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As simple as that, lentils beans and other legumes are sold both with and without husk, and often I make beans with the husk germinate in a jar.
Do those without husk germinate as well? Or are they simply dead?
I'm asking this because my girlfriend told me she's using de-husked legumes, but that they take longer and less of them actually do sprout.
De-husking and industrial storage reduces the germ's longevity quite a lot, although quite a lot of germs will still be viable if the humidity and microbial conditions are very stable, perhaps even a year.
De-husking will accelerate this graph:
The husk is designed to keep the germule in peak condition in the wild until whichever season it is supposed to germ, through droughts, rains, cold weather, changes in humidity, predation, infection.
If the germ is de-husked and left in natural conditions, it rapidly loses vitality and deteriorates. In storage it's kept in conditions made to discourage microbes, so it will last longer, but it will absorb ambient humidity changes and deteriorate much faster than with a husk.
Temperature and seed germination
Temperature can affect the percentage and rate of germination through at least three separate physiological processes. 1. Seeds continuously deteriorate and, unless in the meanwhile they are germinated, they will ultimately die. The rate of deterioration depends mainly on moisture content and temperature. The Q10 for rate of loss of viability in orthodox seeds consistently increases from about 2 at -10 degrees C to about 10 at 70 degrees C. 2. Most seeds are initially dormant. Relatively dry seeds continuously lose dormancy at a rate which is temperature-dependent. Unlike enzyme reactions, the Q10 remains constant over a wide range of temperature at least up to 55 degrees C, and typically has a value in the region of 2.5-3.8. Hydrated seeds respond quite differently: high temperatures generally reinforce dormancy or may even induce it. Low temperatures may also induce dormancy in some circumstances, but in many species they are stimulatory (stratification response), especially within the range -1 degree C to 15 degrees C. Small, dormant, hydrated seeds are usually also stimulated to germinate by alternating temperatures which typically interact strongly and positively with light (and often also with other factors including nitrate ions). The most important attributes of alternating temperatures are amplitude, mean temperature, the relative periods spent above and below the median temperature of the cycle (thermoperiod) and the number of cycles. 3. Once seeds have lost dormancy their rate of germination (reciprocal of the time taken to germinate) shows a positive linear relation between the base temperature (at and below which the rate is zero) and the optimum temperature (at which the rate is maximal) and a negative linear relation between the optimal temperature and the ceiling temperature (at and above which the rate is again zero). The optimum temperature for germination rate is typically higher than that required to achieve maximum percentage germination in partially dormant or partially deteriorated seed populations. None of the sub-cellular mechanisms which underlie any of these temperature relations are understood. Nevertheless, the temperature responses can all be quantified and are fundamental to designing seed stores (especially long term for genetic conservation), prescribing germination test conditions, and understanding seed ecology (especially that required for the control of weeds).
Physiology of Seed Germination
In this article we will discuss about:- 1. Subject-Matter of Seed Germination 2. Factors Influencing in Seed Germination 3. Mobilization of Reserves during Seed Germination 4. How to Test Seed Viability?.
Subject-Matter of Seed Germination:
Seeds develop and mature within the fruits. Once the fruit attains maturity and ripens it is shed and the seeds inside it undergo period of dormancy. In some of the succulent fruits even though the seeds are provided with moisture they do not germinate. This is because of lack of other germination factors.
Dormancy is imposed by several inhibitors present in the seed coat or the seed itself. Sometimes seed coat is thick and highly impervious to water and oxygen. In the latter part of our discussion we shall discuss some of the factors which cause seed dormancy and also how this dormancy is overcome.
Seeds will only germinate if given appropriate environmental conditions including water, air, temperature, free from high salt concentrations, inhibitors and sometimes specific spectral quality of light.
Seeds are highly dehydrated and naturally require water before germination. The first phase of seed germination is water imbibition till critical level of water is attained. Once the imbibition is completed, seeds begin to germinate and seedling emerges out. Radicle or root penetrates the seed coat and is followed by shoot emergence.
This is phase of emergence. Clearly in this phase root and shoot systems develop. Thus germination is preceded by imbibition and followed by emergence. The period of the two vary in different species and may be spread over several days or weeks.
Factors Influencing in Seed Germination:
Several environmental factors influence seed germination and these are described below:
Dry seeds can withstand diverse temperatures but once water is imbibed and germination begins they are sensitive to high temperatures. For every seed minimal, maximal and optimal temperatures exist and can be conveniently worked out. The minimal and maximal temperatures vary for different species and no reasons can be ascribed to such a variability.
Oxygen is essential for seed germination. The initial phase of seed germination may involve anaerobic respiration but immediately it shifts to aerobic state. In a seed where testa is retained the oxygen consumption is much higher than in the seeds where testa has been removed. Several other gases like CO2, CO, N2, H2S and ozone also affect germination by affecting several metabolic processes.
Several different types of compounds are known to affect seed germination and these include phenols, cyanides, alkaloids, herbacides, fungicides, salts of some metals, diverse acids, etc.
Some of the seeds are responsive to light. Light may trigger or inhibit seed germination. Lettuce seeds germinate rapidly when exposed to brief red light period.
The age of seeds is an important factor in germination.
Mobilization of Reserves during Seed Germination:
Seeds may be endospermic or non-endospermic depending upon the state whether endosperm is retained or consumed by the cotyledonary leaves of the embryo. Following seed germination several different types of metabolites e.g. starch, proteins, fats or other polysaccharides have to be hydrolysed and mobilized for the nutrition of the growing embryo and then seedling.
