1. Plant Growth Regulators
Two major influences are at work in controlling growth and reproduction of plants. They are the genes and environment. The important environmental factors include light, temperature, availability of nutrients and water. Ultimately, it is the genes in the form of DNA which have the blueprint for successful growth and reproduction.
Complex internal coordination and control is required, both to express the DNA blueprint and to respond appropriately to the environment. Plants rely entirely on chemicals to control seed dormancy, germination, flowering and fruit maturation. The chemicals are referred to as plant growth regulators or plant growth substances. They are sometimes referred to as hormones since the growth regulators are effective in minute quantities and are basically responsible for enzyme activation and gene expression, similar to hormones in animals.
Growth promoters are auxin, cytokinins and gibberellins. Growth inhibitors are abscisic acid and ethylene.
More than a century ago, Charles Darwin and his son studied the growth of plant shoots towards light. Their findings pointed the way to the eventual discovery of the photoreceptor molecules that capture light signals and the hormones that transmit those signals to other parts of the plant. Light and hormones affect processes in plants as diverse as stem growth, flowering, bud dormancy and the dropping of leaves in autumn. Several of the hormones now find important commercial applications, including the regulation of fruit ripening and enhanced germination of barley for the brewing industry.
Recent advances in understanding plant development have come largely from work with Arabidopsis thaliana, a little mustard like weed. This plant is useful to researchers because its body and seeds are tiny and its genome is unusually small for a flowering plant. It also flowers and form seeds in a relatively short period of time after growth begin. This plant, with altered developmental patterns, provides evidence for the existence of hormones and for the mechanisms of hormone and photoreceptor action.
In this section, we explore the environmental cues, photoreceptors and hormones that regulate plant development and consider the multiple roles that each plays in normal development. The development of a plant (a series of progressive changes like cell expansion, cell division and cell differentiation) that take place throughout its life, is regulated in complex ways.
There are 4 factors that take part in this regulation.
The plant senses and responds to environmental cues.
The plant’s genome encodes enzymes that catalyze the biochemical reactions of development, including he ones that make hormones and receptors, produce chemical blocks and participate in protein synthesis and energy metabolism. No matter what cues direct development, ultimately the plant’s genome determines the limit within which the plant and its parts will develop. The genome encodes the master plan but its interpretation depends on the conditions in the environment.
In order to sense environmental cues, the plant uses receptors. Light acts directly on photoreceptors, which in turn regulates processes such as the many changes accompanying the growth of a young plant out of the soil and into the light.
Chemical messengers or hormones mediate the effects of the environmental cues sensed by the receptors. Hormones are regulatory compounds that act at very low concentrations at sites distant where they are produced. They mediate many developmental phenomena in plants such as stem growth and autumn leaf fall. Unlike animals, which produce each hormone in a specific part of the body, plants produce hormones in many of heir cells. Each plant hormone plays multiple regulatory roles, affecting several different aspects of development. Interactions among hormones are often complex.
1.1 Effects of Plant Growth Regulators on Various Parts of the Plant
There are diagrams which illustrate the importance of plant growth regulators in determining or controlling growth, development and morphogenesis. Plant growth regulators carefully control the physiology of plants to exploit the limited resources in the environment for their survival.
1.2 Interactions of Plant Growth Regulators
There are pictures in textbooks which show this.
1.3 Commercial Applications of Plant Growth Regulators
Plant growth substances or their synthetic equivalents have all sorts of uses. Ever since the chemical structure of auxin was discovered in 1934, scientists have explored the possibility of using it and other growth substances in agriculture and horticulture.
The same approach has been adopted as in the search for medicinal drugs: a naturally occurring substance is discovered, its possible uses are explored and then attempts are made to synthesize it – or an equivalent substance – in the laboratory.
Over the years, numerous artificial growth substances have been made in the lab and are now produced commercially. Although these analogues differ chemically from the natural substances, they share certain features in common.
