Propagation by Different Specialized Plant Parts, Cutting and Layering in Fruit Crops
Authors: K.Usha and Nimisha Sharma (Division of Fruits and Horticultural Technology, IARI, New Delhi)
Most plants reproduce sexually from seeds, but some can be reproduced (or multiplied) by vegetative parts such as stems, roots, and leaves. In addition, some plants produce modified stems and/or roots, referred to here as specialized structures that accumulate food reserves and therefore have built-in regeneration potential and are valuable as vegetative propagates. For example, strawberry plants produce horizontal stems known as runners. Runners can produce roots and develop new plantlets at particular nodes on the runner. Rooted runners can be detached from the mother plant and grown as independent plants. Plants produce a variety of specialized structures useful in plant multiplication or reproduction. Plants with specialized structures are well-suited for asexual propagation. Plants produce specialized structures to survive under unfavourable conditions and resume growth when conditions become favourable for their survival. In this asexual method of propagation plants are obtained from a vegetative portion of the mother plant instead of seeds.
Advantages: In some fruit plants like banana, which do not bear seeds, this is the only method propagation. The plants are generally true-to-type, uniform in growth, yielding capacity and fruit quality. Have short juvenile phase, thus come into bearing earlier than seedling plants. The advantages of rootstocks can be obtained by budding or grafting susceptible varieties on resistant/ tolerant rootstocks. Plants have restricted growth, thus cultural practices and harvesting are easy.
Disadvantages: New variety cannot be evolved by this method. Such plants are not as vigorous and long-lived as the seedling trees. Germplasm conservation requires lot of space and is expensive as compared to storage of seeds.
Sub-aerial (surface) stem modifications
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. Runners produce roots at certain nodal regions of horizontally-growing stems and form new plants. Runners are also considered stolons. When separated from the mother plant with intact roots, runners serve as units of propagation. The runner production is favoured by long day and high temperature. The daughter plants are separated and used as new planting material.
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,bananas, pineapples, raspberries, and blackberries. In banana, sword suckers are commonly used for propagation f plants. Propagate suckers by digging, separating from the mother plant, and growing as individual plants.
Slips, offsets and Crown: Slips are shoots just arising below the crown but above the ground. Pineapple is commercially propagated through this method of propagation. Slips are separated from the mother plant and used for commercial propagation of pineapples. Lateral shoots, often with roots at the base, produced on main stems which can be separated and grown as independent plants. In fact, slips on pineapple can be considered offsets or offshoots.Crowns that develop on the top of pineapple fruit can also be used for propagation by simply detaching and planting in a moist growing medium.
Cutting: 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 favourable conditions develop 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 (Plate 1a) are preferred over angular immature cuttings (Plate 1b). 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 the upper bud to avoid its drying. 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.
Bottom Heat Technique for rooting in cuttings: Some plants are extremely difficult to root while some others are very slow if they root at all using other methods. Bottom heat can be used to induce and speed the rooting of hardwood cuttings of deciduous plants. Bottom heat is used to propagate plant cuttings to keep the top of the cutting dormant and induce root growth at the basal end. The optimum temperature for the soil during propagation is between 70 and 75 degrees Fahrenheit. Commercial growers use forced air furnaces to provide heat under the growing tables. Another commercial solution is a hot water system. For the home gardener with one or two trays of plant cuttings, bottom heat is provided by heating cables or heat mats. The heat mat is less expensive and concentrates the heat on the cuttings.The hardwood cuttings of deciduous plants should first be tied in bundles. Place them right side up in a bed of coarse sand equipped with bottom heat for a period of 14-20 days. The cuttings will develop callous and will be ready to plant out with a much higher degree of success. Make sure you buy a soil warming kit with a built in thermostat. The ideal soil temperature for rooting most cuttings is 69 degrees F.
1. Place the cuttings in a moist, sterile growing medium. The soil should be moist but not saturated. The cuttings may rot when there is too much water in the soil.
2. Cover the growing tray with the lid or use plastic film. The cover keeps the humidity level high for the cuttings and does not allow the soil to dry out. The heat from the bottom heating method causes the water to evaporate and dries the soil before the cuttings root.
3. Set the tray in a bright location with the heat mat directly under the tray. The temperature for the mat should be set between 70 and 75 degrees Fahrenheit. The difference in air temperature and soil temperature allows the cuttings to concentrate energy on forming roots instead of stimulating new leaf growth.
4. Lower the temperature of the heat mat during days when the air temperature is warm. The warm air and the bottom heat may cause the soil to dry quickly and could damage any new roots that may be present.
5. Water to keep the soil moist during the propagation period. Use caution when applying the water. Overspray from a misting bottle may damage the heat mat or electronic thermostat.
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. Most commonly used methods of layering are air layering, ground and mound layering.
Air layering: As name refers, in this method the layering is done in air. To be more precise the rooting is done on the shoot itself when it is still attached to the mother plant. In this method one year old, healthy and straight shoot is selected and ring of bark (Plate 2) measuring about 2.5 cm just below a bud is removed. Moist sphagnum moss is placed around this portion and is wrapped with a polythene strip. It is light in weight, and has a very high water holding capacity. If sphagnum moss is not available, any other material, which can retain moisture for long period of time, can be used for this purpose. The polythene covering does not allow the moisture to come out but permit gas exchange. Moreover the layers need not be watered afterwards, which saves a considerable labour. This method of layering is also known as goottee method. Air layering can be practiced during February-March and July-August in guava, litchi, sapota, loquat etc. After a few weeks the roots are developed which are visible through the polythene covering. Then a half way cut should be given to the rooted layers on the parent branch at least 15 days prior to their permanent removal from the mother plant. At the time of separation, a few leaves or small shoot is retained. It is also advisable to plant these rooted layers in nursery for close attention than to plant them directly in field. These layers can be planted in the fields during the following year in February or September-October.
Ground layering: In this method, a branch of plant, which is near the ground, is chosen and a ring of bark about 2.5 cm is diameter is removed just below the bud. This branch is then bended and buried in soil when still attached to the mother plant. The soil is regularly watered to keep it moist. Within a few weeks, the roots are formed and new plant is separated from the mother plant. Separation should be done in such a way that the roots formed also go with the detached plant. These new plants should preferably be planted in pots or nursery rows for development of better root system and shoot system before planting in the fields. This method is commonly followed for propagation of baramasi lemon.
Mound layering: In this method, plant is headed back either in February or in July. The new shoots come out during April and September, from ground level. A ring of bark is removed from these shoots and they are covered with moist soil. The rooted stools of April stooling are separated during rainy season and those of August are removed in the following spring. These stools, after separating from the parent plant are planted in the nursery fields. This method is also known as stool layering and is used for propagation of guava and apple rootstocks.
Selection of Elite Mother Trees
1. Mother trees should be from known identity/variety
2. They should have high production potential
3. They should have fruit quality traits with commercial acceptance
4. They should be free from pests and pathogens.
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