The following points highlight the six main methods of vegetative propagation of fruit plants. The methods are: 1. Cutting 2. Layering 3. Budding 4. Grafting 5. Specialized Organs 6. Micro Propagation.
Method # 1. Plant Propagation by Cutting:
Many types of plants, both woody and herbaceous, are frequently propagated by cuttings. A cutting is a vegetative plant part which is severed from the parent plant in order to regenerate itself, thereby forming a whole new plant. Take cuttings with a sharp blade to reduce injury to the parent plant.
Dip the cutting tool in rubbing alcohol or a mixture of one part bleach to nine parts water to prevent transmitting diseases from infected plant parts to healthy ones. Remove flowers and flower buds to allow the cutting to use its energy and stored carbohydrates for root and shoot formation rather than fruit and seed production.
With large-leaved cuttings (i.e., Rhododendron) and limited space in the propagation container, trimming up to half the leaf length can improve efficiency, as well as light and air circulation for all the cuttings. To hasten rooting, increase the number of roots, or to obtain uniform rooting (except on soft, fleshy stems), use a rooting hormone, preferably one containing a fungicide.
Prevent possible contamination of the entire supply of rooting hormone by putting some hormone in a separate container for dipping cuttings. Discard this hormone after all the cuttings are treated. Place stem and leaf cuttings in bright, indirect light. Root cuttings can be kept in the dark until new shoots appear.
It is the method of propagating fruit plants in which the part of a plant (generally stem) having at least few buds, when detached from parent plant and placed under favorable conditions develops into a complete plant resembling in all characteristics to the parent from which it was taken. This method is commonly used in plants, which root easily and readily, thus, multiplication of plants is very quick and cheap. The fruit plants like phalsa, baramasi lemon and grapes are commercially propagated by cuttings.
The hardwood cuttings are the common method of propagation, which are prepared from fully mature tissues. Round cuttings are preferred over angular immature cuttings. The shoots of about one year old or more can easily be used for preparing hardwood cuttings.
In case of deciduous fruit plants such as grape, pomegranate, phalsa and fig the cuttings are made after pruning. While in evergreen fruit plants like baramasi lemon, the cuttings can be prepared during the spring (February-March) and rainy season (August-September).
Generally the cuttings of 15-20 cm length and having 3-5 buds are made. The lower cut is given in a slanting manner just below the bud to increase the absorption of nutrients. The upper cut is given at a right angle to reduce the size of the wound and as far as possible away from then upper bud to avoid its drying. In rainy season vice versa cut is made for better rooting and survivability.
After the cuttings are prepared they should be allowed to dry. These cuttings are usually tied in small bundles (20-25 cuttings) and buried in moist soil/sand for a certain period for healing of wounds, which is known as callusing.
Relatively few fruit crops are propagated by cuttings in India, except as clonal rootstocks; as such plants grow on their own roots without utilizing the advantages offered by rootstocks. Stem cuttings are most commonly used although roots have also been found to work well with some plants.
Method # 2. Plant Propagation by Layering:
Layering is a method of vegetative propagation, in which roots are induced on the shoots while they are still attached to the mother plants. This is an alternate method of propagation in fruit plants which do not root easily when detached from the mother plants.
Layering is a method of vegetative propagation by which a good stem is induced to produce roots while it is still attached to the parent plant. In this manner a new plant usually can be developed in a relatively short time and with less trouble than other methods of propagation. It can be used successfully on many fruit trees and woody ornamental shrubs grown in India.
Method # 3. Plant Propagation by Budding:
It is necessary to put cambial tissue of stock and scion in contact with one another in order for budding to be successful. Budding methods fall into 2 categories in terms of the amount of cambial contact which is available initially those in which a broad cambial area is exposed on at least the stock and those in which thin lines of cambium are exposed. Shield and patch bud methods are in the first group and can be done only when the bark is slipping.
Chip budding is representative of the second group and can be used during the dormant season. Budding is a method in which only one bud is inserted in the rootstock. This method is very easy and fast. This method saves bud-wood as compared to grafting.