Clearly during early stages of seed germination hydrolytic enzymes are activated or synthesized. Seemingly gibberellins play a very vital role in their enhancement. In cereal grains the endosperm is starchy and is surrounded by a cellular tissue called aleurone layer. Several of the hydrolases are increased or secreted in this tissue.
β-amylase enzyme concerned with starch digestion is already present in the seed. However, α- amylase and protease appear soon after germination. Several investigators have shown that removal of embryo led to non-appearance of amylases and the addition of GA could replace the embryo removal effect.
It has been concluded that β-amylase was activated whereas α-amylase was synthesized de novo. Both the processes were mediated by gibberellin. Using l4 C-amino acids it was shown that they were incorporated in α-amylase indicating its fresh synthesis.
Gibberellins seemingly act at the molecular level and derepress the genes which cause α-amylase synthesis. Further embryo provides the requisite GA needed to initiate the synthesis or activation of amylases.
On the contrary seeds which have fats as the stored material convert fats into sugars and the latter are translocated to the growing embryo (Fig. 24-1). In such seeds fats are converted to acetyl-CoA through β-oxidation pathway. Acetyl CoA enters glyoxysomes and undergo glyoxylate cycle. In this cycle two molecules of acetyl CoA are converted to one of succinate.
Succinate is converted to oxaloacetic acid (OAA) which gives rise to phosphoenolpyruvate (PEP). Through reversal of glycolysis PEP is converted into sugars. ATP and reducing power needed in reverse glycolysis are obtained from the oxidation in the glyoxylate cycle and during succinate conversion to OAA and also from β-oxidation of fats when NADH is formed.
Figure 24-2 shows diagrammatic representation of mobilization of different nutrients in a germinating seed.
Areas of new growth and translocation of sugars, amides, etc. is clearly shown from the seed to the new centres of growth. The point to be noted is that the nitrogen transported compounds are reassembled in the growing embryo using carbon skeleton obtained from transported sugars. Thus amino acids are constituted and these are used during protein synthesis in the growing embryo.
Seed has been shown to have diverse types of storage products like fats, starch or proteins. The operation of different pathways is clearly indicated. It may be observed that ultimate products of translocation are sucrose, amides, amino acids, etc.
In summary we may state that during seed germination, the following types of metabolic processes are noticed:
(iii) Subcellular organization of the embryo or endosperm,
(iv) Alterations in the activity of phytochrome (if operative)
(vi) Enzymes synthesis de novo,
(vii) Hydrolysis of metabolites e.g. fats, starch, proteins etc.
(viii) Formation of organic molecules and their translocation to the new centres of growth,
(ix) Synthesis of nucleic acids and proteins,
(x) Oxygen uptake and respiration,
(xi) Enlargement of cell and cell division,
(xii) Upsurge of phytohormones,
(xiii) Synthesis of membranes and other cellular constituents,
(xiv) Variation in CO2 and O2 levels.
In the following we shall briefly discuss the chemical changes during germination of maize (monocot) seeds:
Maize seed is a grain filled up with starch and some amount of protein as well. Endosperm is surrounded by aleurone layers. It has one cotyledon which is modified into scutellum. Scutellum cells secrete hydrolases which digest endosperm metabolites.
Once immersed in water, maize seeds imbibe water and increase in diameter. Imbibition phase is completed within 12-14 hours of soaking. This is followed by enlargement of radicle and coleorhiza.
Seed coat is ruptured by the coleorhiza within 20-24 hours of imbibition and soon after radicle emerges out of seed or grain. In maize given favourable conditions and environments, germination is accomplished within a day or so. Biochemically changes begin after 24 to 48 hours of radical emergence. There is change in dry weight indicating loss of some metabolites from the endosperm.
Both fats and starch are digested after 72 hours of germination. Nitrogenous substances change after radical growth. It has been shown that after water imbibition by the seeds there is high metabolic activities and then the second upsurge of activity takes place after 72 hours. In the first phase there is high nucleic acids, protein and enzymes synthesis and activity.
How to Test Seed Viability?
The percentage of viable seeds can be determined through several methods and some of these are briefly mentioned below:
(i) Direct germination. A desired sample of seeds is germinated and viability percentage computed.
(ii) Seeds are soaked in distilled water and the electric conductivity of surrounding medium is determined. If there is high proportion of non-viable seeds then the conductance will increase.
(iii) Seeds soaked in potassium permagnate dilute solution also provide some indication on viability. If the number of seeds in a given sample is large then the KMnO4 solution will rapidly decolorize. Non-viable seeds are permeable and release high amounts of electrolytes and reducing substances into the surrounding medium.
(iv) In seeds with prolonged dormancy embryo is removed from the cotyledons or endosperm and put on sterilized nutrient medium. The viability is known within a week or so.
(v) Some seeds are split open and immersed in some redox dye (TTC) or tested for peroxidase using histochemical staining reaction. The loss of peroxidase or some dehydrogenase reactions also indicate dead nature of the seeds.
Successful Seed Germination
A seed is a miracle waiting to happen. The embryo comes pre-packaged with a food supply and the vital genetic information needed to become a plant just like its parents. Seeds exist in a state of dormancy, absorbing oxygen, giving off carbon dioxide, and slowly using up their stored food reserves. During this process the seed continually monitors the external environment waiting for ideal conditions specific for the particular seed. Once the ideal conditions occur, the seed breaks dormancy and germinates. The seedling gathers energy through its leaves by the process of photosynthesis and absorbs nutrients and water from the soil through the roots. As gardeners, our goal is to provide the optimal environment for germination and seedling growth.