In assessing the usefulness of an analogue, various criteria must be taken into account. In particular, the substance should be effective for its chosen purpose but of low toxicity to other organisms. It should be rapidly destroyed after being released into the environment.
Humans have the responsibility not to release persistent, broad spectrum toxic chemicals into the environment. This is where plant growth substances can be a great help because, as naturally occurring chemicals, they meet many of the requirements which environmentally sensitive manufacturers expect them to have.
1.3.1 As Selective Herbicides
This is probably the best known use of plant growth substances. They are called hoemone weed killers and are taken up by the leaves and transported to all parts of the plant. They therefore kill the roots as well as the leaves. As you might expect, they exert their effect by interfering with the growth and metabolism of the plant.
The beauty of these herbicides is that, when applied in the right concentration, they kill the broad leaved weeds but have no adverse effect on the narrow leaved plants. MCPA and 2,4 – D, are often used to remove broad leaved weeds from lawns or cereal stands. Woody plants are killed by 2,4,5 – T, another chemical related to auxin.
1.3.2 As Growth Promoters
NAA, as an analogue of auxin, induces root formation in cuttings. It is an ingredient of rooting powders. When applied to the cut surface of a stem or branch, it supplements the plant’s own auxin, increasing its concentration relative to cytokinins. This change of balance encourages the undifferentiated cellus tissue which forms at the cit surface to develop into roots. Conversely, the addition of a cytokinin analogue prevents the formation of roots, so that only a mass of simple cells is formed.
1.3.3 As Growth Retardants
Some artificial growth regulators are antagonists to naturally occurring gibberellins and they have the effect of reducing the length of the internodes. When such a substance is applied to a cereal crop, it stops the stalk growing too long. This prevents the plant falling over., making them easier and cheaper to harvest.
When sprayed onto house plants, these growth retardants restrict the growth, making the plants more compact and attractive and easier to manage.
1.3.4 As Flower Inducers
Gibberellin antagonists induce flowering in woody perennials which do not normally flower until they are several years old. Application of the substance causes the vegetative apex to become floral in the first year, thus ensuring a supply of fruit when the tree is still young.
By contrast, biennials which do not flower till the second year, can be made to flower at the end of the first year by applying gibberellins or one of its analogues.
Ethene is also used to induce flowering For example, when applied to commercial pineapple plants, it causes simultaneous flowering of the whole crop.
1.3.5 As Fruit Inducers
Normally, a signal passes from the developing embryo to the ovary wall or receptacle, encouraging it to develop into a fruit. This signal is auxin and it can be mimicked by NAA. When applied to the unpollinated flowers of a tomato plant or pear tree, fruits are formed without prior fertilization. These fruits look very similar to the ones produced naturally when the plant’s own auxin provides the stimulus. But these new fruits are pipless.
Producing pipless fruits us now a big industry. Seedless fruits such as grapes and satsumas are favourites in supermarkets.
When pollination of a flower is poor and some of the ovules escape being fertilized, the quantity of auxin may be insufficient to cause the full development of the fruit. Application of NAA can supplement the natural auxin and ensure the production of high quality of fruit.
1.3.6 As Fruit Ripeners
Ethene is given off naturally by many types of ripening fruit and it accelerates the ripening process. A substance such as ethephon which releases ethane can be used for ripening fruits. This is particularly useful for fruits such as bananas which are picked and shipped green but have to be sold yellow. The fruit is sprayed with ethephon in the ship’s hold and ethane is gradually released, encouraging the ripening process.
2 Seed Dormancy
Common vegetable garden seeds generally are ready to sprout. All they need is some moisture to get their biochemistry activated and temperature warm enough to allow the chemistry of life to proceed. However, there are some seeds that are unable to germinate readily. Seeds taken from the wild are frequently endowed with deeper forms of dormancy.
