The fruits like papaya, phalsa, kaghzilime, jamun and guava are usually propagated from seed. Similarly, the rootstalk for citrus, ber, loquat, mango, pear, peach, plum, etc. are usually raised from seed.
Propagation from seed has been responsible for the production of chance seedlings of highly superior merit and has contributed towards evolution of new varieties. The man made hybrids are first raised from seed. It is the usual means of commercial reproduction in papaya, phalsa and karonda where other methods of vegetative propagation are not satisfactory.
i. Selection of Fruits and Seed Extraction:
Always select mature, uniform and healthy fruits of a known rootstock species\variety. Discard under and oversized and diseased fruit. Extract seeds by removing pulp in the case of mango, guava, loquat, peach, kainth (pear), ber, plum, papaya, etc. In case of citrus, cut the fruit into two halves, squeeze out the seeds along with the juice in non-metallic container. Make the seeds free from pulp-juice, wash with water and dry under shade for 2-3 days. Seeds of mango, citrus, guava, ber, etc. should be sown within a week.
ii. Storage of Seeds:
The seeds generally lose viability when allowed to dry for a long period. If the seeds are to be stored for several months, they should be surface dried quickly after thorough washing and then mixed with equal parts of powdered charcoal, packed in a tight wooden box or tin container and kept in a dry cool place at a temperature of 3 to 12°C.
iii. Dormancy in Seeds:
The dormancy in seeds might be imposed by the hardness of the seed coat, impermeability to water and gases, physiological immaturity of embryo, deficiency of some endogenous growth promoters or excess of endogenous growth inhibitors. Different methods like scarification, acid scarification, chemical treatment, soaking seeds in water, stratification are used for breaking dormancy in seed to improve seed germination.
iv. Testing Seed Viability:
The conventional methods for testing the germination capacity of seeds in petri dishes or in sand are time consuming and laborious. Most of the non dormant fruit seeds require 7-21 days and the dormant seeds require few months to carry out germination tests. Citrus seeds lose their viability within a short period and are stored under special storage conditions. The tetrazolium test makes it possible to determine the seed viability within 4-24 hours. Its special advantage over other biochemical tests is that it can be performed on individual seed organs and tissues vital to growth. The embryo of seed capable of normal germination and growth contain reducing enzymes which convert the almost colourless triphenyle tetrazolium bromide solution into a clearly visible red stain upon the cut surfaces of the embryo tissue.
Sowing of Seeds:
The seeds of fruit like papaya, phalsa, citrus, mango, jamun and loquat do not require any special treatment before sowing. In ber, the germination of seed is quite difficult on account of stony endocarp which contains the seed. Before sowing, seeds should be dipped into 17-18% salt solution for 24 hours. The seeds can also be sown by cracking the hard shell. The seeds of guava should be soaked in water for about two weeks before sowing. Placing the seeds in the boiling water for about five minutes also reduces the time required for germination. The seeds of pear, peach and plum need stratification before they are sown in the field or on raised nursery beds.
The seeds of citrus, guava, phalsa and kainth (pear rootstock) are sown in carefully prepared 15-20 cm raised beds to facilitate drainage. The soil of the seed beds should be well pulverised and mixed with well rotten farmyard manure. Seeds beds usually 1.2 to 1.5 meter wide and 2.0 to 2.5 meter long alternated with 60 cm wide irrigation furrows are prepared with spade and khurpa. The seeds are sown in lines at a distance of 10-15 cm and 2-3 cm deep. Seed is covered with a thick layer of sand or leaf mould mixture to prevent crust formation. In the beginning the beds are watered with watering Can and after germination light and frequent irrigation is applied through furrows. The seed beds should be covered with Sarkanda to preserve moisture.
Germination starts in about 2-3 weeks and thereafter Sarkanda is removed. When the seedlings are 10-15 cm in height, they are transplanted in well prepared nursery plots at 30 x 30 cm spacing in plants and rows. The seedlings of evergreen fruit plants are transplanted in February-March and August-September and deciduous fruit plants in December-January. The seeds of mango, ber, loquat and jamun are sown in flat beds.
