Process of Seed Germination: 5 Steps (With Diagram)
The process of seed germination includes the following five changes or steps.
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.
Process of Seed Germination: 5 Steps (With Diagram) The process of seed germination includes the following five changes or steps. Such five changes or steps occurring during seed
How to Grow Plants From Seeds Step by Step
Maybe you want grow plants from seeds to save money. It’s definitely cheaper than buying transplants. It will also be easier to find seeds of varieties not typically available for sale as transplants. Whatever the reason, starting plants from seeds is probably not a hard as you think. And growing plants all the way from seed to maturity is one of gardening’s most rewarding endeavors.
Here are the basics in 10 steps.
1. Choose a container.
Seed-starting containers should be clean, measure at least 2-3 inches deep and have drainage holes. They can be plastic pots, cell packs, peat pots, plastic flats, yogurt cups, even eggshells. As long as they are clean (soak in a 9 parts water to one part household bleach for 10 minutes), the options are endless. You can also buy seed-starting kits, but don’t invest a lot of money until you’re sure you’ll be starting seeds every year. If you start seeds in very small containers or plastic flats, you’ll need to transplant seedlings into slightly larger pots once they have their first set of true leaves. Keep in mind that flats and pots take up room, so make sure you have enough sunny space for all the seedlings you start.
2. Start with quality soil.
Sow seeds in sterile, seed-starting mix or potting soil available in nurseries and garden centers. Don’t use garden soil, it’s too heavy, contains weeds seeds, and possibly, disease organisms. Wet the soil with warm water before filling seed-starting containers.
3. Plant at the proper depth.
You’ll find the proper planting depth on the seed packet. The general rule of thumb is to cover seeds with soil equal to three times their thickness – but be sure to read the seed packet planting instructions carefully. Some seeds, including certain lettuces and snapdragons, need light to germinate and should rest on the soil surface but still be in good contact with moist soil. Gentle tamping after sowing will help. After planting your seeds, use a spray bottle to wet the soil again.
4. Water wisely.
Always use room-temperature water. Let chlorinated water sit overnight so chlorine can dissipate or use distilled water. Avoid using softened water. It’s important to keep soil consistently moist, but avoid overwatering, which promotes diseases, that can kill seedlings. Try not to splash water on leaves. An easy way to avoid this – as well as overwatering – is to dip base of your containers in water and allow the soil to absorb moisture from the bottom until moist. Some seed-starting kits supply a wicking mat that conducts water from a reservoir to dry soil. This may be the most goof-proof method of watering seedlings but you still have to be careful that the soil doesn’t stay too wet. Whatever you do, don’t miss a watering and let seeds or seedlings dry out. It’s a death sentence.
5. Maintain consistent moisture.
Prior to germination, cover your container to help trap moisture inside. Seed-starting kits typically come with a plastic cover. You can also use a plastic bag, but it should be supported so it doesn’t lay flat on the soil. Remove covers as soon as seeds sprout. Once seedlings are growing, reduce watering so soil partially drys, but don’t let them wilt.
6. Keep soil warm.
Seeds need warm soil to germinate. They germinate slower, or not at all, in soils that are too cool. Most seeds will germinate at around 78°F. Waterproof heating mats, designed specifically for germinating seeds, keep soil at a constant temperature. You can buy them in most nurseries and garden centers. Or, you can place seed trays on top of a refrigerator or other warm appliance until seeds sprout. After germination, air temperature should be slightly below 70°F. Seedlings can withstand air temperature as low as 50°F as long as soil temperature remains 65-70°F.
Start feeding your seedlings after they develop their second set of true leaves, applying a half-strength liquid fertilizer weekly. Apply it gently so seedlings are not dislodged from the soil. After four weeks, apply full-strength liquid fertilizer every other week until transplanting.
8. Give seedlings enough light.
Not enough light leads to leggy, tall seedlings that will struggle once transplanted outdoors. In mild winter areas, you can grow stocky seedlings in a bright south-facing window. Farther north, even a south-facing window may not provide enough light, especially in the middle of winter. Ideally, seedlings need 14-16 hours of direct light per day for healthiest growth. If seedlings begin bending toward the window, that’s a sure sign they are not getting enough light. Simply turning the pots won’t be enough – you may need to supply artificial lighting. Nurseries and mail order seed catalogs can provide lighting kits. Follow instructions carefully.
9. Circulate the air.
Circulating air helps prevents disease and encourages the development of strong stems. Run a gentle fan near seedlings to create air movement. Keep the fan a distance away from the seedlings to avoid blasting them directly.
10. Harden off seedlings before transplanting outdoors.
Before moving seedlings outdoors, they need to be acclimatized to their new, harsher surroundings. This procedure is called “hardening off.” Click here to learn more.
Want to start your plants using seeds? Following these 10 simple steps in the gardening process will teach you how to grow plants from seeds step by step.