Seed dormancy is nature’s way of setting a time clock that allows seed to initiate germination when conditions are normally favourable for germination and survival of the seedlings. It is an adaptive response that evolved in regions with unfavourable seasons for growth. Growth is deferred unless the environment provides the necessary conditions that ensure the survival of the seeding upon germination.
Seeds may not germinate for the following 2 main reasons.
The embryo is quiescent. Quiescence refers to the state when embryos of seeds are able to grow but lack of favourable environment. The embryo of the garden seed packaged in an envelope is quiescent, not dormant. Germination is induced simply by providing favourable growth conditions.
The seed is dormant. Dormancy (after-ripening) refers to the inability of seeds to germinate even when environmental conditions are favourable for growth. Dormancy is imposed by internal or physiological conditions such as thick seed coat, immature embryo, levels of growth regulators and phytochromes etc.
Seeds that are released from a plant in a dormant state exhibit primary dormancy. In several plants, seeds may not be dormant when initially released but become dormant if environmental conditions become unfavourable. Such seeds exhibit secondary dormancy.
2.1 Advantages of Seed Dormancy
2.1.1 Minimize Wastage of Resources in the Seed
2.1.1.1 By limiting the Season of Germination
Dessert seeds will germinate only at the beginning of the rainy season i.e. after substantial rainfall. Seeds of temperate plants will germinate only after they have pass through a long cold period, usually winter.
Seed dormancy allows seeds to discriminate between temporary environmental cues (freaks in weather) that will break dormancy and long-lasting genuine climatic changes that will support later development.
A dessert annual if fooled into germination by some aberrant rainfall even though the actual rainy season is not to happen for some time yet would result in damage and wastage to the species.
To increase productivity, the seeds must only germinate if the environment is good enough to support development of the seedling to maturity to complete its life cycle.
2.1.1.2 By limiting the Location of Germination
Cypress seeds will grow only if there is standing water which is also the preferred environment of the cypress trees. Temperate plants prefer to grow in temperate climates and so on. Plants depend very much on the environmental factors to provide cues for seed germination, flowering, fruit ripening, dispersal etc. Hence it is necessary for growth, development and reproduction of the plant to be synchronized with favourable environmental conditions that allows the plant to complete its life cycle.
2.1.1.3 By Staggering Germination
Each year, only a fraction of the seeds produced are germinated and the rest remain dormant in the soil. This seems to be a successful mechanism to ensure the perpetuation of the species. If there is a disastrous year in which adult plants cannot set seeds, there will also still be viable ungerminated seeds around.
This accounts best for the survival of weeds since the application of herbicide in cultivated land fails to eliminate the weeds in the long term. Staggered germination prevents total elimination or extinction of species due to drastic harmful environmental conditions.
2.1.2 Minimize Competition for Resources in the Environment
2.1.2.1 By Avoiding Overcrowding
Parent plant may produce inhibitors to stop seeds from germinating too close to them, thus minimizing intra-specific competition.
Seed germination is rapid after a fire or clearing of vegetation. Seeds lying on the ground in dense woodland will not germinate until the vegetation above them is removed, This is useful as the shade created by a dense canopy of leaves could prevent the seedling from growing successfully.
2.1.2.2 By Dispersing Seeds
Seeds that delay in germination will allow time to be carried some distance away from the parent plant depending on the mode of dispersal. This would result in reduction of intro-specific and inter-specific competition.
Some seeds require scarification achieved by the corrosive action of the digestive juices in the guts of animals. This will assist in dispersal by the animals that feed on these seeds. There is also possible manuring by faeces of animals when the seed is egested.
2.1.2.3 Dependence of Plant Species on Animals to break Seed Dormancy and Efects of Disrupted Ecosystem on Survival of Plants
This section is left out as there are too many aspects to cover.
2.2 Reasons for Seed Dormancy
2.2.1 Thick Seed Coat
Many kinds of seeds have very thick seed coats. The testa obviously keep water out of the seed so the embryo cannot get the water needed to activate its metabolism and start growing. It is also possible that the testa is too hard for the swelling embryo to burst through. The lotus and apple seeds are examples of this. In natural conditions, the testa is gradually worn down by microbial activity in the soil.
