8.1 Seed germination and seedling growth
3 min read•august 7, 2024
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 and play key roles in regulating these early growth stages.
Seed Germination Stages
Dormancy and Imbibition
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- Seeds remain dormant until favorable conditions for germination are met (adequate , , oxygen)
- Seed coat protects the embryo during dormancy and prevents premature germination
- 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 ()
- 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 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
Key Terms to Review (18)
Abscisic acid: Abscisic acid (ABA) is a plant hormone that plays a crucial role in regulating various physiological processes, particularly during stress responses. It helps plants manage drought conditions, seed dormancy, and influences growth by acting as a signaling molecule that coordinates developmental processes.
After-ripening: After-ripening is the physiological process that seeds undergo after dispersal, allowing them to break dormancy and become capable of germination. This process often requires specific environmental conditions or internal changes, which enable seeds to transition from a dormant state to an active one. It is a crucial step for seeds that cannot germinate immediately upon maturation, ensuring they only sprout under favorable conditions for seedling growth.
Chlorophyll synthesis: Chlorophyll synthesis is the biological process by which plants, algae, and some bacteria produce chlorophyll, the green pigment essential for photosynthesis. This process involves several key steps, including the production of precursors from light and the incorporation of magnesium ions. Chlorophyll plays a crucial role in capturing light energy and converting it into chemical energy during seed germination and the early growth of seedlings.
Cotyledon: A cotyledon is the first leaf or leaves that appear from a seed during germination, serving as a crucial structure for seedling growth. These embryonic leaves provide essential nutrients to the developing plant before true leaves emerge and take over the role of photosynthesis. Cotyledons can vary in number, with some plants having one (monocots) and others having two (dicots), influencing their growth patterns and characteristics.
Endosperm: Endosperm is a tissue produced inside the seeds of most flowering plants, serving as a food reserve for the developing embryo. This tissue plays a critical role during seed germination, providing essential nutrients that fuel the initial growth of the seedling until it can produce its own food through photosynthesis.
Germination phase: The germination phase is the critical process during which a seed absorbs water, swells, and begins to sprout, leading to the emergence of a seedling. This phase marks the transition from a dormant seed to an actively growing plant, which requires specific environmental conditions such as moisture, temperature, and oxygen for successful development. Understanding this phase is vital for comprehending how seeds establish themselves and grow into mature plants.
Gibberellins: Gibberellins are a group of plant hormones that play critical roles in regulating growth and development. They influence processes such as stem elongation, seed germination, and flowering, making them essential for various stages of a plant's life cycle. Their interactions with other hormones highlight their importance in the complex signaling networks that control plant physiology.
Hydroponics: Hydroponics is a method of growing plants without soil, using nutrient-rich water to deliver essential minerals directly to the plant roots. This innovative technique enables controlled growth conditions, leading to faster plant development and higher yields. By eliminating soil, hydroponics can reduce pest and disease issues while allowing for precise management of water and nutrients.
Imbibition: Imbibition is the process by which seeds absorb water and swell, leading to the initiation of germination. This crucial first step allows dormant seeds to activate their metabolism and begin developing into seedlings. Imbibition also influences the structural changes in the seed, enabling it to break dormancy and start growing.
Light-dependent reactions: Light-dependent reactions are the initial stages of photosynthesis that convert light energy into chemical energy in the form of ATP and NADPH. These reactions occur in the thylakoid membranes of chloroplasts, using light to excite electrons in photosynthetic pigments, leading to the production of energy-rich compounds that fuel the subsequent processes in photosynthesis, such as carbon fixation and growth.
Moisture: Moisture refers to the presence of water in the environment, which is crucial for the survival and growth of plants. In the context of seed germination and seedling growth, moisture plays a vital role in activating metabolic processes, facilitating nutrient uptake, and promoting cellular expansion. Adequate moisture levels ensure that seeds can absorb water to initiate germination, while also supporting the young plant's development as it establishes its root system and foliage.
Nutrient solution: A nutrient solution is a water-based mixture that contains essential minerals and nutrients necessary for plant growth and development. This solution provides the vital elements that plants need, such as nitrogen, phosphorus, potassium, and trace minerals, which are not always available in sufficient quantities in the soil. By using nutrient solutions, especially in hydroponic systems, plants can grow more efficiently and healthily without soil constraints.
Orthodox seeds: Orthodox seeds are seeds that can survive desiccation and remain viable under dry conditions. These seeds are capable of being stored for long periods at low moisture levels and can withstand freezing temperatures, which is crucial for their preservation and germination under favorable conditions.
Radicle Emergence: Radicle emergence refers to the initial stage in seed germination when the embryonic root, known as the radicle, breaks through the seed coat and begins to grow into the soil. This crucial event marks the beginning of seedling development, as the radicle anchors the plant and facilitates the uptake of water and nutrients essential for further growth.
Recalcitrant seeds: Recalcitrant seeds are a type of seed that cannot withstand desiccation and require specific environmental conditions to germinate. Unlike orthodox seeds that can be dried and stored for long periods, recalcitrant seeds are sensitive to changes in moisture and temperature, making them crucial for specific ecosystems and plant species. Their unique characteristics significantly influence seed germination and seedling growth processes.
Scarification: Scarification is a process that involves physically or chemically altering the seed coat to promote germination by breaking dormancy. This technique helps seeds overcome physical barriers that might prevent water absorption and gas exchange, ultimately facilitating the germination process and the healthy growth of seedlings.
Temperature: Temperature refers to the measure of thermal energy present in a substance, influencing various physiological processes in plants. It plays a critical role in regulating biochemical reactions, enzyme activities, and overall plant development, affecting vital processes like photosynthesis, growth, and reproduction.
Vegetative phase: The vegetative phase is the stage of plant development characterized by the growth of stems, leaves, and roots, but not yet involving flowering or reproduction. During this phase, plants focus on increasing biomass, establishing a robust structure, and enhancing photosynthetic capacity to prepare for future reproductive stages. The duration and success of the vegetative phase are critical as they directly influence the overall health and productivity of the plant.