Seed germination marks the start of a plant's life cycle, transforming dormant seeds into vibrant seedlings. This process involves complex physiological changes, from water uptake to enzyme activation, setting the stage for growth.

As seedlings emerge, they face critical developmental decisions. Light exposure triggers photomorphogenesis, while darkness leads to skotomorphogenesis. Plant hormones like gibberellins and abscisic acid play key roles in regulating these early growth stages.

Seed Germination Stages

Dormancy and Imbibition

• Seeds remain dormant until favorable conditions for germination are met (adequate moisture, temperature, oxygen)
• Seed coat protects the embryo during dormancy and prevents premature germination
• Imbibition is the uptake of water by the dry seed which activates metabolic processes
  • Water uptake causes the seed to swell and soften, allowing enzymes to become active
  • Respiration rate increases as the seed hydrates providing energy for growth

Radicle Emergence and Cotyledon Development

• The radicle (embryonic root) is the first structure to emerge from the seed during germination
  • Radicle grows downward in response to gravity (gravitropism) to establish the root system
• Cotyledons are the embryonic leaves that store food reserves (starch, proteins, lipids) in many seeds
  • Cotyledons may remain below ground (hypogeal germination) or emerge above ground (epigeal germination)
  • In epigeal germination, cotyledons become photosynthetic and provide nutrients until true leaves develop (beans, lettuce)
  • In hypogeal germination, cotyledons remain below ground and the epicotyl emerges (peas, corn)

Seedling Growth and Development

Hypocotyl and Epicotyl Growth

• The hypocotyl is the stem region below the cotyledons that elongates during germination
  • Hypocotyl growth pushes the cotyledons and shoot tip above the soil surface in epigeal germination
• The epicotyl is the stem region above the cotyledons that gives rise to the first true leaves
  • Epicotyl growth occurs after the cotyledons have emerged or the shoot tip has been pushed above the soil

Photomorphogenesis and Skotomorphogenesis

• Photomorphogenesis is the development of the seedling in the presence of light
  • Light triggers the opening of the apical hook, expansion of the cotyledons, and greening of the seedling (chlorophyll synthesis)
  • Photomorphogenesis is mediated by phytochromes (red/far-red light receptors) and cryptochromes (blue light receptors)
• Skotomorphogenesis (etiolation) is the development of the seedling in the absence of light
  • Seedlings have elongated hypocotyls, closed cotyledons, and underdeveloped chloroplasts (appear pale yellow)
  • Skotomorphogenesis allows the seedling to rapidly grow towards the soil surface to access light

Plant Hormones in Germination and Growth

Gibberellins Promote Germination and Seedling Growth

• Gibberellins (GA) are plant hormones that stimulate seed germination and seedling growth
  • GA is synthesized in the embryo and released during imbibition
  • GA activates enzymes (alpha-amylase) that break down stored starch in the endosperm to provide energy for growth
• GA promotes stem and leaf growth by stimulating cell division and elongation
  • Dwarf mutants (corn, peas) are deficient in GA synthesis or signaling resulting in reduced stem growth

Abscisic Acid Maintains Dormancy and Inhibits Growth

• Abscisic acid (ABA) is a plant hormone that maintains seed dormancy and inhibits germination
  • ABA levels are high in dormant seeds and decrease during imbibition allowing germination to proceed
  • ABA inhibits GA synthesis and activity in the seed preventing premature germination
• ABA is involved in stress responses (drought, salinity) and promotes stomatal closure to reduce water loss
  • Exogenous application of ABA can be used to prolong seed storage and improve stress tolerance in crops