THE PROCESS OF SEED GERMINATION

A.    INTRODUCTION

            Most plants life starts from the humble seed, leaf through this article to understand the seed germination process. Life begins from the seed for all plants alike. To reproduce itself and evolve with time is what a seed offers to the eco-system, and many trees, plants and flowers bear witness to this fact. A seed is basically a kernel that encloses in itself a small embryonic plant covered by a hard seed coat and some stored food that upon receiving the appropriate climatic conditions, will promote growth. The seed is the ripened ovule, (egg) fertilized product of gymnosperm and angiosperm plants. This is the end product of the pollination process in which the embryo develops from the zygote, and the seed coat from the outer covering of the ovule. The ability to consistently and successfully reproduce itself makes trees and plants that use their seeds for propagating themselves have a higher survival rate, than the ones who rely on cuttings, runners, shoots or rhizomes. Let us understand this amazing ability by reading through the seed germination process.

            Germination is the growth of an embryonic plant contained within a seed; it results in the formation of the seedling. The seed of a vascular plant is a small package produced in a fruit or cone after the union of male and female sex cells. All fully developed seeds contain an embryo and in most plant species some store of food reserves, wrapped in a seed coat. Some plants produce varying numbers of seeds that lack embryos; these are called empty seeds and never germinate. Most seeds go through a period of dormancy where there is no active growth; during this time the seed can be safely transported to a new location and/or survive adverse climate conditions until circumstances are favorable for growth. Dormant seeds are ripe seeds that do not germinate because they are subject to external environmental conditions that prevent the initiation of metabolic processes and cell growth. Under favorable conditions, the seed begins to germinate and the embryonic tissues resume growth, developing towards a seedling.

B.     DISCUSSION

            Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the growth of hyphae from fungal spores, is also germination. In a more general sense, germination can imply anything expanding into greater being from a small existence or germ.

            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 others 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. Then the seed coat breaks open and a root or radicle emerges first, followed by the shoot or plumule that contains the leaves and stem.

            Many things can cause poor germination. Overwatering causes the plant to not have enough oxygen. Planting seeds too deeply causes them to use all of their stored energy before reaching the soil surface. Dry conditions mean the plant doesn't have enough moisture to start the germination process and keep it going.

            Some seed coats are so hard that water and oxygen cannot get through until the coat breaks down. Soaking or scratching the seeds will help break down the seed coat. Morning glories and locust seeds are examples. Other seeds need to be exposed to proper temperatures. Apple seeds will not germinate unless they are held at cold temperatures for a period of time.

            The seed contains an immature plant (embryo) that resembles an adult plant, complete with leaves and a root. The seed's leaves are called the cotyledons, seeds that contain one embryonic leaf are known as monocotyledonous or monocots, whereas seeds with two embryonic leaves are termed as dicotyledonous or dicots. The food found in the seed which nourishes the embryonic seedling during its early stages of development, is known as endosperm.
            There are certain basic steps of seed germination. For a seed to germinate successfully, firstly, the right conditions are required. Although, most seeds will germinate under different conditions, the plants or trees will not come true, as it's the quality of the seed that matters, not its age. Lotus seeds as old as 2000 years have germinated, as the quality of their embryo was preserved. Moisture or water is needed by the dried seeds to resume their cellular metabolism and growth. Moisture, combined with warmth, triggers growth, which is probably the reason why sown seeds should be kept in a warm place. Warmth increases humidity, which ensures enough moisture to the seeds. The size of the seed and the depth it is sowed in determines how quick it will sprout through the soil. The larger the seed, more the energy stored in it, and vice-versa. This is the reason why large seeds are sowed more deeper in comparison to smaller seeds. Soil matters as the seed takes its oxygen from its pores, and the right temperature will accelerate its growth. Whether a seed needs light, full or partial, or darkness to sprout depends upon its individual physiological need. The dormancy level of the seeds also determines the time it will take to germinate. Another way to germinate seeds is by growing seeds without soil.

            Once the conditions have been satisfied for the process of seed germination, it is just a matter of time that they turn into a seedling. Some seeds, especially the ones with hard coats like the sunflower, morning glory, dates, acorn, corn, etc. need a couple of hours pre-soaking to speed up the germination of seeds.