For germinating seeds indoors, select a well-drained potting medium designed specifically for germinating seeds. Use clean containers with drainage holes in the bottom. Wash used containers with warm soapy water and rinse with a dilute bleach solution (1 part bleach to 9 parts water). Slightly overfill containers with the potting soil and tap the bottom and sides to encourage even settling.
Create a level surface by scraping excess soil with a board or knife. Do not press or compact the soil which will make it harder for the seeds to get started. Some gardeners will lightly firm the soil with a board to create a level surface. Moisten the soil either by watering carefully from the top or letting water soak up through the bottom. Allow excess water to drain away.
Seeds require a certain temperature in order to germinate. Each plant has a specific optimum and a range within which germination will occur. The closer the temperature is to optimum the quicker germination will occur. Most seeds germinate when the soil temperature is between 68(and 86(F. Once germination occurs, the optimum growing temperature for theseedling is about 10(F cooler than the optimum germination temperature.
Moisture is critical for germinating seeds. They like a moist but not soggy environment. Seeds require oxygen and if kept in a waterlogged state may rot. On the other hand, if the soil dries out, the seed will lose whatever water it has absorbed and will die. Finding the middle ground can be difficult and comes easier with practice. After sowing the seeds, mist the tray with water and cover with plastic wrap, a plastic bag, glass or plexiglass to seal in moisture. As soon as seed germinates remove the covering. Check the seedlings twice a day for moisture. Allow the soil surface to dry between waterings. Ventilation and air circulation are also important to discourage damping off diseases. Some seeds need light in order to germinate, but many do not. Seed packages will usually indicate what your particular selection requires. It is important to follow the directions given on the package for planting depth. In addition to light requirements, seeds that are planted too deep will not have enough stored energy to reach the soil surface and may die in the process. After germination occurs, seedlings require about 12 to 16 hours of light a day. Intense light is necessary to prevent spindly or leggy seedlings. If you are growing under lights, make sure the light source is 4 to 6 inches above the plants. In a sunny window, turn the seedlings regularly to avoid leaning.
If you are sowing seeds in furrows or flats, transplant individual seedlings into cell packs when the first true leaves appear or when they are large enough to handle Seedlings started indoors should be fertilized regularly with a dilute (1/4 strength) water soluble fertilizer. This will help to produce stockier transplants provided enough light is available.
Before planting in the garden, gradually acclimate transplants to the outdoors. Start by putting them outside on cloudy days or in a shaded location then after a few days work them into more light and exposure. Overcast skies or late afternoon is the best time to plant in the garden. Water immediately after transplanting. If plants wilt, provide some protection with an open milk carton or a board for a few days.
As gardeners everywhere begin the gardening season, these suggestions should help in raising strong healthy plants for enjoyment in the months to come.
Germination in Plants: Conditions and Types (With Diagram)
Germination is the awakening of the dormant embryo. In all mature Angiospermic seeds the embryo lies in a dormant state when its physiological activities come to a minimum.
Even its respiration is so slow as to be detectable only by sensitive instruments. As soon as the necessary conditions are satisfied this dormancy is broken and the phenomenon of germination begins.
Like all growth processes, the process of germination is irreversible, i.e., when germination has once commenced it cannot go back and the seed cannot be brought back to the dormant state.
Under germination are included all changes that take place from the time when the dry seed is placed under suitable conditions to the time when the seedling becomes established on the substratum.
Conditions Necessary for Germination:
In order that germination may begin, certain conditions are to be fulfilled. Some of these conditions are external while others are internal.
(A) External Factors:
Water is of primary importance in germination. No seed can germinate unless it is thoroughly moistened. Actual submergence under water is not necessary and may even be harmful as it may choke oxygen supply. Water absorbed by the protoplasm enables the resumption of vigorous physiological activities.
Digestion, -respiration or conduction cannot go on without water. The swelling of the embryo enables it to burst through the seedcoats which again, are softened by water absorption. Oxygen cannot get through the testa unless it is moist.
Oxygen is necessary for the seed as for any other living organ. The need of oxygen is even greater during germination as respiration and all physiological activities are more vigorous at this stage. Germination can go on for some time even without oxygen but it is soon checked.
(3) Optimum temperature:
Like all physiological activities germination is affected by temperate. There is a certain minimum and a certain maximum beyond which germination cannot take place. Within this range there is a certain optimum temperature where germination is most satisfactory.
This range varies from plant to plant. Seeds are not usually expected to germinate below 0°C and above 50°C and the optimum often lies between 25-30°C. Some seeds do not germinate well in any given temperature but prefers a fluctuation of temperature during the germinating period.
Light is not considered as an essential factor since’ germination takes place even without light. But, recent experiments have shown it to be of greater importance than what was hitherto thought.
Its pronounced effect on germination cannot be minimised. Light affects the germination of different seeds in different ways. It was previously thought that light retards germination in most cases.
But experiments by Kinzel (Germany) with about one thousand species of plants showed that the germination of 70% of seeds is favoured by light, in about 26% light inhibits germination while about 4% of the seeds are indifferent to light.
Tobacco, Rumex, mistletoe (Viscum album) seeds do not germinate in darkness while Datura and tomato seeds do not germinate well in light. Sometimes, some special treatment of seed (e.g., removal of seedcoat) may do away with this influence of light.
(B) Internal Factors:
A normal seed is expected to the internally capable of germination.