As soon as the bark starts slipping both on the stock and scion, this is considered to be the optimum time for budding. This shows that the cambium, which is the tissue responsible for union, is active. This method is generally employed during spring and rainy season.
Budding is a grafting technique in which a single bud from the desired scion is used rather than an entire scion containing many buds. Most budding is done just before or during the growing season. However some species may be budded during the winter while they are dormant. Budding requires the same precautions as grafting. Be sure that the scion and rootstock are compatible, that the scion has mature buds and that the cambia of the scion and rootstock match.
Be especially careful to prevent drying or contamination of grafting materials. With practice, the speed with which the process can be performed and the percentage of successful grafts those that “take” – should equal or surpass those of other grafting techniques used on the same species. Generally, deciduous fruit and shade trees are well suited to budding.
Method # 4. Plant Propagation by Grafting:
It is necessary to bring regenerative tissue of stock and scion in close contact in all methods of grafting so that healing will occur. This tissue, called the cambium, is located between the bark and wood. The cambium of both the stock and scion produce callus cells which intermingle and interlock, eventually forming a bridge between the 2 members. Grafting is another method of vegetative propagation, where two plant parts are joined together in such a manner that they unite and continue their growth as one plant.
In this method, the scion twig has more than two buds on it. Grafting is commonly done in pear, peach, plum, almond, mango etc. In temperate fruits like peach, plum and almond grafting is done when the plants are dormant while, in mango it is done when the trees are in active growth. The different methods of grafting are tongue grafting, cleft grafting, approach grafting, side grafting and veneer grafting.
During the selection of the scion wood we have to consider some important aspects:
i. The scion wood must carry healthy buds that will grow into leafy shoot.
ii. It should come from a tree which is free from any pests or diseases.
iii. The plant should have the required characteristics. This includes, that we take scions from plants, which are already bearing fruits.
iv. Buds, which already begin to grow, are useless for grafting. The grafting process will fail.
v. The one year old wood is the best for grafting. These shoots grew during the previous year. Water sprouts from up in the tree make straight scions.
There are some rules, which must be taken consideration for any grafting method to be successful:
i. Two incompatible plants cannot be grafted.
ii. The cambium layers of the rootstock and the scion must touch.
iii. The scion must be the right way up when grafting it.
iv. Grafting can be done in any time of the year, but the best time for deciduous plants is, when the plant drops its leaves and is dormant.
v. Cool, cloudy day without wind prevents the graft from drying out, therefore this type of weather is the best for grafting.
vi. The care activities are very important until the rootstock and scion are properly joined.
This grafting processes use a sharp knife, tape and grafting wax. Grafting wax seals the join wound and avoids water loss and disease infection. Therefore, its use is strongly recommended. If grafting wax is not available, we can use petroleum jelly (Vaseline) or we can produce homemade grafting wax.
There are two ways to make grafting wax:
5 Kg – resin
1 Kg – bee wax
½ Kg – siliceous earth (Fuller’s earth)
or
2 ½ Kg – paraffin wax
1 ½ Kg – siliceous earth
½ Kg – Zinc oxide
In some cases, a section of trunk is introduced between the rootstock and scion. It is called inter-stem or inter stock. This is done in order to have a desired effect or characteristic on the tree. These effects can be disease resistance, winter resistance or effects on the size of the tree. It is also used to join two incompatible cultivars together.
Method # 5. Plant Propagation through Specialized Organs:
1. Runners:
A runner is a specialized stem that develops from the axil of a leaf at the crown of a plant. It grows horizontally along the ground and forms a new plant at one of the nodes e.g. strawberry. The runner production is favoured by long day and high temperature. The daughter plants are separated and used as new planting material.
2. Suckers:
A shoot arising on an old stem or underground part of the stem is known as suckers. In other words, a sucker is a shoot, which arises on a plant below the ground. These shoots, when separated from the mother plant and transplanted produce adventitious roots.
The capacity of a plant to form suckers varies from plant to plant, variety to variety and is even climate dependent. The sucker formation is common in fruit plants like pear and banana. In banana, sword suckers are commonly used for propagation of plants.
