Breeding Methods
    Introduction
    • Introduction of germplasm either from other countries or from one agro climatic region to the other within the country has been one of the most potent improvement methods. The mandarin variety ‘Santra’ is known to have been grown in India for many centuries. It was introduced into the Central Provinces (now Maharashtra) by Ranghojee Bhonsal II from Aurangabad in eighteenth century. Tangerines, St.Michael Blood Orange and Large White Orange were imported and cultivated at Goojranwallah in Punjab during 1880.

    • The present century has seen the introduction of a number of sweet orange varieties including Washington Navel, Valencia, Jaffa, Blood Red Malta and tangerines. The first two were introduced from America and the others from the respective countries of their origin. Grapefruits were introduced from California and Florida, lemons from China and Malta from USA and Italy.
    • ‘Mosambi’ seems to have been introduced in Nagpur during the beginning of the 20th century.The introduction of ‘Kinnow’ mandarin (King x Willow leaf) in 1947 showed great promise in North India. It was introduced in South India in 1958 and Punjab in 1959 and has performed extremely well in Punjab.

    Clonal selection
    Exploitation of natural variability existing in a variety has resulted in the isolation of some promising clones in Citrus.
    • ‘PKM 1 lime is a clonal selection from seedling progenies of kadayam Type of
    • Tirunelveli district of Tamil Nadu.
    • ‘Yuvaraj Blood Red’ is a seedless and early maturing clonal selection from ‘Blood Red’
    • orange.
    • ‘Pramalini’ and ‘Vikaram’, the two kagzi lime varieties were developed through clonal
    • selection at Marathwada University.
    • ‘Chakradha’ is a thornless and seedless selection from Kagzi lime.

    Hybridization
    • Hybridization is confronted with real problems in citrus improvement, both on scion as well as rootstocks because the long juvenile phase delays the assessment of the hybrids.Most of the cultivated varieties are highly polyembryonic, hence the crosses made using these as females result in very few weak hybrids, which are difficult to identify from nucellar seedlings.

    • Electrophoretic techniques separating the isozymes of parents and hybrids may be of great value in scion breeding programme, as no morphological markers are available at present.The heterozygous nature of the crop further leads to wide segregation. The problems are little less complicated of rootstock breeding where the commonly used disease resistant male parent ‘Poncirus trifoliata’ has trifoliate leaves which is dominant over the monofoliate character (in other citrus varieties and all the hybrids), by which distinction of unifoliate nucellar seedlings could be easily made.

    Hybridization Technique
    • The mature flower buds on the female parent are emasculated early in the morning on the day of opening and are bagged. The flowers to be used as male parent are bagged the previous evening. The next morning as the day warms up, the anthers dehisce releasing the pollen grains when these flowers can be plucked to pollinate the receptive stigmas of emasculated flowers.

    • The pollinated flowers are bagged, opened after about a week and allowed to mature into ripe fruits. In some cases, especially when the trifoliate orange is used as male parent, difficulties are encountered as its flowering is over before other citrus varieties flower. Therefore, pollen has to be stored at low humidity and temperature.
    • Seeds from mature fruits are extracted and sown immediately in sterilized sand and soil mixture. When seedlings are about 15 cm high, hybrid seedlings are identified. Particularly those showing some morphological characters of male parent are marked while others are rejected.

    • Electrophoresis methods can also be employed for identification of zygotic seedlings. Identification of hybrid seedlings having P.trifoliata as male parent is easily done by looking for trifoliate character. The hybrid seedlings are grown to mature trees in the field and the seedlings raised from the fruits are evaluated for resistance to various diseases, insect pests, nematodes and for suitability as scion or rootstock.

    Evaluation for rootstock purpose
    • Rootstock hybrids should have desirable attributes like high percentage of nucellar embryony, resistance to different diseases and nematodes. The selected hybrids are then tested with different scion varieties and compared with the commercial rootstock. Various plant and fruit characters, yield and yield contributing characters are recorded.

    Evaluation of scion hybrids
    • In the first round of evaluation, the zygotic seedlings are raised on suitable rootstock and observations on different vegetative and fruit characters are recorded. Meanwhile, the resistance to different diseases is also confirmed. Selected hybrids are tested on different rootstocks at different locations and compared with the commercial varieties.
    • Intergeneric and intrageneric hybrids

    Intergeneric
    • Though intergeneric hybrids are rare in fruit plants, much success has been obtained in Citrus.

    1. Hybrids of Poncirus:
    • Citrange –A group having the parentage of trifoliate orange (Poncirus trifoliata) and sweet orange (C.sinensis), the hybrids showed intermediate characters of the parents. The leaves are mainly trifoliate but unifoliate evergreen leaves are also observed in some plants. The fruits are juicy and flavoured. Some of the cultivars are Troyer, Carrizo, Morton, Etonia, Rusk, Coleman, etc.
    • Citrangequat - This is a tri-generic hybrid of three different genera
    • (Poncirus, Citrus and Fortunella).
    • Citrangor - This hybrid was developed by back crossing Citrange with C.sinensis.
    • Cicitrange - Another back cross hybrid between Citrange and Poncirus trifoliata x C.paradisi.
    • Citrandarin - A hybrid between Poncirus trifoliata and C .reticulata.
    • Citremon - A hybrid between Poncirus trifoliate and C.limon
    • Citradia - Very similar to citrange. A hybrid between Poncirus trifoliata and C.aurantium
    • Citumquat - This is the hybrid between Poncirus trifoliate and Fortunella. japonica or F.margarita, a very difficult hybrid to breed.

