7.3 Germination and seedling development

Seeds are the starting point of plant life, containing all the essential components for a new plant to grow. From the embryo to the protective seed coat, each part plays a crucial role in the seed's survival and eventual germination.

Germination marks the transition from seed to seedling, triggered by environmental cues like water and temperature. This process involves complex changes, including the activation of enzymes, mobilization of food reserves, and the emergence of the radicle, setting the stage for a new plant's growth.

Seed structure and composition

• Seeds are the reproductive units of flowering plants, containing the embryo, stored food reserves, and protective outer layers
• The structure and composition of seeds play crucial roles in their survival, dispersal, and successful germination
• Understanding the key components of seeds is essential for studying their development and the early stages of plant growth

Embryo

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• The embryo is the young, undeveloped plant within the seed that will grow into a new plant upon germination
• Consists of the radicle (embryonic root), plumule (embryonic shoot), and one or two cotyledons (seed leaves)
• The cotyledons may serve as storage organs for food reserves (such as in beans) or may become the first photosynthetic leaves (such as in lettuce)
• The embryonic axis, which includes the radicle and plumule, will develop into the root and shoot systems of the seedling

Endosperm

• The endosperm is a nutritive tissue that surrounds the embryo and provides food reserves for the developing seedling
• In monocots (such as corn and wheat), the endosperm persists and is the primary storage tissue
• In many dicots (such as peas and beans), the endosperm is absorbed by the cotyledons during seed development, and the cotyledons become the main storage organs
• The composition of the endosperm varies among species but typically includes carbohydrates, proteins, and lipids

Seed coat

• The seed coat, also known as the testa, is the protective outer layer of the seed that develops from the integuments of the ovule
• Provides protection against mechanical damage, desiccation, and pathogen attack
• May exhibit various surface features, such as ridges, bumps, or hairs, which can aid in seed dispersal (such as hooks for animal dispersal or wings for wind dispersal)
• The seed coat also regulates water uptake and gas exchange during germination, and its permeability can influence seed dormancy

Seed dormancy

• Seed dormancy is a state in which seeds are unable to germinate even under favorable environmental conditions
• It is an adaptive mechanism that allows seeds to delay germination until conditions are suitable for seedling growth and survival
• Dormancy can be influenced by various factors, including genetic, physiological, and environmental factors

Types of dormancy

• Primary dormancy: Dormancy that is present in the seed at the time of maturation and dispersal from the parent plant
  • Can be caused by factors such as an impermeable seed coat, immature embryo, or presence of germination inhibitors
• Secondary dormancy: Dormancy that is induced in non-dormant seeds by unfavorable environmental conditions after dispersal
  • Can be triggered by factors such as high or low temperatures, water stress, or light conditions
• Combinational dormancy: A combination of primary and secondary dormancy factors that prevent germination

Factors affecting dormancy

• Genetic factors: Some species or genotypes have a higher degree of innate dormancy than others
• Environmental factors during seed development: Temperature, light, and water availability during seed maturation can influence the level of dormancy
• Seed coat properties: Thickness, permeability, and chemical composition of the seed coat can affect dormancy
• Presence of germination inhibitors: Compounds such as abscisic acid (ABA) or phenolic compounds can inhibit germination
• Embryo maturity: In some species, the embryo may be underdeveloped at the time of seed dispersal, requiring an additional period of growth before germination can occur

Breaking dormancy

• Scarification: Mechanical or chemical weakening of the seed coat to allow water uptake and gas exchange
  • Examples include nicking the seed coat with a knife, rubbing with sandpaper, or treating with concentrated sulfuric acid
• Stratification: Exposure of seeds to cold, moist conditions for a specific period to overcome physiological dormancy
  • Often mimics the natural process of overwintering that some seeds require before germination
• After-ripening: A period of dry storage at warm temperatures that can gradually reduce dormancy in some species
• Light exposure: Some seeds require exposure to light, particularly in the red wavelength range, to stimulate germination
• Leaching: Removal of water-soluble germination inhibitors by soaking seeds in water or exposing them to running water

Process of germination

• Germination is the process by which a seed develops into a seedling, marking the beginning of a plant's growth and development
• It involves a series of complex physiological and biochemical changes that are triggered by specific environmental cues
• The process of germination can be divided into three main stages: water uptake, activation of metabolic processes, and radicle emergence

Water uptake and imbibition

• The first stage of germination is the uptake of water by the dry seed, a process called imbibition
• Water is absorbed through the micropyle, a small pore in the seed coat, and gradually hydrates the seed tissues
• Imbibition is driven by the low water potential of the dry seed tissues and the high water potential of the surrounding environment
• As water enters the seed, it activates enzymes, initiates metabolic processes, and softens the seed coat

Activation of metabolic processes

• As the seed hydrates, metabolic processes that were dormant during seed maturation become active
• Enzymes involved in the mobilization of stored food reserves, such as amylases, proteases, and lipases, are synthesized or activated
• Respiratory activity increases, providing energy for the growing embryo through the breakdown of carbohydrates, proteins, and lipids
• DNA and protein synthesis resume, allowing for cell division and growth of the embryo