2.2.2 Immature Embryo
If a seed’s embryo is not completely developed, some additional maturation (e.g. gains the ability to produce GA) may be needed before the seed can sprout. This happens in seeds with little or no storage material invested in the seed.
Orchid seeds are the size of dust and have almost nothing but a very immature embryo on board. Such a seed needs an association with fungi in the soil or other environments to feed the developing embryo until the embryo is mature enough to actually penetrate the seed coat. These seeds are also likely to have a very brief viability. The fungal association must be established rapidly or the embryo dies.
2.2.3 Concentration of Growth Inhibitors
Seeds of plants with succulent fruits such as tomatoes develop surrounded by water and nutrients, conditions which seem ideal for germination. Their seeds fail to germinate because of inhibitor chemicals in the sap of the fruit.
Many plant species invest chemicals in the developing seeds and these chemicals inhibit the development of the embryos. They keep the embryos dormant. Obviously the seed must have some way of eliminating these chemicals before they can sprout. In natural conditions, the inhibitors are broken down or slowly leach out after a rainfall.
2.2.3.1 Abscisic Acid (ABA)
Many temperate zone species that use inhibitors use ABA. This chemical induces dormancy in the embryo. The chemical is produced in abundance in the late summer and early fall. The seeds in the fruits become dormant so, even if they are dispersed in autumn, they cannot sprout. During the winter, enzymes in the seed degrade the ABA. By spring, the ABA is gone and the seed can sprout.
2.2.3.2 Phenolic Compounds
Plants that live in deserts have a different problem. There is no cold, moist, winter to allow vernalization of ABA. These plants instead use more potent toxins, phenolic compounds, to keep their seeds dormant until the proper season for germination. Deserts typically have very long dry seasons and a short wet season accompanied by flash floods and so on. Since phenolic compounds are freely soluble in water, the seed would geminate in the wet season i.e. when the phenolic inhibitors are leached away after prolonged rainfall.
2.2.4 Concentration of Phytochromes
The seed coat in some seeds is so thin that it is no barrier to water. Some other kind of dormancy mechanism is needed. Knowing that light can penetrate thin layers of plant tissue should give you the idea that light may be a signal. That plants can absorb light and respond biochemically is a fact known from your study of photosystems. All we need is a pigment molecule that can absorb light and cause a change in the behavior of the embryo.
The pigment is phytochrome. Like chlorophyll, it is made of a chromophore with tetrapyrole structure and is associated with proteins. This pigment is different from chlorophyll, however, in one critical way. It exists in 2 inter-convertible forms.
One form of phytochrome, named far-red phytochrome (Pfr) is the form of the phytochrome found in plant cells that are exposed to red (660 nm) or common white light. This form of phytochrome is biologically very active and plays a role in all systems when a plant needs to know if the lights are on or off.
The other form of phytochrome, named red phytochrome (Pr) is formed when far-red phytochrome is exposed to far-red (730 nm) light. This form is biologically inactive or inhibits responses. Dormant seeds have high levels of Pr.
2.3 Techniques to Break Seed Dormancy
Strategies used to induce breaking of seed dormancy differ from species to species. In some cases, seeds depend on multiple cues which regulated their dormancy to induce germination.
2.3.1 Leaching
Freely water-soluble growth substances may be easily washed away by large amounts of water. Desert plants produce seeds with high concentration of phenolic compounds that impose dormancy. The phenolic compounds are lost only through leaching by persistent rain in the wet season which will be able to bring about sufficient removal of the inhibitors. The seeds, however, would not germinate after a flash flood in the dry season. This is to ensure that upon germination, the environment would be able to support the continued development of the germinated seed into a full grown plant.
