7.3 Germination and seedling development

Seeds contain everything needed to produce a new plant: an embryo, stored food reserves, and a protective coat. Understanding how seeds are built, how they break dormancy, and how they develop into seedlings is central to plant biology. This topic covers seed structure, dormancy mechanisms, the germination process, and early seedling growth.

Seed structure and composition

Seeds are the reproductive units of flowering plants. Each seed packages three main components: an embryo, stored food reserves, and protective outer layers. These components work together to keep the embryo alive during dispersal and fuel its growth once germination begins.

Embryo

The embryo is the young, undeveloped plant inside the seed. It has three main parts:

• Radicle: the embryonic root, which will become the primary root
• Plumule: the embryonic shoot, which will become the stem and leaves
• Cotyledons: seed leaves (one in monocots, two in dicots)

Cotyledons can serve different roles depending on the species. In beans, they act as storage organs packed with nutrients. In lettuce, they become the first photosynthetic leaves after germination. The radicle and plumule together form the embryonic axis, which develops into the root and shoot systems of the seedling.

Endosperm

The endosperm is nutritive tissue surrounding the embryo that provides food for the developing seedling. Its fate differs between plant groups:

• In monocots (corn, wheat), the endosperm persists as the primary storage tissue
• In many dicots (peas, beans), the endosperm gets absorbed by the cotyledons during seed development, so the cotyledons become the main storage organs

Endosperm composition varies by species but typically includes carbohydrates, proteins, and lipids.

Seed coat

The seed coat (also called the testa) is the protective outer layer that develops from the integuments of the ovule. It serves several functions:

• Protects against mechanical damage, drying out, and pathogen attack
• Regulates water uptake and gas exchange during germination
• Can influence seed dormancy through its permeability

Surface features on the seed coat often aid dispersal. Hooks help seeds hitch rides on animals, while wings help seeds travel on wind.

Seed dormancy

Seed dormancy is a state where seeds won't germinate even under favorable conditions. This is actually an advantage: it prevents seeds from sprouting at the wrong time and lets them wait until conditions give the seedling the best chance of survival.

Types of dormancy

• Primary dormancy: Present in the seed at the time it matures and leaves the parent plant. Causes include an impermeable seed coat, an immature embryo, or the presence of germination inhibitors.
• Secondary dormancy: Develops in non-dormant seeds after dispersal, triggered by unfavorable conditions like extreme temperatures, water stress, or unsuitable light.
• Combinational dormancy: A mix of primary and secondary dormancy factors acting together to prevent germination.

Factors affecting dormancy

• Genetic factors: Some species naturally have deeper dormancy than others
• Environmental conditions during seed development: Temperature, light, and water availability while the seed matures on the parent plant can set the dormancy level
• Seed coat properties: A thicker or less permeable coat tends to increase dormancy
• Germination inhibitors: Hormones like abscisic acid (ABA) and phenolic compounds actively suppress germination
• Embryo maturity: In some species, the embryo is underdeveloped at dispersal and needs additional time to grow before it can germinate

Breaking dormancy

There are several methods to overcome dormancy, each targeting a different mechanism:

1. Scarification: Physically or chemically weakening the seed coat so water and oxygen can enter. This can mean nicking the coat with a knife, rubbing it with sandpaper, or treating it with concentrated sulfuric acid.
2. Stratification: Exposing seeds to cold, moist conditions for a set period. This mimics natural overwintering and breaks physiological dormancy.
3. After-ripening: Storing seeds dry at warm temperatures, which gradually reduces dormancy over time.
4. Light exposure: Some seeds need red light to trigger germination (more on this in the phytochrome section below).
5. Leaching: Soaking seeds in water to wash away water-soluble germination inhibitors.

Process of germination

Germination is the sequence of events that transforms a dormant seed into an actively growing seedling. It can be broken into three main stages.

Stage 1: Water uptake and imbibition

Imbibition is the absorption of water by the dry seed. Water enters through the micropyle, a small pore in the seed coat, and gradually hydrates the seed tissues.

