATION

  • Xylem and phloem differentiation mediated by cytokinin and auxin.

  • WOL (Wooden Leg): Encodes cytokinin receptor; mutations affect vascular development.

  • AXR3: Required for auxin signaling, mutations hinder protoxylem development.

SEEDLING GROWTH

  • Involves increases in cell number and volume through:

    • Cell expansion (tip and diffuse growth).

    • Anisotropic growth (greater enlargement in one direction).

LOOSENING CELL WALL FOR EXPANSION

  • Acid growth hypothesis: Auxin stimulates H+ pumps, activating expansin, increasing cell wall plasticity.

AUXIN AND TROPHISM

  • Tropism: Directional growth responses to stimuli.

    • Phototropism: Shoots grow towards light for photosynthesis.

    • Thigmotropism: Response to touch in climbing plants.

    • Gravitropism: Roots grow downward into soil for water uptake.

ROOT GROWTH AND DIFFERENTIATION

  • Gravitropism: Response to gravity, involving the starch-statolith hypothesis.

  • Root cap: Perceives gravity and secretes compounds to aid soil penetration.

  • Lateral root development: Signals from auxin induce formation of lateral root primordia from pericycle.

ROOT HAIR DEVELOPMENT

  • Extensions of epidermal cells to increase surface area for absorption, promoted by ethylene.

EXAM III DETAILS

  • Date: March 30, 5:00 PM - 6:30 PM in NILS 208.

  • Chapters: 12 - 15.

  • Open-note exam, maximum of two pages allowed.

  • No electronic devices or textbooks permitted. Exam III from 2021 is posted as a study resource.