2.3.2 Vernalization / Stratification (Artificial Chilling)
A large number of plants produce seeds that do not germinate immediately but do so after a period of time under normal conditions for germination. In nature, dormancy occurs during the period between the fall of the seed to the ground in the autumn and its germination the following spring. During this time, while the seeds are covered over with debris and winter snows, there is sufficient time to degrade the growth inhibitors, ABA. Thus, the concentration of ABA decrease after vernalization i.e. exposure to low temperature under normal conditions.
Vernalization can be stimulated by artificial chilling (stratification). Seeds can be placed in moist soil and refrigerated at very low temperatures for several week or months. The optimum temperature and duration of stratification is species specific. Then the planted seeds are placed in a warm greenhouse. The seeds assume winter is over and spring has come and they begin to sprout.
Chilling not only degrades ABA in dormant seeds, it is also necessary for activation of synthesis of growth promoters such as GA and cytokinins.
2.3.3 Scarification or Abrasion
2.3.3.1 Mechanical Scarification
Some seeds have thick and hard seed coats which is impervious to water and gases, pose a physical barrier to the leaching of growth inhibitors and also to increase in volume of swelling cotyledons thus preventing germination.
Any process of abrasion, scratching or mechanically altering the seed coat to make it permeable to water and gases is known as scarification. In nature, this often occurs by fall seeding. Freezing temperatures and microbial activity modify the seed coat during winter such that the seed will be ready to sprout in spring.
Other species might use some pounding along a river or drop seeds into seacoast surf to abrade the thick seed coat. Some of the sea beans do this. Other seeds might need a vertebrate or other animal to attack the seed coat and thereby weaken the coat.
Scarification can also be forced rather than waiting for nature to alter the seed coats. Seed coats can be filed with a metal file, rubbed with sand paper, nicked with a knife or cracked gently with a hammer to weaken the seed coat.
2.3.3.2 Chemical Scarification
A final and very common example of a way to scarify a seed coat is observed in strawberry and raspberry. The thick seed coat is designed to e swallowed by the frugivore. The animal digest the fruit pulp, but the seed coat passes through the digestive system still protecting the viable embryo inside, but weakened enough by the digestive / hydrolytic enzymes and stomach acids to allow sprouting. Besides, small pebbles in the gizzard of animals may grind and rupture the testa. The seed is deposited with a little organic fertilizer in the environment and can now sprout.
Commercial growers scarify seeds by soaking them in concentrated sulphuric acid. Seeds are placed in a glass container and covered with acid. The seeds are gently stirred and allowed to soak for 10 mins to several hours, depending on the species. When the seed coat has been adequately thinned, the seeds are removed, washed and sown. Vinegar is a safer alternative for species that do not have an extremely hard seed coat. Organic solvents such as acetone or alcohol may also be used.
Even boiling water may also be a successful treatment. Abrupt changes in temperature when seeds are placed into boiling water weakens the seed coat or dissolves waxy materials that renders the seed coat impervious to gases.
In some areas of the world, seeds of certain species require fire for scarification. These seeds have a competitive edge immediately after the denudation of a dense vegetative area.
2.3.4 Exposure to Correct Photoperiod and Wavelength
It was found that some species e.g. birch will only germinate when exposed to the correct duration of light and darkness or photoperiods. The response to photoperiods is triggered by changes in concentration of phytochrome (Pfr to Pr). The higher the concentration of Pfr (active form), the higher the percentage of germination.
Dry lettuce seeds have slightly more than half of their phytochrome in the Pfr form. Therefore, if they are moistened and kept in the dark, some of the seeds will actually germinate.
However, if imbibing seeds are illuminated with red or white light, then virtually all the phytochrome is converted to the Pfr (active) form and this photoactivates the genes to initiate the process of germination. On the other hand, if imbibing seeds are illuminated with far-red light then virtually all the phytochrome in the seeds is converted to the Pr (inactive) form and germination is reduced significantly.