This happens because the dry seed has very low water potential compared to the moist soil around it, so water moves in along the gradient. As water enters, it activates enzymes, kicks off metabolic processes, and softens the seed coat.

Stage 2: Activation of metabolic processes

Once the seed is hydrated, processes that were shut down during dormancy restart:

• Enzymes for breaking down stored food (amylases, proteases, lipases) are synthesized or activated
• Respiration ramps up, providing energy by breaking down carbohydrates, proteins, and lipids
• DNA replication and protein synthesis resume, enabling cell division and embryo growth

Stage 3: Radicle emergence

The first visible sign of germination is the radicle (embryonic root) pushing through the softened seed coat. This is driven by cell elongation and division in the root meristem.

Once out, the radicle grows downward in response to gravity (gravitropism) and begins forming the primary root system. Radicle emergence marks the official transition from seed to seedling.

Factors affecting germination

Four main environmental factors determine whether and how quickly germination occurs.

Temperature

Every species has a set of cardinal temperatures for germination:

• Minimum: the lowest temperature at which germination can happen
• Optimum: the temperature where germination is fastest and most uniform
• Maximum: the highest temperature that still permits germination

Temperatures outside this range delay, reduce, or completely prevent germination. Some seeds also need specific temperature patterns (like cold stratification) to break dormancy before they can respond to favorable temperatures.

Water availability

Water is required for imbibition, enzyme activation, and cell expansion. The soil's water potential must be higher than the seed's for water to flow in.

Too little water delays or prevents germination. Too much water can deprive seeds of oxygen and promote decay. Soil texture, organic matter content, and irrigation practices all affect how much water is available to seeds.

Oxygen

Seeds need oxygen for aerobic respiration, which powers germination. Oxygen availability can be limited by:

• Soil compaction
• Waterlogging
• A thick seed coat acting as a physical barrier
• Planting seeds too deep

In some species, the seed coat must be weakened or removed before enough oxygen can reach the embryo.

Light

Light effects on germination vary by species:

• Positively photoblastic seeds require light to germinate (e.g., lettuce, petunias)
• Negatively photoblastic seeds are inhibited by light and need darkness (e.g., onions, some grasses)

These responses are controlled by phytochromes, light-sensitive proteins that detect the ratio of red to far-red light. A low red:far-red ratio signals shade from competing vegetation, which can suppress germination in light-requiring species. Light also influences the balance of hormones like gibberellins and abscisic acid.

Hormonal regulation of germination

The timing of germination is largely controlled by the balance between hormones that promote it and hormones that inhibit it.

Gibberellins

Gibberellins (GAs) are the primary germination-promoting hormones. Here's how they work:

1. GAs are synthesized in the embryo during early germination.
2. They signal the aleurone layer (a cell layer surrounding the endosperm) to produce hydrolytic enzymes.
3. The key enzyme is $\alpha$-amylase, which breaks down starch in the endosperm into simple sugars the embryo can use.
4. GAs also promote cell elongation and division in the embryo, driving radicle emergence.

Applying GAs externally can often overcome dormancy in species with physiological dormancy.

Abscisic acid

Abscisic acid (ABA) is the main germination inhibitor. During seed maturation, ABA levels rise to induce dormancy and prepare the seed for survival. ABA works by:

• Blocking synthesis of germination-promoting enzymes like $\alpha$-amylase
• Suppressing embryo growth
• Increasing the seed's sensitivity to environmental stresses like drought and cold

As the seed takes up water during germination, ABA levels drop, tipping the hormonal balance toward germination. The GA:ABA ratio is the critical switch: high GA relative to ABA favors germination, while high ABA relative to GA maintains dormancy.

Ethylene

Ethylene is a gaseous hormone that promotes germination in some species, especially under stress. It can:

• Increase the seed's sensitivity to GAs
• Promote the breakdown of ABA
• Stimulate germination when seeds encounter mechanical resistance or low oxygen

Ethylene-releasing compounds are sometimes applied commercially to promote germination in specific crops.

