What is propagation
Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants.
Seeds and spores can be used for reproduction (through e.g. sowing). Seeds are typically produced from sexual reproduction within a species, because genetic recombination has occurred a plant grown from seeds may have different characteristics from its parents. Some species produce seeds that require special conditions to germinate, such as cold treatment. The seeds of many Australian plants and plants from southern Africa and the American west require smoke or fire to germinate. Some plant species, including many trees do not produce seeds until they reach maturity, which may take many years. Seeds can be difficult to acquire and some plants do not produce seed at all.
Plants have a number of mechanisms for asexual or vegetative reproduction. Some of these have been taken advantage of by horticulturists and gardeners to multiply or clone plants rapidly. People also use methods that plants do not use, such as tissue culture and grafting. Plants are produced using material from a single parent and as such there is no exchange of genetic material, therefore vegetative propagation methods almost always produce plants that are identical to the parent. Vegetative reproduction uses plants parts such as roots, stems and leaves. In some plants seeds can be produced without fertilisation and the seeds contain only the genetic material of the parent plant. Therefore, propagation via asexual seeds or apomixis is asexual reproduction but not vegetative propagation.
Different Types of Propagation
Layering is a means of plant propagation in which a portion of an aerial stem grows roots while still attached to the parent plant and then detaches as an independent plant. Layering has evolved as a common means of vegetative propagation of numerous species in natural environments. Layering is also utilised by horticulturists to propagate desirable plants. Natural layering typically occurs when a branch touches the ground, whereupon it produces adventitious roots. At a later stage the connection with the parent plant is severed and a new plant is produced as a result.
The layering process typically involves wounding the target region to expose the inner stem and optionally applying rooting compounds. In ground layering, the stem is bent down and the target region buried in the soil. This is done in plant nurseries in imitation of natural layering by many plants such as brambles which bow over and touch the tip on the ground, at which point it grows roots and, when separated, can continue as a separate plant. In either case, the rooting process may take from several weeks to a year. Layering is more complicated than taking cuttings, but has the advantage that the propagated portion can continue to receive water and nutrients from the parent plant while it is forming roots. This is important for plants that form roots slowly, or for propagating large pieces.
Ground layering is the typical propagation technique for the popular Malling-Merton series of clonal apple rootstocks in which the original plants are set in the ground with the stem nearly horizontal, which forces side buds to grow upward. After these are started the original stem is buried up to the tip. At the end of the growing season, the side branches will have rooted, and can be separated while the plant is dormant. Some of these will be used for grafting rootstocks, and some can be reused in the nursery for the next growing season's crop.
In air layering, the target region is wounded and then surrounded in a moisture-retaining wrapper such as sphagnum moss, which is further surrounded in a moisture barrier such as polyethylene film. Rooting hormone is often applied to encourage the wounded region to grow roots. When sufficient roots have grown from the wound, the stem from the parent plant is removed and planted. It takes about three months for the new plant to become mature.
Division, in horticulture and gardening, is a method of asexual plant propagation, where the plant (usually an herbaceous perennial) is broken up into two or more parts. Both the root and crown of each part is kept intact. The technique is of ancient origin, and has long been used to propagate bulbs such as garlic and saffron. Division is mainly practiced by gardeners and very small nurseries, as most commercial plant propagation is now done through plant tissue culture.
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together. This vascular joining is called inosculation. The technique is most commonly used in asexual propagation of commercially grown plants for the horticultural and agricultural trades.
In most cases, one plant is selected for its roots and this is called the stock or rootstock. The other plant is selected for its stems, leaves, flowers, or fruits and is called the scion or scion. The scion contains the desired genes to be duplicated in future production by the stock and scion plant. In stem grafting, a common grafting method, a shoot of a selected, desired plant cultivar is grafted onto the stock of another type. In another common form called bud grafting, a dormant side bud is grafted onto the stem of another stock plant, and when it has inosculated successfully, it is encouraged to grow by pruning off the stem of the stock plant just above the newly grafted bud.
For successful grafting to take place, the vascular cambium tissues of the stock and scion plants must be placed in contact with each other. Both tissues must be kept alive until the graft has 'taken', usually a period of a few weeks. Successful grafting only requires that a vascular connection take place between the grafted tissues. Joints formed by grafting are not as strong as naturally formed joints, so a physical weak point often still occurs at the graft, because only the newly formed tissues inosculate with each other. The existing structural tissue of the stock plant does not fuse.
Micropropagation is the practice of rapidly multiplying stock plant material to produce a large number of progeny plants, using modern plant tissue culture methods. Micropropagation is used to multiply novel plants, such as those that have been genetically modified or bred through conventional plant breeding methods. It is also used to provide a sufficient number of plantlets for planting from a stock plant which does not produce seeds, or does not respond well to vegetative reproduction. Frederick Campion Steward was the person who discovered and pioneered micropropagation and plant tissue culture in the late 1950s and early 1960s.