Lettuce seeds artificially exposed to far red light do not germinate. This condition is experienced by seeds buried deep in the soil. Germination is inhibited due to high concentration of Pr in the seed. Given that lettuce has a small seed, it is unwise for germination to take place. Depletion of food resource before the seed leaves emerge from the soil is wastage. Unable to photosynthesize, the seedling will die soon after germination.
Lettuce seeds germinate only when placed in white or red light. It produces Pfr which causes the seeds to begin to germinate. Red light is detected when the seeds are moved to the surface of the soil due to the disturbance e.g. wind, animal burrowing, human activity.
Thus phytochrome and light tell the lettuce when they are exposed to sunlight and can begin to germinate. Small seeds do not have reserves enough to start germinating without being able to quickly get to light, it is no surprise that small-seeded species have evolved this mechanism for photoactivation of seed germination. It prevents a deeply buried seed from trying to germinate.
2.3.5 Exposure to Growth Promoters
Low concentrations of growth promoters impose dormancy. Exogenous application of GA eliminates the need for chilling to promote seed germination. Other growth promoters involved are cytokinins (increase the size of cotyledons which will rupture the seed coat through stimulation of cell division).
3. Seed Germination
A seed certainly looks dead. It does not seem to move, to grow or do anything. In fact, even with biochemical tests for the metabolic processes we associate with life, the rate of these processes is so slow that it would be difficult to determine whether there really was anything alive in the seed.
Indeed if a seed is not allowed to germinate or sprout within certain length of time, the embryo inside will die. Each species of seed has a certain length of viability. Some maple species have seeds that need to sprout within 2 weeks of being dispersed or they die. Some seeds of the lotus plants are known to be up to 2000 years old and can still be germinated.
Assuming the seed is still viable, the embryo inside the seed coat needs something to get its metabolism activated to start the embryo growing. The process of getting a seed to germinate can be simple or complicated.
Germination refers to the onset of growth of the embryo usually after a period of dormancy. A seed is said to have germinated when the growth of the radical bursts the seed coat and protrudes as a young root. Later, the plumule will emerge and become the shoot. Germination involves 3 major processes. They are the inbibition of water, formation of enzyme systems and the growth of the seedling. To initiate germination, the seeds need to have water, oxygen, warmth, nutrients and in some cases, light.
3.1 Effects of Environmental Factors
3.1.1 Water
Water is absorbed into the seed through the micropyle and testa. This process is known as imbibitions. It is purely a physical process due to the more negative water potential in the dry seed. Water moves from cell to cell by osmosis. The absorption of water by proteins, starch and cell wall materials such as hemicellulose and pectin causes the seed to swell and rupture its testa surrounding the seed. The seed coat when split open, will allow water to enter even faster.
Water is required to activate hydrolytic enzymes which catalyze metabolic / biochemical reactions in the dormant embryo associated with germination as these reactions occur in aqueous solution. Water is also an important reactant in the hydrolysis of food stores. Water is also required for the activation of growth substances and the leaching of growth inhibitors.
3.1.2 Oxygen
Although the initial metabolic reactions are anaerobic, the switch to aerobic respiration is rapid. Aerobic respiration occurs as soon as the seed coat is ruptured as this allows entry of oxygen to the respiring tissues. Synthesis of new cellular components for growth at the spices requires free energy supplied by ATP.
3.1.3 Light
Light is important as it is required for photosynthesis by the first foliage leaves. Light is perceived by molecules known as phytochromes. In some species, red light stimulates while far red light inhibits germination.
3.1.4 Temperature
Temperature requirements are species-specific. The higher the temperature the higher the rate of germination as Q10 of enzymatic reactions is 2.
Other temperature requirements include a chilling temperature before germination can occur. The process of stratification or chilling is important for breaking seed dormancy as it lowers the concentration of growth inhibitors such as ABA.