Raising the Seedlings in Polythene Bags:
The seedlings of sexually propagated plants raised in polythene bags can stand transplanting better than those raised in seed beds. Perforated polythene bags of 22.5 x 15 cm size with 100 gauge thickness can be used for sowing the seed. These bags are filled with a mixture of farmyard manure, soil and sand in equal proportion. The rooting media of equal proportion should be treated with Brassicol 75 WP. Two or three seeds are sown in one bag and after germination only one seedling is retained. These seedlings in the polythene bags should be treated with Captan (0.2 per cent) after their emergence.
Identification of Nucellar Seedlings in Citrus Nursery:
Seeds of most citrus species are endowed with the ability to produce more than one embryo. One of the embryos arises from the union of male and female gametes and is called gametic or sexual embryo whereas the others are produced by simple mitotic division of cells of the nucellus without the male gamete taking part in their formation are known as nucellar embryo. Since these nucellar embryos arise from the tissue of the mother parent alone, they are genetically alike and exhibit the same characteristics as their mother parent.
The phenomenon of nucellar embryo is of common occurrence in the genus citrus. Most citrus fruits of commercial importance like sweet orange, loose skinned oranges, grapefruit, lime, lemons and some of the species used as rootstocks like rough lemon, kharna khatta, Rangpur lime etc. exhibit a high percentage of nucellar embryony. In some citrus varieties like Troyer citrange, tangelos and Unshiu mandarin, even the gametic embryo aborts after a certain stage of development and the seed is left with only the vegetative or a sexual embryos that are capable of producing cent per cent nucellar seedlings.
Selection of Nucellar Seedlings:
In a mixed population of gametic and nucellar seedlings of a particular variety, it will be observed that whereas a large majority of seedlings fall within one vigour group and are more or less of the same size, others will be either too small or too tall than the commonly prevailing type. The small proportion of seedlings that differ from prevailing type in regard to vigour may on careful examination, also show differences in other characters like internodal length, branching habit, shape and size of foliage, serration on leaf margin and development of wings etc.
These variants are apparently seedlings produced from the sexual embryos. Roguing out of these variants at different stages in the nursery will ensure the availability of nucellar seedlings. In actual practice, roguing out of gametic seedlings is mostly done on the basis of vigour differences.
First roguing to eliminate gemetic seedlings should be done when they are about 10 to 12 cm tall and ready for transplanting in the nursery. All small sized, slow growing and weak seedlings should be discarded at this stage. Likewise some of the seedlings which manifest extra vigour and are very tall should also be discarded. Only seedlings of uniform vigour should be transplanted in the nursery rows and allowed to grow there till they are ready for budding.
At the time of budding again, all such seedlings which have been lagging behind in growth and exhibit morphological differences should be discarded. Some of the gametic seedlings which might have escaped notice at the time of first roguing will show vigour differences in the nursery rows when they are allowed to grow for a longer period and should be eliminated. In this way a very large population of gametic seedlings shall have been discarded and the remaining seedlings will largely consist of nucellars which should be budded with appropriate scion variety during budding season.
A third and final roguing should be done at the time of selecting bud-lings for transplanting in the field. Experience has shown that even the growth of scion buds on gametic seedlings is not quite as good as on nucellar seedlings. In case, therefore, some gametic seedlings which escaped notice at earlier stages of roguing were budded, the scion buds on them will make relatively less growth. Bud-lings which have made poor growth, therefore, should not be transplanted in the field.
If after conducting roguing at three different stages, bud-lings showing uniform growth are selected for transplanting, then the grower can safely assume that he has eliminated practically all the gametic seedlings. The resultant plant material will be of a very good quality and will not give rise to heterozygosity in orchard trees. Nucellar embryo thus provides the citrus nurserymen valuable tool, producing superior quality of uniform genetic make-up.