The following factors are of importance in determining this internal capacity:
All normal seeds contain a supply of food ‘which is necessary for the growing embryo and the young seedling. It has already been seen that this food may be contained in the cotyledon (mainly protein and starch to pulses oil in mustard, groundnut, sesame, linseed, cucurbits, sunflower, etc.), endosperm (starch and a little protein in cereals oil in castor, coconut, etc. cellulose in date, areca-nut, etc. mucilage in mallow or Malva sp., etc.), perisperm (black pepper, cardamom, etc.) or testa (pomegranate). Auxins are growth-promoting substances whose presence is essential for growth during germination.
(2) Completion of resting period (dormancy):
Many Angiospermic seeds cannot germinate as soon as they are formed. They have to undergo a period of dormancy or resting period. The period of dormancy varies from plant to plant. It may be a few days or some months.
Most cereals are capable of germination immediately after harvesting while some other seeds do not-germinate till after a year. Some plants do not need any resting period. The period of rest may be necessary for various reasons.
In some seeds the seedcoat is so hard that it takes time to wither. In this case the period of dormancy may be cut short by breaking open the seedcoat.
In others, the embryo may take time to be fully differentiated or some after-ripening may be necessary for the seed. Different treatments may hasten this ‘after-ripening.
Seeds retain their viability (capacity to germinate) for a definite period of time after which the embryo becomes dead for all practical purposes. Viability test is necessary to ascertain the germinating capacity of any seed. Conditions of storage (temperature, humidity, etc.) and circumstances in which the seed matured often determine the period for which the seed remains viable. Seeds usually keep well when kept dry, cold and free from insects or fungi.
Proper drying of seed is extremely important in the retention of viability although there are a few seeds (e.g., willow, poplar, maple) which fail to germinate if too dry. Weak or immature seeds lose their viability quickly. Hard-coated seeds often remain viable for a long time. Even when everything is satisfactory, it is found that the period during which the seed remains viable varies in different plants. The period may vary from one growing season to many years.
The longest authentic record of this period of viability is that of some lotus (Nelumbo nucifera) seed found within peat at the dried up bottom of a lake in Manchuria whose age has been calculated to be some eight hundred years.
Stories of 5,000 year old wheat or other cereal grains found in tombs in Egypt or Mohenjodaro proving viable are without any foundation. Seeds that old, are found to be in a carbonised condition. When viability is apparently lost, it may sometimes be restored by different treatments. This shows that viability may be lost even when the embryo is not really dead.
Changes during Germination:
When all the necessary conditions are satisfied, the first change noticed is swelling of the seed by rapid imbibition and osmosis of water. This may cause a bursting of the seed-coat. Absorption of water causes a vigorous resumption of physiological activities by the protoplasm. There is rapid respiration and copious secretion of enzymes which causes digestion of stored food. Insoluble food is rendered soluble and complex food made simple.
This simple food solution is diluted by water and conducted towards the growing epicotyl, hypocotyl, radicle and plumule. Food is translocated from perisperm to endosperm, from endosperm to cotyledon and from cotyledon to the growing organs according as which of them are present in the seed. Assimilation of this food by the growing organ enables growth and the seedling soon assumes its ultimate shape.
When the seed is placed in the soil and growth has become vigorous the radicle is the first organ to grow vigorously. It comes out through the micropyle and fixes the seed to the soil. After this, either the hypocotyl or the epicotyl begins to grow. When the hypocotyl grows first, it pushes the cotyledonary node and all other parts of the seed (with or without the seedcoat) out of the soil and the mode of germination is called epigeal or epigeous.
When the epicotyl grows first only the plumule is pushed out of the soil while the cotyledonary node, cotyledons and all other parts remain under the soil.
This type of germination is called hypogeal or hypogeous. In Monocots, however, the hypocotyl does not grow but the epigeous and hypogeous nature is determined by the growth of the cotyledon itself.
Types of Germination:
A. Epigeal Germination:
Epigeal germination is shown by some dicotyledonous plants and a few monocots. Common examples of this type of germination are found in:
Dicotyledonous exalbuminous: Cucurbits, mustard, tamarind, French bean (Phaseolus vulgaris), Lablab (Dolichos lablab), sunflower.
Dicotyledonous albuminous: Castor.
Monocotyledonous albuminous: rare, found in onion.
Monocotyledonous exalbuminous: Alisma plantago.
(1) Gourd (Cucurbita Maxima):
As germination begins, the straight radicle comes out and fixes the seed to the soil with the secondary roots coming out of the radicle. The hypocotyl next grows so quickly that it forms a loop which comes out of the soil and pulls out the rest of the seed.
Frequently, the seedcoat gets caught to a peg-like projection at the base of the hypocotyl so that it is cast off easily and the cotyledons are brought out in the air.
Next, the cotyledons open out like two leaves, become green, large and thin so that they look and behave like ordinary leaves in every way though differing in form from normal cucurbita leaves. The plumule within the cotyledons becomes exposed and soon grows into, the aerial shoot.
(2) Mustard (Brassica spp.):
The seedcoat is thinner, the two cotyledons are much oily and the radicle is curved in the tiny seed. The stages of germination are essentially the same as in the cucurbits.
(3) Tamarind (Tamarindus Indica):
The testa in this case is very hard. Nevertheless, the radicle comes out first after the testa is burst and fixes the seed by forming the root system. The hypocotyl now grows fast and soon pulls out the two large and thick cotyledons.
The plumule then grows out into the aerial shoot. The cotyledons turn greenish, gradually shrivel up and finally drop off as the food matter within them is used up. The cotyledons, although turning greenish, never look like ordinary leaves as they do in Cucurbita.
(4) & (5) Lablab (Dolichos lablab) and French Bean (Phaseolus vulgaris):
Both of them germinate like tamarind. The fleshy cotyledons do not become leafy but behave like tamarind cotyledons. Lablab is the common flat bean of the plains.