            After the seeds are sowed, and the soil misted with water, it (water) gets absorbed by the seeds through its coat, and provides moisture to the embryo nestled in it. This activates enzymes that help in duplication of plant cells, and also gets them to use the energy or food stored in the seed to start nourishing the embryonic plant. With all the nourishment, the embryo becomes too large, and bursts open through the seed coat, in search of light to start its process of photosynthesis, and thus, the growing plant emerges. During the same time, even the roots sprout and head down in search of more food from the soil. Both the root and plant shoot move downwards and upwards, simultaneously and respectively. In no time then, you will see the seedling force its way through the soil.

There are basically three steps of seed germination:

• Step 1-Water imbibation results in rupture of seed coat, uniform imbibation is important and approximately optimum temperatures are required
• Step 2-The imbibition of the seed coat results in emergence of the radicle and the plumule, the cotyledons get unfolded. It is important that the temperature and photo period are required in optimum amounts
• Step 3-This marks the final step in the germination of the seed where the cotyledons are expanded which are the true leaves.

Factors affecting seed germination 

            Seed germination depends on both internal and external conditions. The most important external factors include temperature, water, oxygen and sometimes light or darkness. Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of a plant's natural habitat. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.

• Water - is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called inbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals. After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.

• Oxygen - is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it grows leaves. Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.

• Temperature - affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above room-temperature 60-75 F (16-24 C), while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28-40 F (-2 - 4 C), and some when the soil is warm 76-90 F (24-32 C). Some seeds require exposure to cold temperatures (vernalization) to break dormancy. Seeds in a dormant state will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they take in water in the fall and experience cooler temperatures. Four degrees Celsius is cool enough to end dormancy for most cool dormant seeds, but some groups, especially within the family Ranunculaceae and others, need conditions cooler than -5 C. Some seeds will only germinate after hot temperatures during a forest fire which cracks their seed coats; this is a type of physical dormancy.

        Most common annual vegetables have optimal germination temperatures between 75-90 F (24-32 C), though many species (e.g. radishes or spinach) can germinate at significantly lower temperatures, as low as 40 F (4 C), thus allowing them to be grown from seed in cooler climates. Suboptimal temperatures lead to lower success rates and longer germination periods.

• Light or darkness - can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for growth of the seedling.

            Scarification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others need to be passed through an animal's digestive tract to weaken the seed coat enough to allow the seedling to emerge.

Germination rate

            In agriculture and gardening, the germination rate describes how many seeds of a particular plant species, variety or seedlot are likely to germinate. It is usually expressed as a percentage, e.g., an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions. The germination rate is useful for calculating the seed requirements for a given area or desired number of plants.

Dicot germination

The part of the plant that first emerges from the seed is the embryonic root, termed the radicle or primary root. It allows the seedling to become anchored in the ground and start absorbing water. After the root absorbs water, an embryonic shoot emerges from the seed. This shoot comprises three main parts: the cotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs among plant groups.

Epigeous

In epigeous (or epigeal) germination, the hypocotyl elongates and forms a hook, pulling rather than pushing the cotyledons and apical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air. Beans, tamarind, and papaya are examples of plants that germinate this way.

Hypogeous

Another way of germination is hypogeous (or hypogeal), where the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose. Peas, for example, germinate this way.

Monocot germination

In monocot seeds, the embryo's radicle and cotyledon are covered by a coleorhiza and coleoptile, respectively. The coleorhiza is the first part to grow out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. There, it stops elongating and the first leaves emerge.

Precocious germination

While not a class of germination, precocious germination refers to seed germination before the fruit has released seed. The seeds of the green apple commonly germinate in this manner.

One-step seed germination of Brassica and pea seeds: testa rupture and initial radicle elongation

• The endosperm is completele obliterated during the seed development of Brassica spp. (see figure below) or pea and the mature seeds of these species are therefore non-endospermic. Uptake of water by a seed is triphasic with a rapid initial uptake (phase I, i.e. imbibition) followed by a plateau phase (phase II). A further increase in water uptake (phase III) occurs only when germination is completed, as the embryo axes elongates and breaks through the testa. Thus, besides radicle elongation, testa rupture is the only visible landmark during Brassica spp. and pea seed germination.
• Abscisic acid (ABA) does not inhibit imbibition and testa rupture (see figure below), but ABA inhibits phase III water uptake and the transition from germination to postgermination growth.

