Plant propagation is carried out by sexual or asexual mode of life cycles. The asexual or vegetative reproduction produces genetically identical plants. Multiplication of several horticulturally important crops follows vegetative methods.
Production of genetically identical plant cultivars by asexual method is called clonal propagation and genetically identical individuals derived from a single individual by asexual mode referred as clone.
Contrast to conventional methods of vegetative propagation, tissue culture offers novel techniques for the mass multiplication of elite species maintaining genetic uniformity. In vitro propagation also opens up possibilities of exploiting every plant parts particularly shoot tip, leaf, petiole and nodal explant for the production of millions of plants. Thanks to the potentials of totipotency in plant system.
The concept of micro-propagation or clonal propagation stems from the regeneration potentials of orchid propagation. When orchid explants are cultured on the medium, it produces top shaped structure known as protocorm like bodies (PLBs). For scientific curiosity when protocorms (PLBs) are sliced into number of pieces, all are regenerated into set of protocorms.
In the entire process of protocorm multiplication, few millions of orchid plants were produced in culture. This technique was later extended to other horticulturally important plants. Micro-propagation technique has been employed in the propagation of tree species, woody species, immature embryos, hypocotyl, young leaves, cotyledons, ovary, ovule and other juvenile tissues as good source of tissue for the propagation of several woody plants.
In vitro flowering in micro-propagation techniques can shorten juvenile period. Micro-propagation is a new technique and has to override several barriers, but still improvising the techniques of well-defined micro-propagation technique can overcome problem owing to deforestation and dwindling population of useful plants. Presently, most of the commercial laboratories followed micro-propagation techniques for propagation of plants.
Micro-Propagation of Ornamentals:
One of the broadest applications of plant tissue culture is multiplication of ornamental plants for horticultural industry. Several commercial companies in USA, Europe and Australia are involved in micro-propagation of ornamental plants extensively intended for sale as pot plants, cut flowers or as bulbs and corms or as plants intended for replanting (Table 7.1).
A great number of herbaceous ornamental species are propagated by shoot tip culture. In Europe, extensive work has been done for delivering methods for propagating anthuriams due to its high market potential. Shoot tip culture has been extensively used for commercial propagation. Several genera belongs to the family gessneriaceae such as Santpaulia, Navtilocalyx can be readily propagated from adventitious shoots formed on lamina petioles supplemented with BAP and NAA.
The same method is extended for other genera. European researcher standardized methods for in vitro propagation of bulbs and corms. The techniques are based on the proliferation of axillary shoots, induction of adventitious shoots on organs and callus mediated adventitious formation. In Narcissus at least 1000 bulbs can be produced by in vitro methods in one year, whereas it takes 6 years to get the same number through conventional method.
Bulbils form directly on bulb scale in response to exogenous supply of auxins. Tulips have proved to be more difficult to propagate through tissue culture than other bulbils species. However, two methods like culturing sections of immature flower and 3-5 mm axillary buds dissected from sprouted buds overcomes the problem of multiplication. In vitro propagation of ornamentals such as Gerbera and Geranium has been proved to be propagated successfully using modified Murashige and Skoog media.
Among ornamental plants, orchids have been found to occupy special place in view of its long lasting beautiful flowers. In vitro flasking and mericloning techniques are commonly employed for orchid micro-propagation.
Mericlone is referred to as orchid plants originating from single mother plants that were produced in culture from somatic protocorms. Mericloning is distinct from shoot tip culture. Several orchid genera such as Cymbidium, Dendrobium, phaelonopsis, Aranda have been successfully propagated using micro-progation methods.
Micro-Propagation of Tree Species:
Generally tree species are difficult to propagate in tissue culture due to various reasons. Some of the major barriers among tree specific propagations are difficult to root, polyphenol exudation and early maturation status of the plant parts. The age of the tree plant plays a significant role in root regeneration capacity. Therefore, in vitro methods demands juvenile tissue for initiation of culture.
Tree species are required for bioenergy plantation. The clonal propagation a forest tree species has progressed significantly over last few years. Numerous species are now propagated by vegetative means including conifers which are difficult to root.
Several major woody ornamentals and tree species include Abies, Sequoin, Pinus, Thuja, Pophar, wild cherry tree, etc. In most cases, the methods employed are adventitious budding or axillary budding.
Adventitious budding method has been implicated in multiplication of seedling of coniferous species. Certain juvenile tissues such as cotyledon, hypcotyl are excised and embryos have been the main source of juvenile explants.
The most cases where regeneration has been accomplished. Axillary budding is particularly used for in vitro multiplication of Aspen, Poplar, Thuta, coast red wood, etc. In Pinus and other young trees, use of needle as initial explant has been developed.
Ecological and economic importance of woody legumes proved necessary for tissue culture propagation. The successful propagation of woody perennial species through tissue culture has been recently developed. The woody species in leguminous family that have been successful cultured in vitro from apical or axillary meristems.
Commercial tissue culture lab has many micro-propagated woody legumes such as glymocladus. The difficulties encountered during the micro-propagation of woody legumes are common to woody plant micro-propagation in general and are directly linked to woody nature of the explant, maturation status and excessive phenolic link and recalcitration nature of shoot or root formation.
Regeneration of several woody plants has been obtained through somatic embryogenesis such as Santalum album and Cornls avellana. Somatic embyrogenesis mediated regeneration have been obtained in frost resistant species of Eucalyptus for coniferous species.
Somatic embryogenesis has rapid development process held in the case of Pinus, Norway sprice. Somatic embryogenesis culture of woody legumes have been initiated mainly from seed or seedling tissue, for example, Acacia and Albivia were all initiated from seedling hypocotyl and Robinia from immature embiyos.