BREEDING OF FRUIT CROPS - Lecture.4
The phenomenon in which more embryos are present within a single seed is called polyembryony. It may result due to (a) nucellar embryony e.g., Citrus (b) development of more than one nucleus within the embryo sac (in addition to the egg embryo during the early stages of development) leading to multiple embryos (e.g. conifers).
Occurrence of polyembryony is widespread in all citrus species but the number of embryos per seed varies from species to species. In rough lemon, it varies from 3 to 5. In mango certain cultivars are reported to be polyembryonic with the number of embryos ranging from 2 to 10 and the germination per cent from 40 to 87. Polyembryonic seedlings can be identified from its true seedlings by their uniformity and vigorous growth, while the seedling arising from fertilized embryo will be weak. The greater vigor in polyembryonic nucellar seedlings is probably due to the elimination of viruses. In mango polyembryony was determined by single dominant gene (Anon, 1996). In citrus, all the species are polyembryonic in nature except C.medica (Citron) and C.grandis (Pumelo) which are monoembryonic. Though nucellar embryony in citrus is of great value for producing vigorous, uniform and virus free plants, the phenomenon is 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.
Parthenocarpy and Seedlessness
In the recent years, the consumer preference towards seedless fruits is increasing among the consumers. The seedless nature of certain fruits is due to the phenomenon of ‘parthenocapy’ which refers to the development of fruits without fertilization or even without the stimulus that comes from pollination. Parthenocarpic fruits are usually seedless but need not be always.
If a fruit develops even without the stimulus of pollination, then the phenomenon is referred to as vegetative parthenocarpy (automatic) eg. Banana and Japanese persimmon.
If a fruit develops from the mere stimulus of the pollination (but without fertilization), the phenomenon is known as stimulative parthenocarpy. The female flowers of triploid watermelon require the pollen grains of diploid varieties to develop into a seedless fruit. Diploid pollen grain gives a stimulus to the ovary of guava when self pollinated, which result in the development of parthenocarpic fruit due to the stimulation provided by pollen hormones. E.g) Thompson Seedless variety of Grapes and papaya
In “Black Corinth” variety of grapes, pollination and fertilization take place but the embryo gets aborted subsequently resulting in seedlessness. This phenomenon of development of seedless fruits is referred to as ‘steno-spermocarpy’.
The seedlessness or parthenocarpic fruits are advantageous since there is a greater preference among the consumers for the seedless fruits of the same kind (e.g. seedless grapes, guava or oranges). Besides the problem of unfruitfulness due to pollination failure, sterility and incompatibility may not arise if a fruit develops parthenocarpically and the grower is assured of good crop (e.g. banana). One drawback with the seedless fruits is that they are usually small in size (e.g. Black Corinth variety of grapes) and irregular in shape (guava).
Induction of seedlessness in fruits
The seedlessness can be induced by the following methods
1. Use of growth regulators
Application of GA at 8000 ppm in lanolin paste on the cut end of the style of the emasculated flowers of guava resulted in the development of seedless fruits.
Similarly, seedlessness in loquat was induced by spraying GA 100 to 200 ppm on the emasculated flowers.
2. Changing the ploidy level
It was first demonstrated in Japan that by developing a triploid water melon 2n= 33 by crossing tetraploids x diploid varieties, seedlessness could be achieved. Naturally available seedless guava varieties are due to auto polyploidy (triploid) and not due to parthenocarpic fruit development.
In some plants, fruits develop parthenocarpically, still they produce viable seeds. (e.g. Mangosteen and Strawberry). This phenomenon is referred to as parthenogenesis. The seedlings of such fruits are genetically uniform. In certain cases, seeds develop partenogentically but they are non-viable (e.g. Apple) When female flowers of jack are pollinated with the pollen grains of bread fruit, seeds do form in jack but they did not germinate as they are non-viable.