Brassica napus seed germination is one-step. The mature seeds of these species are without endosperm and so testa rupture plus initial radicle elongation result in the completion of germination. ABA does not inhibit testa rupture, but inhibits subsequent radicle growth.

Two-step seed germination of Lepidium and Arabidopsis (Brassicaceae): testa and endosperm rupture

• For the Lepidium and Arabidopsis seed anatomy see the webpage "Seed Structure".
• Rupture of the testa (seed coat) and rupture of the endosperm are separate events in the germination of Lepidium and Arabidopsis seeds (see figures below). Arabidopsis and Lepidium exhibit a two-step germination, in which testa rupture and endosperm rupture are sequential events. 

• Such two-step germination is widespread over the entire phylogenetic tree and has been described for many species, e.g. for Trollius, Chenopodium, Nicotiana and Petunia.
• We found that the plant hormone ABA inhibits endosperm rupture, but not testa rupture, of Arabidopsis and Lepidium. This inhibitory effect of ABA is counteracted by GA, supporting the view that endosperm rupture is under the control of an ABA-GA antagonism.
• We found that ABA inhibits endosperm weakening of Lepidium, and this inhibitory effect is counteracted by GA. This supports the view that weakening of the micropylar endosperm occurs in Arabidopsis and Lepidium seeds (Brassicaceae, Rosid clade), is under ABA-GA control, and is functioning in controlling the germination of endospermic Brassicaceae seeds.
• We show that ethylene promotes endosperm cap weakening of Lepidium sativum and endosperm rupture of the close Brassicaceae relatives Lepidium sativum and Arabidopsis thaliana and that it counteracts the inhibitory action of abscisic acid (ABA) on these two processes. Cross-species microarrays of the Lepidium micropylar endosperm cap and the radicle shows that the ethylene-ABA antagonism involves both tissues and has the micropylar endosperm cap as a major target. Ethylene counteracts the ABA-induced inhibition without affecting seed ABA levels. The Arabidopsis loss-of-function mutants ACC oxidase2 (aco2; ethylene biosynthesis) and constitutive triple response1 (ctr1; ethylene signaling) are impaired in the 1-aminocyclopropane-1-carboxylic acid (ACC)-mediated reversion of the ABA-induced inhibition of seed germination. Ethylene production by the ACC oxidase orthologs Lepidium ACO2 and Arabidopsis ACO2 appears to be a key regulatory step. Endosperm cap weakening and rupture are promoted by ethylene and inhibited by ABA to regulate germination in a process conserved across the Brassicaceae.

C.    CONCLUSION

            Germination is a fascinating process. Seeing a tiny seedling emerge from a dry, wrinkled seed and watching its growth and transformation, is observing the mystery of life unfolding. The first sign of germination is the absorption of water until lots of water. This activates an enzyme, respiration increases and plant cells are duplicated. Soon the embryo becomes too large, the seed coat bursts open and the growing plant emerges. The tip of the root is the first thing to emerge and it's first for good reason. It will anchor the seed in place, and allow the embryo to absorb water and nutrients from the surrounding soil.

            Some seeds need special treatment or conditions of light, temperature, moisture, etc. to germinate. Seed dormancy is very complex, but it protects that living plant material until conditions are right for it to emerge and grow.

            For the growth and development of seeds ,different kinds of food like carbohydrates, fats and proteins are required in stored form. Besides the growth promoting substances like auxins, heteroauxins are also formed at the time of germination which controls the growth and development of seedlings during germination.

REFERENCES

Anonim^a. 2012. Germination. en.wikipedia.org/wiki/Germination.

Anonim^b. 2012. Germination. http://urbanext.illinois.edu/gpe/case3/c3facts3.html

Buzzle. 2012. Seed Germination Process. http://www.seedbiology.de/germination.asp

Kennel, Holly S. 2012. Seed Germination. gardening.wsu.edu/library/.../vege004.htm

Koning, Ross. 2012. Seed Germination. www.seedbiology.de/germination.asp

Leubner, Gerhard. 2011. Seed Germination. http://plantphys.info/seedg/seed.html

Tutorvista. 2010. Process of Seed Germination. http://www.tutorvista.com/content/biology/biology-iv/plant-growth-movements/seed-germination-process.php