(6) Castor (Ricinus Communis):
The shell-like testa first bursts near the caruncle and the radicle grows out. Subsequent growth of the hypocotyl pulls out of the soil the two thin cotyledons enclosed in the endosperm. The testa is cracked and is soon cast off but the cotyledons do not come out of the endosperm until the latter is almost consumed by the former.
The cotyledons then open up and become green and leafy while the plumule slowly develops into the leafy shoot. The remnant of the endosperm withers and drops off.
Epigeous germination is very rare among the Monocots. Onion is one of the very few examples. In this case the radicle as well as the base of the curved cotyledon (scutellum) grows out of the seed.
The radicle penetrates the soil while the other end of the cotyledon remains within the endosperm and sucks the food material. The base of the cotyledon grows further, turns green and pushes the seed out of the soil.
The plumule is not visible so long, as it is covered by the base of the cotyledon in the form of a sheath just above the radicle. The plumule now pierces this cotyledon sheath and forms the first cylindrical foliage leaf.
Meanwhile, adventitious roots develop from above the radicle forming a fibrous root system which is characteristic of monocots.
It should be noted in this case that the seed is pushed out of the soil not by the growth of the hypocotyl as in the other cases but by that of the base of the cotyledon itself.
(8) Peperomia Peruviana:
This is a dicot (Piperaceae) with endosperm and perisperm showing a peculiar type of germination. During germination one of the cotyledons remains hypogeal within the endosperm sucking the food material from the latter as well as from the perisperm while the other cotyledon becomes epigeal and green.
B. Hypogeal Germination:
Hypogeal germination is shown by some dicotyledons and by most of the monocotyledons. Common examples are:
Dicotyledonous exalbuminous: Pea, gram, broad bean (Vicia faba), scarlet runner bean (Phaseolus multiflorus), mango, jack-fruit.
N.B. In pea and runner bean sometimes a tendency to epigeous germination is noticed.
Monocotyledonous albuminous: Rice, maize, wheat, coconut, date, areca-nut, fan (palmyra) palm.
The radicle comes out first, penetrates the soil and forms a root system by giving out secondary branches. It is the epicotyl which grows first here.
It arches out and carries the plumule above ground. The plumule soon forms the aerial shoot. The cotyledons, therefore, remain under soil throughout.
(2) & (3) Gram (Cicer Arietinum) and Broad Bean (Vicia Faba):
The mode of germination is -essentially the same as in pea. The cotyledons remain underground and are gradually used up.
(4) Mango (Mangifera Indica):
The seed is covered by the hard endocarp. Absorption of water causes swelling and rupture of the endocarp and the seedcoat. The radicle comes out and forms a root system.
The epicotyl then grows, comes out through a slit in the cotyledons and takes the plumule out of the soil while the cotyledons remain within the endocarp below. The first leaves are copper-coloured which gradually become green.
(5) Jack-fruit (Artocarpus Heterophyllus):
Germination is hypogeous. Although the two unequal cotyledons remain underground, they develop a green colour.
A day or two after the seed is placed in moist soil, the coleorhiza pierces the base of the fruit and appears as a glistening knob. The radicle next penetrates the soil after splitting the coleorhiza.
The coleoptile comes out now. (If the seed remains submerged in water, sometimes the coleoptile may come out first, the emergence of the radicle being delayed.) Immediately after the emergence of the radicle two other roots grow from its base and these are called seminal roots.
The radicle and seminal roots give rise to” secondary branches but, as opposed to the dicotyledons, the radicle does not form the root system. The base of the coleoptile and the mesocotyl now lengthen somewhat and the plumule soon comes out piercing the -coleoptile.
Meanwhile, adventitious roots are formed from the base of the plumule (top of the mesocotyl) or from the lowermost nodes of the stem. These adventitious roots form the fibrous root system of the mature plant. The pierced coleoptile soon withers away.
(7) Maize or Corn (Zea Mays):
The mode of germination is essentially the same. The radicle emerges first by piercing the fruit wall and the coleorhiza. The coleoptile follows. The coleoptile and plumule develop as in rice. Three seminal roots develop from above the radicle (one opposite scutellum and two others from slightly above that point). In exceptional cases the number of seminal roots may vary from 0 to 10.
The radicle and the seminal roots with their branches persist throughout the life of the plant and are not short-living as was previously thought. The adventitious roots are formed from the lowermost nodes above the mesocotyl.
Germination as in rice and maize. Seminal roots number 4 to 5. The number of seminal roots is sometimes found to be variable. Adventitious roots, as in others, develop from above the mesocotyl though some of them may develop on the coleoptile. The branched radicle and the seminal roots probably persist throughout the life of the plant along with adventitious roots higher up.
(9) Coconut (Cocos Nucifera):
The small embryo below one eye on the shell on the top of the endosperm is undifferentiated at first. During germination the lower (actually the upper end as the fruit remains in a hanging upside down position) end of the embryo forms the cotyledon which begins to grow as a spongy structure inside the endosperm.
This spongy cotyledon increases in size as it absorbs the food material stored within the endosperm. The upper end of the embryo develops through the eye carrying the radicle and the plumule. The plumule pierces the fibrous pericarp and emerges like a horn. This develops the aerial shoot even before the roots have come in contact with the soil. The radicle fails to develop any further but several adventitious roots grow from the base of the plumule. The seedling becomes established when the adventitious roots penetrate the soil.
(10) Date (Phoenix Sylvestris):
The stony seed is formed mainly of a cellulose endosperm on one side of which is the small embryo. During germination the cotyledon begins to grow.
