Dr. Rashad M. Balal, *Dr. M. M. Khan, Dr. M. Afzal and *M. Adnan Shahid
University College of Agriculture, University of Sargodha, Sargodha.
*Institute of Horticultural Sciences, University of Agriculture-38040 Faisalabad.
Citrus, a member of family Rutaceae refers to all edible and rootstock species and a few closely related genera. Citrus fruit are grown throughout the world and are known for their fine flavor and quality. The importance of citrus in agriculture and the world’s economy is demonstrated by its wide distribution and large-scale production. Citrus fruit ranks second after grapes in terms of area and production in the world. Citrus is grown in tropical and subtropical climate where suitable soil and sufficient moisture is available for sustained tree growth and development. World is producing 103821 thousand metric tons of citrus per annum. Brazil (South America) is the leading (19215 thousands metric tons) citrus producing and exporting country followed by U. S. A. (F. A. O, 2004).
Citrus is a prized fruit of Pakistan and hold number one position among all fruits both in area and production. In Pakistan citrus crop has also gained importance. Pakistan also stands among one of the major citrus producing countries of the world, having an area of 200 thousand hectares with yearly production of 1995 thousand tones. About 957 of total (95%) citrus production is concentrated in Punjab province. Citrus fruits are the outstanding source of vitamin C along with appreciable amounts of vitamins A and B. They also contain calcium, phosphorus, iron, traces of copper, magnesium and manganese. It also contains 10-12 % carbohydrates in the form of dextrose and sucrose. The fruit of citrus is a special type of berry known as hesperidium. It is a good source of hesperidine, a vitamin like substance.
Among all, kinnow mandarin (Citrus reticulata Blanco) is highly popular and commercially cultivated for its processing quality, fresh consumption and aromatic flavor. Despite its excessive cultivation, kinnow plantation still has some problems such as slow growth and long juvenility, insects, pests, diseases, alternate bearing, pre and post harvest losses, large number of seeds per fruit, short season of supply and short storage life etc. As its demand is increasing day by day inside and out side the country their export can be increased by producing disease free and good quality kinnow.
In vitro propagation has therefore been a great potential tool to overcome problems related with the field culture of such species. The main objective of callus induction is genetic homozygosity and diversity. The main objective of multiple shoot induction and regeneration is to produce the new cultivars with genetic improvements of fruit crop. Tissues obtained from young plant parts have relatively more regenerative capacity than old tissues. The physical factors like light, surface, temperature, pH and chemical composition of the medium are important for organogenesis in vitro. The root and bud formation in tissue culture is dependent on a specific equilibrium between the auxins and the cytokinins, gibberellins and cytokinins ratio, which control the shoot and leaf development. The tissue culture techniques are the most suitable and beneficial which can help to screen out salt tolerant disease free citrus rootstock plants with in short period of time compared to ordinary propagation methods. For in vitro propagation of citrus rootstocks various tissue culture techniques are used but among these techniques micropropagation technique is most efficient and successful. A basic requirement for the application of tissue culture in crop improvement is either to make use of or devise an in vitro plant regeneration system. In vitro regeneration of citrus rootstock has been a subject of research for many years because of the economic importance of the genus and the potential for further improvement via genetic manipulation. Citrus in vitro regeneration has been described in several species using stem and epicotyl segments, reproductive organs, explants type and orientation, different composition of the culture medium and conditions of incubation.
Advances in biotechnology have generated new opportunities for the conservation and utilization of genetic resources. Molecular markers are increasingly used for screening of germplasm to study genetic diversity. It provides plant breeders with option to develop new and more productive crops that are resistant to virulent pests and disease and is well adapted to changing environment. A plant reproduces naturally through the development of zygotic embryos. Formation of the embryo begins with the division of the fertilized egg or zygote within the embryo sac of the ovule. Through an orderly progression of cell division, the embryo eventually differentiates, matures, and develops into a new plantlet. Alternatively, the plant can be derived from a single somatic cell or a group of somatic cells. This regeneration process which differs from the natural pathway is called somatic embryogenesis. Somatic embryogenesis can efficiently be used as a tool in citrus improvement programs.