    II.Hybrids of Fortunella:
    • Procimequat - (F.japonica x C.aurantifolia cv. Mexican) x F. hindisii
    • Limequat - C.aurantifolia x F.japonica
    • Orangequat - C.reticulata cv.Satsuma x F.japonica x F.margarita cv. Meiwa.
    • Intrageneric Tangor - Mandarin x sweet orange (C.sinensis x C.reticulata) hybrids
    • .g.cv.Temple, Clementine, and Monreal are some important cultivars, mostly monoembryonic.
    • Tangelo - Mandarin x grapefruit, (C.reticulata x C.paradisi) e.g.cv.Orlando,
    • Sampson, Minneola, Seminole, etc.
    • Lemon - Lemon x lime, (C.limon xC.aurantifolia) e.g.cv.Parrine Lemonnage
    • (C.limon x C.sinensis) Lemandarin (C.Limon x C.reticulata)
    • In India, very little work has been done on citrus improvement through hybridization. At the PKV, Akola, hybridization work has been undertaken to evolve hybrids of kagzi lime. As a result, Hybrid 2, Hyrbid 4 and N52 were found resistant to canker.
    • Breeding for improvement of citrus rootstock was initiated in 1972 at the Central Horticultural Experiment Station, Chethali, and IIHR, Bangalore. Trifoliate orange was used as a donor source for Phytophthora and citrus nematode resistance.
    • Hybridization programme resulted in the production of 1183 hybrids from 16 different cross combinations. Of these, CRH.3, CRH.5 and CRH.41 resistant to citrus nematode have been evolved. A hybrid between Rangpur lime and trifoliate orange (Australia) having high resistance to nematodes and Phytophthora, and highly polyembryonic in nature is being evaluated for its suitability as rootstock for mandarin and sweet orange.

     Mutation Breeding
    • Somatic mutations are common in citrus and through selection of the natural mutants, quite a few number of desirable clones have been obtained. The frequent occurrence of chimera may lead to clonal impurity and thus bud selection work in propagation becomes important for ensuring clonal purity. Selections of natural mutants have been successfully employed for seedlessness (lyo tangor), season of ripening (Satsuma, Navel), improvement of colour (Ray Ruby grapefruit) etc. in Citrus.
    • Besides natural mutations, many induced mutants have been developed in Citrus. For instance, ‘Star Ruby’ and ‘Rio Red’ varieties of grapefruit were developed in Texas, USA through x ray and thermal neutron treatments of seeds of cv. ‘Ruby red’ whose red flesh colour faded at harvest. In Japan, a few closely related clones of Satsuma mandarin with varied fruit colour and fruit ripening times were obtained through mutation.

    • In USA also mutations had produced Satsuma seedling lines differing in productivity, fruit shape and the ripening time. The grapefruit clones like Thompson and Foster Pink arose as limb sports on white grapefruit. Gamma irradiation of seeds and bud woods performed in Orlando, Florida, resulted in Seedless fruits on certain trees of seeded cultivars like Pineapple orange as well as Duncan and Foster grapefruit. In Israel, Shamouti trees of compact habit and early fruiting types and seedlessness have been developed in Eureka lemon through irradiation of bud wood with gamma rays.

    Polyploidy breeding
    • Most of the species and varieties of Citrus are diploids but occurrence of polyploidy has been reported in many cultivars. The Hongkong wild kumquats, Fortunella hindsii may have been the first reported tetraploid. Polyploidy breeding seems to offer prospects to obtain large sized fruit with dwarf plant types. Production of triploids by crossing tetraploid with diploids may be useful in obtaining seedless varieties. The seedless lime (C. latifolia) a triploid. Triploids have favorable characteristics and yield well but they are sterile. The development of triploid through breeding is very limited.

    • Production of 3x is normally achieved by crossing of 4x with 2x which is often not feasible for want of sexual parents. The reciprocal cross ((2x) x (4x)) produces many tetraploid individuals. Polyploidy manipulation by crossing of tetraploids with diploids yielded some valuable triploid varieties like ‘Oroblanco’ and ‘Melogold’. A large diversity of autotetraploid parents with desirable characters expressed in the progeny will be of high value to any citrus cultivar breeding program.

    • Spontaneous autotetraploids occur among many polyembroyonic citrus varieties. Tetraploid trees of monoembryonic cultivars can be obtained by colchicine treatment. Triploids also, occasionally occur spontaneously as sexual seedlings. In most cases the egg provides the double chromosome number.

    Biotechnology in improvement of citrus
    • Transformation of fruit species by biotechnological tools is a potential approach to develop disease resistant cultivars. Woody plants are known to be difficult to work in-vitro than herbaceous plants but citrus is exceptional. Though nucellar embryony in citrus is of great value for producing vigorous, uniform and virus free plants, it appears to be an obstacle in hybridization. In polyembryonic cultivars, the vigorous growth of nucellar embryos inhibits the growth of the zygotic embryo and causes its degeneration prior to seed maturation.

    • Such abortive embryos can be rescued by tissue culture. Tissue culture has effectively been used in obtaining hybrid Poncirus plantlets from polyembryonic citrus cultivars. Poncirus trifoliata not only carries a genetic marker, but also possess resistance to tristeza, Phytophthora, nematode and cold stress. Inter – generic hybridization with the aid of cell/tissue culture offers possibility of incorporation of multiple desirable characters found in different genera for improvement of citrus root stocks and scion cultivars.
    • Cell and tissue culture and specially protoplast manipulations have effectively been explored in citrus improvement by regeneration of citrus trees from protoplast, somatic hybridization (cybridization) and organelle transfer. In an attempt to develop protoplast derived plants in the last one decade, Israel and Florida have shown protoplast system in a dozen genera and interestingly citrus is the only woody plants among them. Efficient protocols have been developed to obtain protoplasts with cell diversion capability from all major citrus cultivars and some of their wild relatives.

Last modified: Thursday, 23 February 2012, 7:16 PM