Offshoots or suckers arise near the base of the stem or from roots of some plants. These can be removed by hand in some cases, by twisting or with the aid of a large, sharp knife or shovel if necessary. They can be used as rootstocks to be grafted or as plants to be grown on their own roots. Suckers which are succulent should be allowed to “dry off” for several days before planting to reduce problems associated with rotting.
3. Slips:
Slips are shoots just arising below the crown but above the ground. Pineapple is commercially propagated through this method of propagation.
4. Division:
Propagation from the following plant parts can be considered a modification of layering, as the new plants form before they are detached from their parent plants.
5. Stolons:
A stolon is a horizontal, often fleshy stem that can root, and then produce new shoots where it touches the medium. A runner is a slender stem that originates in a leaf axil and grows along the ground or downward from a hanging basket, producing a new plant at its tip.
Plants that produce stolons or runners are propagated by severing the new plants from their parent stems. Plantlets at the tips of runners may be rooted while still attached to the parent, or detached and placed in a rooting medium. Examples – strawberry, spider plant.
6. Offsets:
Plants with a rosetted stem often reproduce by forming new shoots at their base or in leaf axils. Sever the new shoots from the parent plant after they have developed their own root system. Un-rooted offsets of some species may be removed and placed in a rooting medium. Some of these must be cut off, while others may be simply lifted off the parent stem. Examples – date palm, Haworthia, bromeliads, many cacti and succulents.
7. Separation:
Separation is a term applied to a form of propagation by which plants that produce bulbs or corms multiply.
8. Bulbs:
New bulbs form besides the originally planted bulb. Separate these bulb clumps every 3 to 5 years for largest blooms and to increase bulb population. Dig up the clump after the leaves have withered. Gently pull the bulbs apart and replant them immediately so their roots can begin to develop. Small, new bulbs may not flower for 2 or 3 years, but large ones should bloom the first year. Examples – tulip, narcissus, hyacinth, amaryllis, lilies
9. Corms:
A large new corm forms on top of the old corm, and tiny cormels form around the large corm. After the leaves wither, dig up the corms and allow them to dry in indirect light for 2 or 3 weeks. Remove the cormels, and then gently separate the new corm from the old corm. Dust all new corms with a fungicide and store in a cool place until planting time. Examples – crocus, gladiolus, freesia.
10. Crowns:
Plants with more than one rooted crown may be divided and the crowns planted separately. If the stems are not joined, gently pull the plants apart. If the crowns are united by horizontal stems, cut the stems and roots with a sharp knife to minimize injury. Divisions of some outdoor plants should be dusted with a fungicide before they are replanted. Examples – snake plant, iris, prayer plant, day lilies, sansevieria, Boston fern, cast iron plant, peace lily.
11. Root Sprouts:
Red Raspberries and many (most) shrubs can produce root sprouts. The roots of most plants produce cytokinins. As these accumulate in the roots, the cytokinins induce shoot formation. Near the base of the shrub, a new shoot will begin to grow. The new shoot is called a root sprout or “sucker.”
A large number of these growing up together can make quite a thicket. In wetlands of Connecticut, the spicebush (Lindera benzoin) and sweet pepper bush (Clethra alnifolia) propagate naturally this way.
i. Rhizome:
Botanically a rhizome is an elongated, horizontal fleshy underground stem having nodes and internodes bearing buds in the axils of reduced scale leaves e.g. banana etc. The rhizomes are cut in to pieces, each with one or two viable bud and planted in favourable atmospheric conditions. Adventitious root and shoots develop from the nodes.
ii. Tuber:
A tuber is a swollen underground modified stem, containing food reserves and functions as an organ for vegetative propagation. A tuber consists of all the parts of stem i.e. nodes, internodes, lateral and terminal buds. Each eye consists of one or more buds subtended by a leaf scar. Tubers can be planted as a whole or cut into pieces, each having at least one eye and used as a planting material.
iii. Tuberous Root:
Tuberous roots are fleshy modified roots acting as a storage organ, bearing growing point on the crown or stem end without nodes and internodes. Care should be taken while separating the tuberous roots from the mother plant so that each carries at least a small portion of the stem as the new growth originates from the point of attachment with the stem.