The base (sheath and stalk) of the cotyledon grows out by forcing open the soft tissue above the embryo (which often comes out in the form of a lid) while the upper part remains inside the endosperm gradually increasing in size and absorbing more and more of the reserve cellulose transforming the latter into sugar.
The base of the cotyledon which penetrates the soil is like a sheath enclosing the axis at its extremity.
This sheath may be called the cotyledonary sheath. The radicle pierces the coleorhiza at the lower end and forms a more or less strong primary root system in the soil which is stronger than in other monocots although it does not form the main root system.
Next, the plumule bursts out from one side of the sheath anchdevelops aerial leaves. The upper part of the cotyledonary sheath acts as a stalk for the part of the cotyledon which is still inside the endosperm sucking the nutrients.
Adventitious roots are later given out from the base of the aerial shoot and form the main root system of the growing plant.
(11) & (12) Palmyra or Fan Palm (Borassus Fiabellifer) and Betel- or Areca-Nut (Areca Catechu):
Both of these have got hard cellulose endosperms. The betel-nut endosperm is also ruminated. The manner of germination is practically the same as in date palm.
Vivipary means germination of the seed within the fruit while still attached to the mother plant. In the animal world, mammals are viviparous as the embryo differentiates into the young individual while still in the mother’s womb.
Birds lay eggs and ordinary plants develop seeds and not seedlings on the parent body, so they are not viviparous. Vivipary, however, is found in a number of plants. In the vegetable Sechium edule of Cucurbitaceae (locally called squash), common in Indian hill towns, it is often seen that the seed germinates while inside the fruit still attached to the mother plant.
So is the case of coconut. Paddy grains germinate on the mother plant if they get sufficient moisture. Such vivipary is always dependent on excessive moisture in the atmosphere or within the fruit (e.g., lemon and oranges, tomatoes, melons) and on the absence of any period of dormancy. Vivipary may also take place through vegetative organs, e.g., bulbils of Agave (discussed in connection with ‘bud’).
Beside the above, a special kind of vivipary is noticed in the mangrove plants found in estuarine tidal shores of the tropics, e.g., the Sundarbans in the Gangetic delta and similar mangrove formations in the estuaries of different Indo-Burmese rivers.
The seed embryos in these plants do not have any resting stage and continue to grow uninterruptedly inside the fruit. The radicle first comes out of the fruit and then the hypocotyl begins to grow very vigorously so that it looks like a club which is usually 2 to 9 Inches long and may sometimes attain 18 inches.
The length varies according to the species of plant and may be determined by the depth of the water below. The plumule also grows somewhat while the cotyledons remain inside the fruit acting as haustoria.
Some fast growing mangrove plants (e.g., Sonneratia caseolaris of Sonneratiaceae) do not show any vivipary while in a few others, e.g., Avicennia (Verbenaceae) and Aegiceras (Myrsinaceae), the type of vivipary is rudimentary.
In markedly viviparous plants the process of germination is slow. Common examples are Rhizophora mucronata, Ceriops decandra, Bruguiera gymnorhiza and Kandelia candel all belonging to the family Rhizophoraceae. Of these, Rhizophora grows in the deepest water and shows the largest hypocotyl while Ceriops comes next.
When the hypocotyl grows very heavy the fruit gets detached from the plant or, in some cases, the axis (i.e., radicle, hypocotyl and plumule) gets detached from the cotyledons and, because of the heaviness and the shape of the hypocotyls, it falls vertically down like a dart so that the radicle penetrates the soil below the shallow water.
It soon forms a root system while the plumule grows safely above the water level. Sometimes, if the water be too deep, the viviparous fruit may float with the hypocotyl hanging down and get rooted where the depth of water is just right enabling the radicle to get fixed to the soil.
This is a peculiar natural adaptation particularly suiting this type of plants .
Seed Germination & Detergents
Seeds come in different sizes, shapes, and colors. Some are edible and some are not. Some seeds germinate readily while others need specific conditions to be met before they will germinate. Within every seed lives a tiny plant or embryo.The outer covering of a seed is called the seed coat. Seed coasts help protect the embryo from injury and also from drying out. Seed coats can be quite thin and soft as in beans or very thick and hard as in locust or coconut seeds. Endosperm, which is a temporary food supply, is packed around the embryo in the form of special leaves called cotyledons or seed leaves. These generally are the first parts visible when the seed germinates. Plants are classified based upon the number of seed leaves (cotyledons) in the seed. Plants such as grasses and grass relatives can be monocots, containing one cotyledon. Dicots are plants that have two cotyledons.
Seeds remain dormant or inactive until conditions are right for germination. All seeds need water, oxygen, and proper temperature in order to germinate. Some seeds require proper light also. Some germinate better in full light while other require darkness to germinate.When a seed is exposed to the proper conditions, water and oxygen are taken in through the seed coat. The embryo’s cells start to enlarge and the seed coat breaks open and root or radicle emerges first, followed by the shoot or plumule which contains the leaves and stem.
Many factors contribute to poor germination. Over-watering results in a lack of proper oxygen levels. Planting seeds to too deep results in the seed using up all of its stored energy before reaching the soil surface, and dry conditions result in the lack of sufficient moisture to start and sustain the germination process.
The students will be able to describe how some environmental factors affect seed germination.
Masking tape, Scissors, 3 ziplock bags, Marker, Forceps, Paper Towels, Metric Ruler, 3 colored pencils, 25 seeds, distilled water, 50 ml graduated, 1% detergent solution, 10% detergent solution, graph paper
Materials Required For Seed Germination Experiment :
This project can be made from different materials available in our house. We have made this science experiment using very simple materials. Some of materials needed for this germination of seed are :
- Take a medium size beaker. If we don’t have baker then medium size water glass can also be used.