Mobilization of seed reserves

The stored food in the endosperm or cotyledons must be broken down into forms the growing embryo can use. Three major types of reserves are mobilized during germination.

Starch hydrolysis

Starch is the primary carbohydrate reserve in many seeds, especially cereals and legumes. The breakdown process works as follows:

1. GA signaling triggers the aleurone layer to produce $\alpha$-amylase.
2. $\alpha$-amylase cleaves $\alpha$-1,4-glycosidic bonds in starch, producing oligosaccharides and maltose.
3. $\beta$-amylase and debranching enzymes further break these down into glucose.
4. Glucose is transported to the growing embryo as an energy source.

Protein degradation

Storage proteins provide amino acids for building new enzymes and structural proteins in the seedling. During germination, proteases and peptidases break storage proteins into peptides and amino acids, which are transported to actively growing regions of the embryo.

Lipid metabolism

Seeds of oil crops (sunflower, rapeseed, soybean) store significant energy as triglycerides. The conversion pathway has several steps:

1. Lipases hydrolyze triglycerides into fatty acids and glycerol.
2. Fatty acids undergo $\beta$-oxidation in glyoxysomes (specialized peroxisomes), producing acetyl-CoA.
3. Acetyl-CoA enters the glyoxylate cycle, producing succinate.
4. Succinate is converted to glucose through gluconeogenesis.

This pathway converts fats into sugars, fueling the embryo until it can photosynthesize on its own.

Seedling development

Once the radicle emerges, the embryo transitions into a seedling. Development involves coordinated growth of the shoot, cotyledons, and roots.

Hypocotyl elongation

The hypocotyl is the stem segment between the radicle and the cotyledons. Its behavior during germination defines two patterns:

• Epigeal germination (beans, lettuce): The hypocotyl elongates significantly, pulling the cotyledons above the soil surface. The cotyledons then open and may begin photosynthesis.
• Hypogeal germination (peas, corn): The hypocotyl stays short, and the cotyledons remain underground. Instead, the epicotyl (the shoot above the cotyledons) pushes upward.

Hypocotyl elongation is driven by cell expansion and is influenced by light, temperature, and moisture.

Cotyledon expansion

After the seedling emerges from the soil, the cotyledons expand and unfold. In many dicots, cotyledons become the first photosynthetic organs, providing energy until true leaves develop. In species where cotyledons serve as storage organs, they gradually transfer their nutrients to the growing seedling and then wither.

Root growth

Root development is critical for anchoring the seedling and absorbing water and nutrients.

• The primary root develops from the radicle and grows downward via gravitropism.
• Lateral roots branch off from the pericycle (a cell layer inside the primary root), increasing the surface area for absorption.
• Root architecture adapts to soil conditions: roots branch more densely in nutrient-rich patches and extend further in dry soils.

Root growth is influenced by soil moisture, temperature, and nutrient availability.

Photomorphogenesis

Photomorphogenesis is the process by which light controls seedling growth and development. It governs the transition from heterotrophic growth (living off seed reserves) to autotrophic growth (photosynthesis).

Light perception by phytochromes

Phytochromes are photoreceptor proteins that exist in two interconvertible forms:

• Pr: absorbs red light (660 nm) and converts to Pfr
• Pfr: absorbs far-red light (730 nm) and converts back to Pr

Pfr is the biologically active form. When red light is abundant (as in direct sunlight), Pfr accumulates and triggers light-dependent developmental responses. These include:

• Inhibition of stem elongation (preventing the seedling from becoming tall and spindly)
• Promotion of leaf expansion
• Stimulation of chlorophyll synthesis
• Triggering germination in positively photoblastic seeds

In shade or darkness, Pfr levels drop, and the seedling adopts a skotomorphogenic (dark-grown) pattern: elongated stems, unexpanded leaves, and no chlorophyll. This etiolated growth strategy helps buried seedlings reach light quickly before investing in photosynthetic machinery.

The red:far-red ratio detected by phytochromes gives the seedling information about its light environment, including whether it's shaded by other plants, since leaves absorb red light but transmit far-red light.