Seed Germination Steps

Why This Matters

Seed germination isn't just a checklist of events—it's a masterclass in how plants coordinate biochemical signaling, resource allocation, and developmental timing to transition from dormancy to active growth. You're being tested on your understanding of enzyme activation, hormone regulation, water relations, and energy metabolism—all packed into this single process. Every AP question about germination is really asking: do you understand how seeds sense their environment and mobilize resources at precisely the right moment?

The steps of germination also demonstrate core physiological principles you'll see throughout the course: turgor pressure driving cell expansion, hydrolysis reactions converting storage molecules to usable forms, and the shift from heterotrophic to autotrophic nutrition. Don't just memorize the sequence—know what mechanism each step illustrates and how disrupting any single phase would cascade through the entire process.

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Water Relations and Physical Changes

The first phase of germination centers on water uptake and the physical transformations it triggers. Without adequate hydration, no biochemical machinery can activate.

Imbibition (Water Absorption)

• Imbibition—the passive absorption of water through the seed coat—causes seeds to swell by 2-3 times their dry weight
• Seed coat softening allows oxygen penetration and prepares for radicle emergence; some species require scarification to enhance this process
• Triggers the rehydration of cellular membranes and organelles, which is essential before any metabolic activity can resume

Embryo Cell Expansion

• Turgor pressure—the force of water pushing against cell walls—drives cell enlargement without requiring new cell division initially
• Water uptake into vacuoles creates the hydraulic force necessary for the radicle to break through the seed coat
• Cell expansion is regulated by cell wall loosening, controlled by proteins called expansins that allow walls to stretch

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Biochemical Activation and Nutrient Mobilization

Once water enters the seed, dormant enzymes reactivate and begin converting stored macromolecules into transportable, usable forms. This phase represents the metabolic "wake-up call."

Activation of Enzymes

• Gibberellic acid (GA) signals the aleurone layer to synthesize hydrolytic enzymes—this hormone-enzyme relationship is a classic exam topic
• Amylases break starch into maltose and glucose; proteases convert storage proteins into amino acids; lipases hydrolyze fats into glycerol and fatty acids
• Enzyme activation follows a lag phase during imbibition, meaning germination timing depends on both water availability and temperature

Mobilization of Stored Nutrients

• Endosperm or cotyledon reserves—primarily starch, proteins, and lipids—are broken down via hydrolysis reactions
• Breakdown products are transported to the embryonic axis (radicle and plumule) to fuel respiration and biosynthesis
• This heterotrophic phase sustains the seedling until photosynthetic independence; seeds with larger reserves can survive longer in low-light conditions

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Structural Emergence and Directional Growth

With energy available, the embryo physically breaks dormancy through coordinated emergence of root and shoot structures, each responding to different environmental cues.

Radicle Emergence

• Radicle (embryonic root) emerges first in nearly all species—this priority ensures water and mineral uptake before the shoot demands resources
• Emergence is guided by positive gravitropism, with auxin redistribution causing differential cell elongation on the lower side
• Successful radicle emergence is the operational definition of germination; everything before is preparation, everything after is seedling development

Plumule Growth

• Plumule (embryonic shoot) grows upward via negative gravitropism and positive phototropism, orienting leaves toward light
• In dicots, the hypocotyl hook protects the delicate plumule as it pushes through soil; light triggers hook straightening
• Plumule growth rate depends on available reserves and environmental signals—etiolation occurs in darkness as the shoot elongates rapidly seeking light

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Transition to Autotrophy

The final phase marks the seedling's shift from dependence on seed reserves to self-sufficient photosynthesis—a critical survival threshold.

Cotyledon Expansion

• Cotyledons (seed leaves) serve dual functions: nutrient storage in some species and early photosynthesis in others
• Epigeal germination brings cotyledons above ground where they photosynthesize; hypogeal germination keeps them buried as pure storage organs
• Cotyledon function determines how quickly a seedling must produce true leaves—species with photosynthetic cotyledons have a longer buffer period

Seedling Establishment

• Establishment requires successful root anchorage, functional vascular connections, and positive carbon balance from photosynthesis
• The seedling must withstand environmental stresses—drought, herbivory, competition—that the protected seed did not face
• This phase has the highest mortality rate in plant life cycles; understanding establishment explains why plants produce thousands of seeds

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Quick Reference Table

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Self-Check Questions

1. Which two steps both involve water uptake but differ in whether the process is passive or actively regulated? Explain the mechanism behind each.

2. If a mutation prevented gibberellic acid synthesis, which germination step would fail first, and what downstream effects would you predict?

3. Compare and contrast radicle emergence and plumule growth in terms of their tropism responses and the role of auxin in each.

4. A seedling germinates in complete darkness. Describe how cotyledon expansion and plumule growth would differ from a seedling germinating in light, and explain why.

5. An FRQ asks you to explain why seedling establishment represents a "critical survival threshold." Using at least two germination steps, construct an argument linking reserve mobilization to establishment success.