Radicle emergence

• The first visible sign of germination is the emergence of the radicle (embryonic root) from the seed coat
• The radicle is the first part of the embryo to elongate and push through the weakened seed coat
• Radicle emergence is driven by cell elongation and division in the embryonic root meristem
• Once the radicle emerges, it begins to grow downward in response to gravity (gravitropism) and establishes the primary root system
• The emergence of the radicle marks the transition from seed to seedling and the beginning of the plant's independent growth

Factors affecting germination

• Germination is influenced by various environmental factors that can either promote or inhibit the process
• Understanding these factors is crucial for optimizing seed germination in agricultural and horticultural practices
• The main factors affecting germination include temperature, water availability, oxygen, and light

Temperature

• Temperature plays a critical role in regulating seed germination, as it affects the rate of metabolic processes and the activity of enzymes
• Each plant species has a specific range of temperatures within which germination can occur, known as the cardinal temperatures
  • The minimum temperature is the lowest temperature at which germination can occur
  • The optimum temperature is the temperature at which germination is most rapid and uniform
  • The maximum temperature is the highest temperature at which germination can occur
• Exposure to temperatures outside the cardinal range can result in delayed, reduced, or inhibited germination
• Some seeds require specific temperature regimes to break dormancy, such as a period of cold stratification

Water availability

• Water is essential for seed germination, as it is required for the activation of metabolic processes, enzyme activity, and cell expansion
• Seeds need to absorb water to initiate germination, and the amount of water available in the environment can greatly influence the success of germination
• Insufficient water can delay or prevent germination, while excessive water can lead to oxygen deprivation and seed decay
• The water potential of the soil or growing medium must be higher than that of the seed for imbibition to occur
• Factors such as soil texture, organic matter content, and irrigation practices can affect water availability for seed germination

Oxygen

• Oxygen is required for aerobic respiration, which provides energy for the growing embryo during germination
• Seeds need an adequate supply of oxygen to support the increased metabolic activity associated with germination
• Oxygen availability can be limited by factors such as soil compaction, waterlogging, or the presence of physical barriers (such as a thick seed coat)
• In some species, the seed coat may need to be weakened or removed to allow for sufficient oxygen uptake
• Planting depth can also influence oxygen availability, as seeds planted too deeply may have difficulty obtaining enough oxygen for germination

Light

• Light can have a stimulatory or inhibitory effect on seed germination, depending on the species and the specific light requirements
• Some seeds are positively photoblastic, meaning they require light for germination (such as lettuce and petunias)
• Other seeds are negatively photoblastic, meaning they are inhibited by light and require darkness for germination (such as onions and some grasses)
• The response to light is mediated by phytochromes, which are light-sensitive proteins that detect the presence and quality of light
• The red to far-red light ratio is particularly important in regulating germination, as it can indicate the presence of competing vegetation or shading
• Exposure to light can also influence the synthesis and degradation of plant hormones involved in germination, such as gibberellins and abscisic acid

Hormonal regulation of germination

• Plant hormones play a crucial role in regulating seed germination by controlling various physiological processes within the seed
• The balance between germination-promoting hormones (such as gibberellins) and germination-inhibiting hormones (such as abscisic acid) determines the timing and success of germination
• Understanding the hormonal regulation of germination is essential for developing strategies to control and optimize seed germination in agricultural and horticultural practices

Gibberellins

• Gibberellins (GAs) are a class of plant hormones that promote seed germination by stimulating the mobilization of stored food reserves and the growth of the embryo
• GAs are synthesized in the embryo and aleurone layer (a layer of cells surrounding the endosperm) during germination
• They induce the production of hydrolytic enzymes, such as α-amylase, which break down starch in the endosperm into simple sugars that can be used by the growing embryo
• GAs also promote cell elongation and division in the embryo, leading to radicle emergence and seedling growth
• Exogenous application of GAs can often overcome seed dormancy and promote germination in species with physiological dormancy

Abscisic acid

• Abscisic acid (ABA) is a plant hormone that acts as a germination inhibitor, maintaining seed dormancy and preventing precocious germination
• During seed maturation, ABA levels increase, inducing dormancy and preparing the seed for dispersal and survival in unfavorable conditions
• ABA inhibits the synthesis of germination-promoting enzymes, such as α-amylase, and suppresses the growth of the embryo
• It also increases the sensitivity of the seed to environmental stresses, such as drought or cold, which can prevent germination in unfavorable conditions
• As the seed imbibes water during germination, ABA levels decrease, allowing the germination process to proceed
• The balance between ABA and GAs is crucial in regulating the transition from dormancy to germination

Ethylene

• Ethylene is a gaseous plant hormone that can promote seed germination in some species, particularly in response to environmental stresses
• It is involved in the regulation of seed dormancy release and the stimulation of germination-related processes
• Ethylene can enhance the sensitivity of seeds to GAs and promote the breakdown of ABA, thus favoring germination
• In some species, ethylene production increases during germination, particularly in response to mechanical resistance or oxygen deprivation
• Exogenous application of ethylene or ethylene-releasing compounds can be used to promote germination in certain species or to overcome dormancy