3.2 Mobilization of Food Reserves in Barley Seed
3.2.1 Imbibition of Water
After exposure to special environmental conditions, the first step in barley seed germination is imbibitions. In this process, water penetrates the seed coat / testa and begins to soften the hard, dry tissues inside. The grain begins to swell up physically to rupture the seed coat. The testa, when split open, will allow water to enter even faster.
The water begins to activate the biochemistry of the dormant embryo. This is achieved through activation of the hydrolytic enzymes that are present. Water is also required for the activation of the growth substances and the leaching of the growth inhibitors.
3.2.2 Formation of Enzyme Systems
The energy for seed germination probably comes from respiration of the food stored in the endosperm. The endosperm of the seed has 2 parts. The bulk of the volume is a starch storage area. The covering layer is called the aleurone layer which is made of cells that store protein in abundance. Since, the embryo and the starch are separated from each other, there must be some chemical communications that allow the embryo to mobilize the food storage during germination. These chemicals are the growth regulators and enzymes.
Water coming into the seed and embryo dissolves the growth regulator, GA, or gibberellin made inside the embryo. The release of gibberellin from the embryo signals the seeds to break dormancy and germinate.
The dissolved GA is transported with the water through the rest of the seed tissues until it arrives at the aleurone layer, the thin outer layer of the endosperm.
The GA crosses into the cytoplasm of the aleurone cells and turns on certain genes in the nuclear DNA. The precise mechanism of how GA turns on the DNA is unknown at present. GA stimulates the transcription of mRNA leading to translation of hydrolytic enzymes like alpha amylase and also other enzymes such as proteases and lipases that mobilize stored nutrients.
3.2.3 Growth of Seedling
The growth of the seedling depends on the use of the nutrients such as lipid, starch and protein which are stored in either the cotyledons or endosperm.
Embryo also produces other hormones at the growing apices such as cytokinin which promotes cell division and auxin which promotes cell enlargement. Influences of different regulators allow cells to divide, lengthen and differentiate.
Therefore, the net dry mass of germinating seed decreases during these early stages of germination prior to photosynthesis since the rate of catabolic reactions is greater than the rate of anabolic reactions.
At the growth apices, first the radical then the plumule emerge from the seed.
The radical, embryonic root, is positively geotropic and grows downwards. It absorbs more water and inorganic ions from the soils allowing the rest of the seedling to grow even more rapidly.
The plumule, embryonic shoot, is negatively geotropic and positively phototropic and grows up above the soil.
In monocots, a coleoptile, protective sheath around the embryonic leaves pushes upwards through the soil towards the light. In dicots, a hook forms in the hypocotyls and growth pushes the hook above the ground. Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyls. Thus, the delicate shoot apex and bulky cotyledons are pulled above ground rather than being pushed tip first through the abrasive soil.
Once the plumule gets above the ground, the epicotyls spreads its leaves, which expand, become green and photosynthesis can begin. Carbon dioxide in the air are fixed and become incorporated into new substances in the plant so its dry mass increases.
The cotyledons then shrivel and fall away from the seedling, their food reserves having been consumed by the germinating embryo.
3.3 Types of Seed Germination
The terms used to describe the mode of germination bear relation to the position of the cotyledons during the process.
3.3.1 Epigeal Germination
The cotyledons emerge above the ground during the germination process. The cotyledons are carried up and out of the soil by the growth of the hypocotyl (the part of the embryo between the cotyledon stalks and the radical). The cotyledons appear as simple green leaves. During the growth through the soil, the hypocotyl remains looked and the plumule tip lies between the cotyledons, protected from abrasion damage by the soil.
3.3.2 Hypogeal Germination
The
cotyledons remain underground. The plumule is carried aloft by the growth of
the epicotyls (the base of the plumule). The hooked shape taken on by the
plumule as it is drawn up through the soil protects the tip from damage by soil
abrasion. In monocotyledons, the plumule grows through the soil, with the first
leaves protected by the coleoptile.