12. Epiphyllous Bud:
In plants such as Bryophyllum (also known as “mother of thousands”) or the piggyback plant, the leaf supports development of a tiny shoot buds. These buds may begin to form roots at their base. As these shoots break off from the original leaf, they fall onto the ground and take root. Cytokinins accumulating at the leaf margins stimulate cell division in the notches to produce these adventitious shoots.
Advantage of Plant Propagation through Specialized Organs:
1. It is very easy to perform and economical.
2. Rapid multiplication is possible within a short time.
3. Does not require much space.
4. The complicated stionic relationship can be avoided.
5. Cuttings can be transported to any place because the stem or root pieces remain fresh 5 to 7 days if packed properly.
Disadvantages:
1. All the plant species do not root very easily.
2. The benefits of the stock plant cannot be exploited.
Method # 6. Micro Propagation:
Apart from conventional methods of propagation such as sexual and asexual methods, there are special techniques of propagation, which have played an important role in enhancing efficiency of plant production. One of such methods is in vitro, also commonly referred as micro propagation.
The vegetative propagation of plants has been practised for centuries and many improvements in conventional methods have been made over the years. Recently, the tissue culture technique i.e. micro propagation has expanded their scope and potential on commercial scale. Micro propagation is suitable for the rapid and large-scale clonal multiplication of elite germplasm.
The technique has been referred as micro propagation because the size of the tissue in culture is very minute as compared to conventional vegetative cutting or any other plant part. The meristem explants used for micro propagation is about 0.1-0.5 mm size having only one or two leaf primordia. Morel and Martin (1952) for the first time demonstrated that virus free plants can be obtained by culturing shoot meristems.
Later on with the discovery of the hormonal control of organogenesis by Shoog and Miller (1957) and finding of most commonly used T/C media by Murashigue and Skoog (1962), the scope of micro propagation was further extended to vast range of plant species, including fruit and plantation crops.
With the advancement in science and technology, micro propagation technique has also been standardized for many plants, and it, is now widely used for multiplication of many horticultural plants.
It is estimated that there are now more than 200 Tissue Culture laboratories in the world each producing more than 1 million plants per annum. U.S.A. is the largest producer followed by Asia, the Netherlands, other European countries, Australia and New Zealand, France, Italy and Israel. The reports indicate that more than 500 million plants belonging to different plant species are being produced through micro propagation annually in different parts of the world.
Pathway of Regeneration:
There are number of pathways for the regeneration of whole plantlet from excised plant parts.
The three main pathways are:
i. Regeneration from Existing Meristems:
This is also known as axillary shoot proliferation. The existing meristems such as shoot tip or nodal bud is cultured on the medium, containing cytokinins. The shoot proliferation depends on the cytokinin used. The commonly used cytokinins are Benzyl aminopurine (BAP), kinetin, 2-isopentanyl adenine (2- ip). The regenerants are considered to be genetically stable, as compared to regeneration from adventitious meristems.
ii. Regeneration from Adventitious Meristems:
Shoot multiplication either directly or by callus formation can be obtained by inducing adventitious shoot production on mature plant organs such as leaves, stems and roots. For initiation of adventitious meristems, a proper balance of auxin and cytokinin is needed in culture medium. In general, shoots are formed when a high ratio of cytokinin to auxin is present and reverse is true for root formation.
The plants regenerated via this method are not always genetically stable, due to accumulation of aneuploid and polyploid cells with continuous sub culturing of callus causing mixoploids. The repeated subculture of callus also reduces its morphogenetic potential/regenerative capacity.
iii. Regeneration by Somatic Embryogenesis:
The induction of somatic embryos is the best technique for rapid and true to type multiplication of plants. The somatic embryos originate from somatic or vegetative cells, and are bipolar structures, which possess both shoot and root meristem. The induction of embryo requires a high level of auxin in culture medium, followed by low auxin and cytokinin medium.