Micropropagation begins with the selection of plant material to be propagated. Clean stock materials that are free of viruses and fungi are important in the production of the healthiest plants. Once the plant material is chosen for culture, the collection of explant(s) begins and is dependent on the type of tissue to be used; including stem tips, anthers, petals, pollen and others plant tissues. The explant material is then surface sterilised, usually in multiple courses of bleach and alcohol washes, and finally rinsed in sterilised water. This small portion of plant tissue, sometimes only a single cell, is placed on a growth medium, typically containing sucrose as an energy source and one or more plant growth regulators (plant hormones). Usually the medium is thickened with agar to create a gel which supports the explant during growth. Some plants are easily grown on simple media, but others require more complicated media for successful growth; The plant tissue grows and differentiates into new tissues depending on the medium. For example, media containing cytokinins are used to create branched shoots from plant buds.
Multiplication is the taking of tissue samples produced during the first stage and increasing their number. Following the successful introduction and growth of plant tissue, the establishment stage is followed by multiplication. Through repeated cycles of this process, a single explant sample may be increased from one to hundreds or thousands of plants. Depending on the type of tissue grown, multiplication can involve different methods and media. If the plant material grown is callus tissue, it can be placed in a blender and cut into smaller pieces and recultured on the same type of culture medium to grow more callus tissue. If the tissue is grown as small plants called plantlets, hormones are often added that cause the plantlets to produce many small offshoots that can be removed and re-cultured.
Banana plantlets transferred to soil from plant media. This process is done for acclimatisation of plantlets to the soil as they were previously grown in plant media. After growing for some days the plantlets are transferred to the field. This stage involves treating the plantlets and shoots produced to encourage root growth and hardening. It is performed in vitro, or in a sterile ‘test tube’ environment.
Hardening refers to the preparation of the plants for a natural growth environment. Until this stage, the plantlets have been grown in "ideal" conditions, designed to encourage rapid growth. Due to the controlled nature of their maturation, the plantlets often do not have fully functional dermal coverings. This causes them to be highly susceptible to disease and inefficient in their use of water and energy. In vitro conditions are high in humidity, and plants grown under these conditions often do not form a working cuticle and stomata that keep the plant from drying out. When taken out of culture, the plantlets need time to adjust to more natural environmental conditions. Hardening typically involves slowly weaning the plantlets from a high-humidity, low light, warm environment to what would be considered a normal growth environment for the species in question.
Transfer from culture
In the final stage of plant micropropagation, the plantlets are removed from the plant media and transferred to soil or (more commonly) potting compost for continued growth by conventional methods. This stage is often combined with the pre-transplant stage.
Micropropagation has a number of advantages over traditional plant propagation techniques:
· The main advantage of micropropagation is the production of many plants that are clones of each other.
· Micropropagation can be used to produce disease-free plants.
· Micropropagation produces rooted plantlets ready for growth, saving time for the grower when seeds or cuttings are slow to establish or grow.
· It can have an extraordinarily high fecundity rate, producing thousands of propagules while conventional techniques might only produce a fraction of this a number.
· It is the only viable method of regenerating genetically modified cells or cells after protoplast fusion.
· It is useful in multiplying plants which produce seeds in uneconomical amounts, or when plants are sterile and do not produce viable seeds or when seed cannot be stored.
· Micropropagation often produces more robust plants, leading to accelerated growth compared to similar plants produced by conventional methods - like seeds or cuttings.
· Some plants with very small seeds, including most orchids, are most reliably grown from seed in sterile culture.
· A greater number of plants can be produced per square meter and the propagules can be stored longer and in a smaller area.
Micropropagation is not always the perfect means of multiplying plants. Conditions that limit its use include:
· It is very expensive, and can have a labour cost of more than 70%
· A monoculture is produced after micropropagation, leading to a lack of overall disease resilience, as all progeny plants may be vulnerable to the same infections.
· An infected plant sample can produce infected progeny. This is uncommon if the stock plants are carefully screened and vetted to prevent culturing plants infected with virus or fungus.
· Not all plants can be successfully tissue cultured, often because the proper medium for growth is not known or the plants produce secondary metabolic chemicals that stunt or kill the explant.
· Sometimes plants or cultivars do not come true to type after being tissue cultured. This is often dependent on the type of explant material utilised during the initiation phase or the result of the age of the cell or propagule line.
· Some plants are very difficult to disinfest of fungal organisms.
