|MadSci Network: Agricultural Sciences|
There are several ways that seedless fruits can arise. They are usually an exception to the normal fertilization process. Most seedless fruits have what is termed parthenocarpy, where fruit form without fertilization, but pollination is sometimes required as stimulation. Seedless banana and seedless watermelon are both triploid so cannot go through meiosis to form sperm and eggs. Seedless watermelon development does require pollination from another watermelon with functional pollen. Plants with separate male and female individuals, such as persimmon, may form seedless fruit if the female plant is not near a male plant and cannot be pollinated. Pineapples are seedless because they are grown in fields of one variety, and they are self infertile, meaning they require cross-pollination to set seed. With cross pollination, they would produce seeded fruit. Many citrus are seedless for the same reason. The famous 'Washington Navel' orange is a mutation and does not seem to produce seeds even if cross pollinated. Many plants can be stimulated to produce seedless fruit by treating the flowers with plant hormones, such as auxins, gibberellins, or cytokinins. Home tomato growers often spray their tomato flowers with an auxin to get them to set fruit. Seedless grapes are not technically seedless, although they are seedless for practical purposes. Seedless grapes are different than most seedless fruits because the seeds initially develop but abort when they are very tiny embryos. Thus, in seedless grapes, normal fertilization does occur. You can often see the remains of the aborted seed in the mature fruit. Seedless grape breeders can remove the tiny embryos before they abort and grow them in tissue culture to produce a mature plant. This is termed embryo rescue. Seedless grapes have what is termed stenospermocarpy. Not all plants are prone to form seedless fruit. Seedlessness has been exploited in plants where it occurs because it is so desirable commercially. The underlying reason for seedless fruit appears to be that the tissues that form the fruit are the parent tissues and can act independently from the seed. Fruit tissue development does not always depends on successful fertilization or seed development. Seedless grapes are normally sprayed with gibberellin to increase the size of the seedless fruit but apparently there is enough gibberellin or other hormones to make the seedless fruit form in the first place even after the embryo aborts. A new seedless grape variety, 'Melissa', does not even require the gibberellin spray to produce large fruit. Contrary to several reports, there does seem to be an advantage to the plant to produce seedless fruit under natural conditions. If a plant failed to produce fruit some years because of poor weather conditions that prevented pollination, it could lead to starvation or migration of animals that dispresed its seeds. Therefore, producing seedless fruit could be beneficial to the plant by maintaining the plant's seed dispersers. In pineapple, the leafy top of the fruit can root and form a new plant. Thus, even a seedless pineapple may serve to propagate the plant. References How do seedless fruits arise and how are they propagated? Trio of New Seedless Grapes on the Way to Consumers Biological and molecular aspects of seedlessness in grapes (Vitis vinifera) Pineapple (Ananas comosus)
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