I have just spent a few hours cutting up oranges and lemons to make marmalade, and was startled to see that none of the oranges had seeds. How do the trees they come from reproduce?
• On the premise that seeds detract from the appeal of fruit to consumers, plant breeders have developed seedless varieties of a number of fruits, including oranges and other citrus fruits, grapes and watermelons. These varieties can’t reproduce from seeds, of course, but this isn’t a problem.
Most commercial fruit trees, whether seedless or not, are propagated by grafting. A cutting is taken from a parent plant and grafted onto root stock from which it then develops into a new tree that (from the graft outwards) is genetically identical to the parent. In this way, large numbers of genetically identical trees with the desired properties of flavour and yield can be produced.
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Jonathan Wallace,
Newcastle upon Tyne, UK
• A fruit usually forms after a flower’s ovules have been pollinated. But in some cases, fruit can form without pollination. This phenomenon, known as parthenocarpy, can occur as a natural mutation in some plants. Seedless oranges come from parthenocarpic plants.
Pollination failures happen in different ways. For example, navel orange plants have a mutation that stops their anthers from producing pollen. Other varieties, like Jaffa oranges, have a mutation that leads to self-incompatibility between the plant’s sperm and egg cells.
Because these plants can’t reproduce through pollination, they would die out very quickly if left alone in nature. However, they are cultivated by farmers who take cuttings from an original, seedless orange tree and graft them onto another, normal orange tree. Each graft is a clone of the original tree, which preserves the seedless trait in the oranges.
So seedless orange plants are reproducing asexually through grafting by humans.
Interestingly, all navel oranges are believed to have come from a single tree in Brazil that was found in the early 1800s. Similarly, according to legend, Jaffa oranges are based on a mutation from a single branch of one tree that was discovered around 1844 in Palestine. They are named after their main export point, the port of Jaffa.
Tiffany and Damian Pang,
Discovery Bay, Hong Kong, China
• Navel oranges are so called because the end opposite the stalk looks like a belly button. The fruit, first discovered in Brazil, was juicy, sweet, easily peeled and, having no seeds, a potential money-spinner.
Cuttings were taken and grafted onto citrus root stock. These grew into orange trees that were genetically identical to the original, giving consistency of fruit production. This cultivar was named the navel orange as its development was financially supported by the US government.
Another parthenocarpic fruit is the Cavendish banana, which supplies over 99 per cent of the bananas in our supermarkets.
David Muir,
Edinburgh, UK
• The implications of this question are worrying. It sounds like the questioner isn’t using Seville oranges to make their marmalade. Though these tart, thick-skinned oranges are more pip and peel than pulp, they are generally considered to make the best marmalade.
Adrian Foulds,
Glasgow, UK
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