Mobilization of seed reserves

• During germination, the stored food reserves in the endosperm or cotyledons are mobilized to support the growth and development of the embryo
• The mobilization of seed reserves involves the breakdown of complex macromolecules, such as starch, proteins, and lipids, into simpler forms that can be transported and utilized by the growing seedling
• This process is mediated by various hydrolytic enzymes that are activated or synthesized during germination

Starch hydrolysis

• Starch is the primary carbohydrate reserve in many seeds, particularly in cereals and legumes
• During germination, starch is hydrolyzed into simple sugars, such as glucose and maltose, by the action of amylolytic enzymes
• The main enzyme involved in starch breakdown is α-amylase, which is synthesized in the aleurone layer in response to GA signaling
• α-amylase cleaves the α-1,4-glycosidic bonds in starch, producing smaller oligosaccharides and maltose
• Other enzymes, such as β-amylase and debranching enzymes, further degrade these products into glucose, which can be readily used by the growing embryo

Protein degradation

• Proteins are another important reserve in seeds, providing amino acids for the synthesis of new proteins and enzymes during germination and seedling growth
• Protein degradation is catalyzed by proteolytic enzymes, such as proteases and peptidases, which are activated or synthesized during germination
• These enzymes break down storage proteins into smaller peptides and amino acids, which are then transported to the growing regions of the embryo
• The amino acids are used for the synthesis of new enzymes, structural proteins, and other essential compounds required for seedling development

Lipid metabolism

• Lipids, particularly triglycerides, are important energy reserves in some seeds, such as those of oil crops (e.g., sunflower, rapeseed, and soybean)
• During germination, lipids are hydrolyzed into fatty acids and glycerol by the action of lipases
• The fatty acids are then converted into acetyl-CoA through the process of β-oxidation, which takes place in the glyoxysomes (specialized peroxisomes)
• Acetyl-CoA is further metabolized through the glyoxylate cycle, producing succinate, which is converted into carbohydrates (such as glucose) through gluconeogenesis
• The carbohydrates produced from lipid metabolism are used to support the growth and development of the embryo until it becomes photosynthetically active

Seedling development

• Seedling development is the process by which the embryo within the seed grows and differentiates into a young plant
• It involves a series of morphological and physiological changes that are regulated by genetic and environmental factors
• The main stages of seedling development include hypocotyl elongation, cotyledon expansion, and root growth

Hypocotyl elongation

• The hypocotyl is the stem region of the embryo that connects the radicle to the cotyledons
• During seedling development, the hypocotyl undergoes rapid elongation, pushing the cotyledons and epicotyl (the shoot region above the cotyledons) above the soil surface
• Hypocotyl elongation is driven by cell expansion and is influenced by various environmental factors, such as light, temperature, and moisture
• In epigeal germination (e.g., in beans and lettuce), the hypocotyl elongates significantly, bringing the cotyledons above the soil surface
• In hypogeal germination (e.g., in peas and corn), the hypocotyl remains short, and the cotyledons remain below the soil surface

Cotyledon expansion

• Cotyledons are the embryonic leaves that store food reserves or perform photosynthesis in the early stages of seedling development
• After the hypocotyl elongates and the seedling emerges from the soil, the cotyledons expand and unfold
• In many dicotyledonous species, the cotyledons become the first photosynthetic organs, providing energy for the growing seedling until true leaves develop
• The expansion of cotyledons is driven by cell enlargement and is influenced by factors such as light, temperature, and water availability
• In some species, the cotyledons may also serve as storage organs, gradually transferring nutrients to the growing seedling

Root growth

• Root growth is essential for seedling establishment, as it allows the plant to anchor itself in the soil and absorb water and nutrients
• The primary root, which develops from the radicle, is the first root to emerge from the seed and grows downward in response to gravity (gravitropism)
• As the primary root elongates, lateral roots develop from the pericycle, a layer of cells within the primary root
• Lateral roots further increase the surface area for water and nutrient uptake and provide additional anchorage
• Root growth is influenced by various environmental factors, such as soil moisture, temperature, and nutrient availability
• The root system architecture, which includes the spatial arrangement and branching patterns of roots, can adapt to different soil conditions and optimize resource acquisition

Photomorphogenesis

• Photomorphogenesis is the process by which light influences the growth and development of seedlings
• It involves the perception of light signals by photoreceptors, such as phytochromes, and the subsequent regulation of gene expression and plant growth
• Photomorphogenesis is crucial for the transition from heterotrophic to autotrophic growth and the adaptation of seedlings to their environment

Light perception by phytochromes

• Phytochromes are a family of photoreceptors that detect red and far-red light in plants
• They exist in two interconvertible forms: the red light-absorbing form (Pr) and the far-red light-absorbing form (Pfr)
• Pr is converted to Pfr upon absorption of red light (660 nm), while Pfr is converted back to Pr upon absorption of far-