Stolon and runners
In botany, stolons are stems which grow at the soil surface or just below ground that form adventitious roots at the nodes, and new plants from the buds. Stolons are often called runners. Thus, not all horizontal stems are called stolons. Plants with stolons are called stoloniferous. Rhizomes, in contrast, are root-like stems that may either grow horizontally at the soil surface or in other orientations underground. A stolon is a plant propagation strategy and the complex of individuals formed by a mother plant and all its clones produced from stolons form a single genetic individual, a genet.
Stolons arise from the base of the plant. In strawberries the base is above the soil surface; in many bulb-forming species and plants with rhizomes, the stolons remain underground and form shoots that rise to the surface at the ends or from the nodes. The nodes of the stolons produce roots, often all around the node and hormones produced by the roots cause the stolon to initiate shoots with normal leaves. Typically after the formation of the new plant the stolon dies away in a year or two, while rhizomes persist normally for many years or for the life of the plant, adding more length each year to the ends with active growth. The horizontal growth of stolons results from the interplay of different hormones produced at the growing point and hormones from the main plant, with some studies showing that stolon and rhizome growth effected by the amount of shady light the plant receives with increased production and branching from plants exposed to mixed shade and sun, while plants in all day sun or all shade producing fewer stolons.
A number of plants having soil level or above ground rhizomes including Iris species and many orchid species. The term rhizome to a horizontal, usually subterranean, stem that produces roots from its lower surface and green leaves from its apex, developed directly from the plumule of the embryo. He recognised stolons as axillary, subterranean branches that do not bear green leaves but only membranaceous, scale-like ones.
A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have an underground storage organ are called geophytes in the Raunkiær plant life-form classification system. Storage organs often, but not always, act as perennating organs which enable plants to survive adverse conditions (such as cold, excessive heat, lack of light or drought).
Storage organs may act as 'perennating organs' these are used by plants to survive adverse periods in the plant's life-cycle (e.g. caused by cold, excessive heat, lack of light or drought). During these periods, parts of the plant die and then when conditions become favourable again, re-growth occurs from buds in the perennating organs. For example geophytes growing in woodland under deciduous trees die back to underground storage organs during summer when tree leaf cover restricts light and water is less available.
However, perennating organs need not be storage organs. After losing their leaves, deciduous trees grow them again from 'resting buds', which are the perennating organs of phanerophytes in the Raunkiær classification, but which do not specifically act as storage organs. Equally, storage organs need not be perennating organs. Many succulents have leaves adapted for water storage, which they retain in adverse conditions.
Underground storage organs
In common parlance, underground storage organs may be generically called roots, tubers, or bulbs, but to the botanist there is more specific technical nomenclature:
· Storage taproot — e.g. carrot
· Modified stems: Corm — e.g. Crocus
· Stem tuber — e.g. Zantedeschia (arum lily), potato
· Rhizome— e.g. Iris pseudacorus (yellow flag iris)
· Pseudobulb— e.g. Pleione (windowsill orchid)
· Caudex— e.g. Adenium (desert-rose)
· Others: Storage hypocotyl (the stem of a seedling) — sometimes called a tuber, as in Cyclamen
· Bulb (modified leaf bases) — e.g. Lilium, Narcissus, onion
Some of the above, particularly pseudobulbs and caudices, may occur wholly or partially above ground. Intermediates and combinations of the above are also found, making classification difficult. As an example of an intermediate, the tuber of Cyclamen arises from the stem of the seedling, which forms the junction of the roots and stem of the mature plant. In some species roots come from the bottom of the tuber, suggesting that it is a stem tuber; in others roots come largely from the top of the tuber, suggesting that it is a root tuber. As an example of a combination, juno irises have both bulbs and storage roots. Underground storage organs used for food may be generically called root vegetables, although this phrase should not be taken to imply that the class only includes true roots.
How to take a typical cutting
Typically, striking is a simple process in which a small amount of the parent plant is removed. This removed piece, called the cutting, is then encouraged to grow as an independent plant. Since most plant cuttings will have no root system of their own, they are likely to die from dehydration if the proper conditions are not met. They require a moist medium, which, however, cannot be too wet lest the cutting rot. A number of media are used in this process, including but not limited to soil, perlite, vermiculite, coir, rock wool, expanded clay pellets, and even water given the right conditions. The environment should be humid (this generally means placing the cuttings under a plastic sheet or in another confined space where the air can be kept moist) and partial shade should be provided, also to prevent the cutting from drying out. After cuttings are placed in the medium, they are watered thoroughly with a fine mist, such as from a nozzle sprayer or a spray mister bottle. After the initial watering, the medium is allowed to almost dry out before misting again, with the aim to keep the soil moist but not wet and waterlogged. A fine mist is used to avoid disturbing plants.