- We need a one liter clean water.
- It is good to use color Popsicle stick. If not a normal 15 cm scale can also be used.
- As our seed is germination we need a seed. For this we have used gram seed. You can use other seed too.
- Lastly we also need a string. In our case we have used three different color thread.
Process of Seed Germination: 5 Steps (With Diagram)
Such five changes or steps occurring during seed germination are: (1) Imbibition (2) Respiration (3) Effect of Light on Seed Germination(4) Mobilization of Reserves during Seed Germination and Role of Growth Regulators and (5) Development of Embryo Axis into Seedling.
The first step in the seed germination is imbibition i.e. absorption of water by the dry seed. Imbibition results in swelling of the seed as the cellular constituents get rehydrated. The swelling takes place with a great force. It ruptures the seed coats and enables the radicle to come out in the form of primary root.
Imbibition is accomplished due to the rehydration of structural and storage macromolecules, chiefly the cell wall and storage polysaccharides and proteins. Many seeds contain additional polysaccharides, not commonly found in vegetative tissues. Seeds packed dry in a bottle can crack it as they imbibe water and become swollen.
Imbibition of water causes the resumption of metabolic activity in the rehydrated seed. Initially their respiration may be anaerobic (due to the energy provided by glycolysis) but it soon becomes aerobic as oxygen begins entering the seed. The seeds of water plants, as also rice, can germinate under water by utilizing dissolved oxygen.
The seeds of plants adapted to life on land cannot germinate under water as they require more oxygen. Such seeds obtain the oxygen from the air contained in the soil. It is for this reason that most seeds are sown in the loose soil near the surface. Ploughing and hoeing aerate the soil and facilitate seed germination. Thus the seeds planted deeper in the soil in water-logged soils often fail to germinate due to insufficient oxygen.
(iii) Effect of Light on Seed Germination:
Plants vary greatly in response to light with respect to seed germination. The seeds which respond to light for their germination are named as photoblastic. Three categories of photoblastic seeds are recognized: Positive photoblastic, negative photoblastic and non-photoblastic. Positive photoblastic seeds (lettuce, tobacco, mistletoe, etc.) do not germinate in darkness but require exposure to sunlight (may be for a brief period) for germination.
Negative photoblastic seeds (onion, lily, Amaranthus, Nigella, etc.) do not germinate if exposed to sunlight. Non-photoblastic seeds germinate irrespective of the presence (exposure) or absence (non-exposure) of light.
In these light sensitive seeds, the red region of the visible spectrum is most effective for germination. The far-red region (the region immediately after the visible red region) reverses the effect of red light and makes the seed dormant. The red and far-red sensitivity of the seeds is due to the presence of a blue-coloured photoreceptor pigment, the phytochrome. It is a phycobiloprotein and is widely distributed in plants.
Phytochrome is a regulatory pigment which controls many light-dependent development processes in plants besides germination in light- sensitive seeds. These include photo-morphogenesis (light-regulated developmental process) and flowering in a variety of plants.
Phytochrome and Reversible Red-Far-red Control of Germination:
The pigment phytochrome that absorbs light occurs in two inter-convertible forms Pr and Pfr. Pr is metabolically inactive. It absorbs red light (660 nm.) and gets transformed into metabolically active Pfr (Fig. 4.10). The latter promotes germination and other phytochrome-controlled processes in plants. Pfr reverts back to Pr after absorbing far-red (730 nm.).
In darkness too, Pfr slowly changes to Pr. Owing to this oscillation of phytochrome between Pr and Pfr status, the system has been named as “reversible red—far-red pigment system” or in brief phytochrome system. Treatment with Red light (R) stimulates seed germination, whereas far-red light (FR) treatment, on the contrary, has an inhibitory effect.
Let US examine seed germination in positive photoblastic seeds e.g. lettuce (Lactuca sativa). When brief exposure of red (R, 660 nm.) and far-red (FR, 730, nm.) wave lengths of light are given to soaked seeds in close succession, the nature of the light provided in the last exposure determines the response of seeds. Exposure to red light (R) stimulates seed germination. If exposure to Red light (R) is followed by exposure to far-red light (FR), the stimulatory effect of Red light (R) is annulled.
This trick can be repeated a number of times. What is crucial for seed germination is the quality of light to which the seeds are exposed last. This also indicates that responses induced by red light (R) are reversed by far-red light (FR).
Whole of this can be shown as given ahead:
Light requirement for seed germination may be replaced by hormones such as gibberellins or cytokinins. Several development processes of plants controlled by phytochrome may be mimicked by appropriate hormones given singly or in combination with other hormones at the correct time.
(iv) Mobilization of Reserves during Seed Germination and Role of Growth Regulators:
During germination the cells of the embryo resume metabolic activity and undergo division and expansion. Stored starch, protein or fats need to be digested. These cellular conversions take place by making use of energy provided by aerobic respiration.
Depending upon the nature of the seed, the food reserves may be stored chiefly in the endosperm (many monocotyledons, cereal grains and castor) or in the cotyledons (many dicotyledons such as peas and beans). Thorough investigations in the mobilisation of reserves from the endosperm to the embryo via a shield-like cotyledon (scutellum) has been done in several cereal grains (Fig. 4.11).
The outer layer of special cells (aleurone layer) of endosperm produces and secretes hydrolyzing enzymes (such as amylases, proteases). These enzymes cause digestion i.e. breakdown of the stored food such as starch and proteins in the inner endosperm cells.
