Breeding of Fruit and Plantation Crops 2 + 1

Problems in citrus breeding

• There are three major problems which hinder the success of citrus breeding.
Time

• Citrus being perennial in nature takes more time for bearing. However this period can be reduced to a maximum of half by top working the seedling on an old tree.

Polyembryony

• It is peculiar feature found in citrus in which seed consist of more than one embryo. In addition to the zygotic embryo, one or more sometimes as many as fifteen additional embryos are developed from the nucellar tissue called nucellar embryos and found in the embryo sac.
  
• Most often, the zygotic seedling is crowded out by the vigorous nucellar seedlings. Forgetting large number of hybrids, citrus breeder should select a seed parent known to be either monoembryonic citrus species or polyembryonic except (C.medica, C.latifolia and C.grandis) which are monoembryonic restricts the choice of breeder and complicate the procedure required to attain the desirable objectives.

Sterility

• Sterility is inability of gametic or sexual reproduction. Prevalence of high generative sterility is obviously a serious hindrance in the use of a particular parent for hybridization. Complete pollen sterility is problematic, where proportion of nucellar embryos are very high. High level of sterility often leads to production of seedless fruits which is serious hindrance to develop varieties.

Self incompatibility

• Self incompatibility and cross incompatibility is a common phenomenon which occurs widely in citrus. Most of the varieties of grape fruit (C.grandis) are found to be self incompatible besides, some varieties of lemon, sweet orange and mandarins exhibit self incompatibility of gametophytic type governed by oppositional alleles.
• Hybrid cultivars including Clementine, Orlando, Minneola, Sukega, Nova, Robinson are cross incompatible. Nova and Robinson is also suspected to be cross incompatible. Sweet orange varieties like Washington Navel and Satsuma mandarin are having sterile pollen, thereby they produce parthenocarpic fruits if cross pollination is not done through viable pollens.

Long juvenility

• It is a major barrier in the progress of citrus breeding in India. General treatments to shorten the period or induce early flowering have not been generally effective. It was reported that neither chemical treatments nor incorporation by genetic transfers has been effective in combating long juvenility in citrus.

Breeding objectives

• Producing early maturing citrus fruits with high yield and fruit quality.Developing rootstocks having disease and nematodes resistance, wider adaptability, etc.In rootstock breeding, the main emphasis has been given on the development of root stock resistant to tristeza virus, Phytophthora, nematodes, etc. Most of the breeding programmes make use of Poncirus, which is a carrier of resistance to tristeza, Phytophthora and nematodes besides cold hardiness. Salt tolerant rootstocks have also been found possible in some progenies involving Cleopatra and Sunki mandarin and Rangpur lime.

Floral biology

• Flowering in citrus takes place during February –April. In North India, sweet orange and mandarins bloom only once in March. However, it is reported that sweet oranges bloom twice in a year under Bihar conditions i.e.February –March and June –July. Inflorescense in citrus species is of cymose type. Generally anthesis takes place in the morning between 9.00 am to 12.00 noon. Flowers on shaded side of the tree have been observed to open later than those exposed to sunshine.