Somatic embryos may arise in culture directly on explants or via callus formation or liquid suspension cultures. The somatic embryos can be encapsulated, thus producing artificial or synthetic seeds, which is an attractive alternative for propagation of plants.
Among two methods viz., hydrated or desiccated, for artificial seed production, the production of hydrated seeds is more popular. In this method, the individual somatic embryo is encapsulated in a water based gel (hydrogel; such as calcium alginate).
Embryos developed through tissue culture technique are mixed with sodium alginate and dropped with pipette into a calcium salt (calcium chloride) solution to form calcium alginate capsules. The capsules are washed in water and then placed on culture medium for germination. Artificial seeds have been produced in Banana, Citrus, Mango, Apple, Olive and kiwi fruit.
Hardening of Tissue Culture Plants:
The tissue culture plants need acclimatization or hardening before they are transferred in the field. The acclimatization is necessary because there is vast variation in the environment surrounded by in vitro plants and the field environment. In culture vessels the in vitro plants are exposed to high humidity, heterotrophic mode of nutrition, high ethylene concentration and constant temperature throughout the year.
These conditions lead to the development of plants having low epicuticular wax, low stomatal density and stomatal malfunction, which make these plants more vulnerable to mortality in field conditions. To prevent this mortality, it is must to harden or acclimatize tissue culture plants.
Mist Propagation of Plants:
The plants produced through tissue culture or the soft wood leafy cuttings do not survive, when transferred to open field conditions, primarily due to excessive evaporative loss of water from above ground portion of plants/ cuttings. The excessive loss of water from leaves of cuttings and from the tender tissue culture plants can be prevented by use of misting. The mist propagation involves spraying of water in the form of mist, which is either done continuously, or intermittently to maintain high humidity.
The aim of misting is to maintain continuously a film of water on the leaves, thus reducing transpiration and keeping the cuttings turgid until rooting takes place. In this way, leafy cuttings can be fully exposed to light and air because humidity remains high and prevents damage even from bright sunshine. Mist also prevents disease infection in cuttings by way of washing off fungus spores before they attack the tissues.
While the leaves in this process must be kept continuously moist, it is important that only minimum water should be used. This is because excessive water leaches out nutrients from the compost which may cause starvation. Moreover, a directly injurious effect on the cuttings may occur from over watering. Hence, it is necessary to utilize nozzles capable of producing a very fine mist.
The mist unit is controlled by a timer, operating a magnetic solenoid valve and is set in a way to turn on the mist for 3-5 seconds to wet the leaves and turn off for some time and when the leaves are dry, the mist is again turned on. In general, the mist may have any of the 5 control mechanisms timer, electronic leaf, thermostat and timer, screen balance and photoelectric cell.
The two types of timers are used in mist unit, one turns “on” in the morning and “off” at the night and the second operate during day hours to produce an intermittent mist. Usually 6 seconds “on” and 90 seconds “off”, in electronic leaf, a plastic with two terminals is placed under the mist along with cuttings. The alternate drying and wetting of the terminals breaks the current which in turn controls the solenoid valve. A thermostat controls the temperature of the mist.
In screen balance control mechanism, a stainless steel screen is attached to a lever with mercury switch. When mist is on, water is collected on the screen and when weight of water is more, it trips the mercury switch. The photoelectric controls are based on the relationship between light intensity and the transpiration rate. The mist unit can be set up in a glasshouse or in a polyethylene tunnel. Usually, it is set up on the propagation beds with 1.2 m width.
The layout of the misters is very important. The water supply and quality is equally important to have uniform misting. The water should have good pressure and it must be free from salts. The optimum pH of water to be used in mist unit is 5.5 to 6.5. Hard water or alkaline water may be avoided as it may block the nozzles of the mist chamber and it may accumulate on the young leaves of the cuttings and thus inhibit the growth and development of roots. While installing mist propagation unit, all the jets should be at equal height.
Further, it is essential that a well-drained rooting medium is used and there should be a proper provision for adequate removal of excess water. Similarly, development of blue and green algal growth is very common in mist propagation structures, which is considered very harmful to the propagating material and thus every care should be taken to keep mist propagating unit free from any type of algae.