The insoluble food is rendered soluble and complex food is made simple. These simpler food solutions, comprising of sugars and amino acids thus formed, are diluted by water and passed towards the growing epicotyl, hypocotyl, radicle and plumule through the cotyledon.
Gibberellic acid plays an important role in initiating the synthesis of hydrolyzing enzymes. Gibberellin, therefore, promotes seed germination and early seedling growth. Assimilation of this food by the growing organ induces growth and the seedling soon assumes its ultimate shape.
It is very significant to note that the dormancy inducing hormone, abscisic acid (ABA), prevents the germination. The concentration of ABA has been shown to increase during the onset of dormancy of the embryo during seed development in several kinds of seeds.
When young embryos of cotton are removed and grown in culture, they continue to grow without the development of any dormancy. Dormancy in such cases can be induced by the addition of ABA at a crucial stage of growth.
(v) Development of Embryo Axis into Seedling:
After the translocation of food and its subsequent assimilation, the cells of the embryo in the growing regions become metabolically very active. The cells grow in size and begin divisions to form the seedling.
Factors Affecting Seed Germination: External and Internal Factors
The below mentioned article will highlight the factors affecting seed germination.
Some of the important factors are: (1) External factors such as water, oxygen and suitable temperature. (2) Internal factors such as seed dormancy due to internal conditions and its release.
I. External Factors:
A dormant seed is generally dehydrated and contains hardly 6-15% water in its living cells. The active cells, however, require about 75-95% of water for carrying out their metabolism. Therefore, the dormant seeds must absorb external water to become active and show germination. Besides providing the necessary hydration for the vital activities of protoplasm, water softens the seed coats, causes their rupturing, increases permeability of seeds, and converts the insoluble food into soluble form for its translocation to the embryo. Water also brings in the dissolved oxygen for use by the growing embryo.
Oxygen is necessary for respiration which releases the energy needed for growth. Germinating seeds respire very actively and need sufficient oxygen. The germinating seeds obtain this oxygen from the air contained in the soil. It is for this reason that most seeds sown deeper in the soil or in water-logged soils (i.e. oxygen deficient) often fail to germinate due to insufficient oxygen. Ploughing and hoeing aerate the soil and facilitate good germination.
(iii) Suitable Temperature:
Moderate warmth is necessary for the vital activities of protoplasm, and, therefore, for seed germination. Though germination can take place over a wide range of temperature (5-40°C), the optimum for most of the crop plants is around 25-30°C. The germination in most cases §tops at 0°C and 45°C.
II. Internal Factors:
(iv) Seed Dormancy Due to Internal Conditions and Its Release:
In some plants the embryo is not fully mature at the time of seed shedding. Such seeds do not germinate till the embryo attains maturity. The freshly shed seed in certain plants may not have sufficient amounts of growth hormones required for the growth of embryo. These seeds require some interval of time during which the hormones get synthesized.
The seeds of almost all the plants remain viable or living for a specific period of time. This viability period ranges from a few weeks to many years. Seeds of Lotus have the maximum viability period of 1000 years. Seeds germinate before the ending of their viability periods.
In many plants, the freshly shed seeds become dormant due to various reasons like the presence of hard, tough and impermeable seed coats, presence of growth inhibitors and the deficiency of sufficient amounts of food, minerals and enzymes, etc.
Importance of Water for Grass Seed Germination
As stated above, grass seed is very dry at the time of planting. On average, less than 10 percent of a seed’s total weight is water. Therefore, before a seed can germinate it must absorb water. This is called "imbibition". Water is necessary to activate the seed and begin the process of germination.
During imbibition, the seed swells considerably. Dormant enzymes are activated. New enzymes are also developed during imbibition that help the seed to utilize stored food created during embryo development. As internal cell division begins, there must be sufficient soil moisture to supply a continuous source of water and nutrients to the seed. This is the reason why the soil must remain damp until the seed has completed germination and has sprouted.
When grass seed germination is completed and the seed has sprouted, sufficient soil moisture is still necessary. The short roots are still developing, so frequent, but not heavy irrigation will be needed until the roots have developed some depth. Saturated soils can lower oxygen levels and encourage root diseases.
The Importance of Oxygen in Grass Seed Germination
The seed operates in an anaerobic manner during the initial stages of water absorption at the start of germination. This continues until the seed coat ruptures. Once the seed coat ruptures the seed moves from being anaerobic (not requiring oxygen) to aerobic (requires a steady supply of oxygen). This process of oxygen exchange in plants is called “transpiration.”
Therefore, it is very important to make sure the soil is damp, but not waterlogged. Waterlogged soil decreases the amount of oxygen available to the seed. If saturated soil conditions are prolonged, the seed can perish.
Planting in good soil is also important for oxygen exchange. Heavy clay soil may hold water well, but can easily compact, which reduces oxygen levels. Loamy soils are better for planting seed. Soils with a high volume of sand may have good oxygen levels, but do not hold water very well.
The Importance of Temperature in Seed Germination
Different seed types have slightly different preferred temperature ranges for germination. However, the important thing to remember is that seed will have a minimum and maximum temperature. If the temperature is outside of those limits the seed will not germinate.
The majority of lawn grass seed will germinate in an average soil temperature range of 80 degrees. Keep in mind that most seed types will still germinate in temperature variations of plus or minus 15 degrees from average. A little research on your seed variety will give you the best temperature range for planting
In contrast, some early season vegetable plant seeds can germinate in considerably cooler soil temperatures.
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