In addition, the cutting needs to be taken correctly. It must be taken at the right time; in temperate countries, stem cuttings of young wood need to be taken in spring, of hardened wood they need to be taken in winter. It must have the right size and amount of foliage; length of stem cuttings of soft wood for example need to be between 5–15 cm and of hard wood between 20–25 cm. Also, two thirds of the foliage of soft wood stem cuttings should be removed. For hard wood stem cuttings, complete foliage removal is necessary.
If the plant is unlikely to grow then a rooting hormone to encourage the plant to grow and mature may be administered. Though not essential, several compounds may be used to promote the formation of roots through the signalling activity of plant hormone auxins, and is helpful with especially hard plant species. Among the commonly used chemicals is indole-3-butyric acid (I.B.A.) used as a powder, liquid solution or gel. This compound is applied either to the cut tip of the cutting or as a foliar spray. Rooting hormone can be manufactured naturally - one method is to soak the yellow-tipped shoots of a weeping willow tree in water, or to prepare a tea from the bark of a willow tree. When using the shoots or bark, they should be soaked for 24 hours prior to using. Honey, though it does not contain any plant hormones, can also make an effective rooting substance. Some plants form roots much more easily. Most succulent cuttings can be left on a table and small roots will form, and some other plants can form roots from having their cuttings placed in a cup of water.
Types of cuttings
Many vegetative parts of a plant can be used. The most common methods are:-
· Stem cuttings, in which a piece of stem is part buried in the soil, including at least one leaf node. The cutting is able to produce new roots, usually at the node.
· Root cuttings, in which a section of root is buried just below the soil surface, and produces new shoots
· Scion cuttings, which are dormant ligneous woody twigs.
· Eye cuttings, which are pieces of foliated or defoliated stalks with one or more eyes.
· Leaf cuttings, in which a leaf is placed on moist soil. These have to develop both new stems and new roots. Some leaves will produce one plant at the base of the leaf. In some species, multiple new plants can be produced at many places on one leaf, and these can be induced by cutting the leaf veins.
Although some species, such as willow, blackberry and pelargoniums can be grown simply by placing a cutting into moist ground, the majority of species require more attention. Most species require humid, warm, partially shaded conditions to strike, thus requiring the approach above to be followed. Particularly difficult species may need cool air above and warm soil. In addition, with many more difficult cuttings, one should use the type of cutting that has the most chance of success with that particular plant species
The right soil for cuttings
Depending on the type of soil being used, several additives may need adding to create good soil for cuttings. These additions may include:
· Chalk; to increase the pH-value of the soil; a pH of 6-6,5 is to be maintained
· Organic substance/humus; to increase nutrient load; keep to a bare minimum though
· Sand or gravel; to increase the soil's water permeability
Providing the right humidity
Although several options can be used here, usually semi-white plastic is used to cover the cuttings. The soil below and from the cuttings themselves is kept moist, and should be aerated once in a while to prevent formation of moulds.
Stem cuttings or truncheons
In temperate countries, stem cuttings may be taken of soft (green or semi-ripe) wood and hard wood which has specific differences in practice. Stem cuttings of soft wood is taken in spring, while from hard wood, they are taken in winter. Also, of soft wood the upper branches are taken (with a length of 5–15 cm) and with hard wood, the lower branches are taken instead (with a length of 20–25 cm). Finally, soft wood cuttings are planted above ground and hard wood cuttings are totally submerged with soil. With hard wood cuttings, several cuttings are also bound together (to a bushel).
Twin-scaling is practiced by professional growers and skilled amateurs to increase bulbs that would naturally propagate very slowly, or to speed up the production of desirable cultivars. Using twin-scaling it is possible to multiply one bulb into 16 to 32 (or more) viable bulbs in a couple of years, whereas natural propagation might only lead to a doubling every two years or so. It is one of a number of propagation techniques based on the fact that an accidentally damaged bulb will often regenerate by forming small bulblets or bulbils on the damaged surface. Commercial growers have obtained as many as 100 twin-scales from a single bulb.
Scaling and twin-scaling
The dormant bulb which is to be twin-scaled has its surface sterilised by removing its dry tunic and carefully trimming off its roots and any dead tissue, while leaving a layer of sound basal plate intact, then dipping the clean bulb in dilute bleach. The bulb is then sliced cleanly from top to bottom several times, creating 8 or 16 segments, depending on the size of the bulb. At this stage the segments are called chips.
True twin-scaling involves further subdivision of the chips to create pairs of scales, joined together by a small part of the basal plate. The twin-scales are then treated with fungicide before being mixed with moist, sterile Vermiculite, sealed in plastic bags and left in a fairly warm, dark location until new bulblets form. Some species may require alternate periods of warm and cool storage to initiate bulblet growth. The bulbs are planted into pots or trays of compost and allowed to grow on for a year or more, until they are large enough to survive in individual pots or the open ground. They usually take several years to reach flowering size, although some bulblets of Galanthus have been known to flower in their first year.