Nearly every fruit carries seeds, because holding seeds is what a fruit is for. But some plants can swell a fruit without ever finishing a seed, and the seedless bananas, grapes and watermelons in shops are crops chosen and bred for that trait. People have been doing this for a very long time: they were apparently planting seedless figs over 11,000 years ago.1National Geographic Society. The Development of Agriculture
What Makes Something a Fruit
To a botanist, a fruit is the ripened ovary of a flower together with the seeds inside it.2The Science of Plants. Fruit The ovary is the part of the flower that holds the tiny unfertilized seeds-to-be, and after pollination it swells into the fruit.3University of Nevada, Las Vegas. Fruit – the ripened ovary of an angiosperm flower Conifers work differently: pine trees and their relatives leave their seeds exposed rather than enclosed in an ovary, so berry-like parts such as juniper berries are not true fruits.4In Defense of Plants. Gymnosperms and Fleshy “Fruits” — In Defense of Plants
Cooks sort food by sweetness; botanists sort by which part of the flower it came from. By the botanical rule, tomatoes, cucumbers, peppers and pea pods are fruits, and so are squash and eggplant, even though they end up in savory dishes. Rhubarb and carrots are not. Even the “seeds” on the outside of a strawberry are really tiny individual fruits.
The test a botanist applies is simple: did the food grow from a flower’s ovary, and does it hold seeds? You can do the same by cutting a food open and looking for seeds inside a ripened wall of tissue. Carrots and beets fail because they are swollen roots, and celery fails because it is a leaf stalk. Rhubarb is the classic trick case: people bake it into pies like fruit, but it is a stalk.
A plant wraps its seeds in fruit for a reason. Fleshy fruit attracts animals that eat it and carry the seeds away, and the fruit also protects the seeds while they mature.
How a Fruit Can Grow Without Seeds
The growth hormones that drive fruit to swell are thought to come mainly from the developing seeds.5PubMed Central. Tissue-Specific Transcriptome and Hormonal Regulation of Pollinated and Parthenocarpic Fig (Ficus carica L.) Fruit Suggest that Fruit Ripening Is Coordinated by the Reproductive Part of the Syconium So in most plants, an unpollinated flower simply drops off instead of forming fruit. Pollination normally starts a chain: pollen fertilizes the ovule, the seed begins to develop, and the seed’s hormones tell the surrounding ovary wall to swell.
In some plants, though, fruit can develop with no pollination or seed formation. These plants skip the seed step and get the swelling signal from another source.6PubMed Central. Domestication, Genomics and the Future for Banana Two hormones, auxin and gibberellin, play the central role in starting this seedless growth.7PubMed. Cytokinin, gibberellin and auxin regulate parthenocarpy in cucumber via modulating downstream auxin signaling network Cytokinin and, to a lesser degree, ethylene and other hormones help decide whether a fruit sets at all.8PubMed Central. Hormonal interactions underlying parthenocarpic fruit formation in horticultural crops
There are two routes to a seedless fruit. In one, the flower sets fruit with no pollination at all, so no seed ever begins; this is the case for cultivated bananas. In the other, pollination or fertilization may happen, but the seed stops developing early, as in many seedless grapes, and the fruit carries on growing regardless.
Some “seedless” fruit still shows traces. The white specks in a seedless watermelon are soft, empty seed coats that formed but never filled in with a hard seed, and they are safe and normal to eat.
This ability has a natural purpose too. In fruits that normally hold many seeds, growing fruit anyway may help when pollinators are scarce.9PubMed Central. The Occurrence of Seedlessness in Higher Plants; Insights on Roles and Mechanisms of Parthenocarpy A poorly pollinated flower can still finish its fruit and spread the few seeds it did make instead of losing them.10PubMed Central. The Occurrence of Seedlessness in Higher Plants; Insights on Roles and Mechanisms of Parthenocarpy
Bananas, Grapes, Figs and Pineapples
Bananas are the classic natural exception. Wild bananas are full of hard seeds, while cultivated bananas are seedless and grow fruit without pollination.11PubMed Central. Domestication, Genomics and the Future for Banana Early edible bananas in Southeast Asia lost fertility, and later crosses produced vigorous seedless types with extra chromosome sets.12Office of the Gene Technology Regulator. The Biology of Musa L. (banana) The small dark dots in the middle of a supermarket banana are leftover remains of seeds that never developed, soft enough to chew without noticing.
In many seedless grapes, the seed starts forming but the embryo dies early as the berry grows.13MDPI / Horticulturae. Optimization of an In Vitro Embryo Rescue Protocol for Breeding Seedless Table Grapes (Vitis vinifera L.) in Italy Growers add a hormone to the process: they spray gibberellin 10 to 20 days before bloom, which makes most berries in a bunch seedless and helps more berries set.14Japan Agricultural Research Quarterly. Production of Seedless Grapes by Gibberellin Treatment They often use it again after bloom to make berries larger.
Pineapples can’t fertilize themselves with their own pollen, so commercial fields of one variety set no seeds.15Current Botany. Studies on crossability among different varieties of Ananas comosus (L.) Merr. (Bromeliaceae) If pollen arrives from a different variety, for instance carried by hummingbirds where they live, the fruit can develop small hard seeds. A seedless pineapple is partly a result of how the field is planted, not only of the plant’s biology.
Figs are stranger still. A fig is not a single ovary but a hollow flower cluster turned inside out, and the gritty bits you chew are the actual small fruits. Some fig varieties ripen fruit without pollination, and their seed parts stay small and empty.16PubMed Central. Tissue-Specific Transcriptome and Hormonal Regulation of Pollinated and Parthenocarpic Fig (Ficus carica L.) Fruit Suggest that Fruit Ripening Is Coordinated by the Reproductive Part of the Syconium That is how seedless-style figs are grown where the tiny wasp that pollinates other types does not live. Other types need that wasp to ripen, so whether a fig has seeds depends on the variety.
Because these crops have no seeds to plant, they are grown as clones. Bananas grow from shoots off the parent plant, grapes and figs from cuttings, and pineapples from their leafy tops or side shoots.
How Seedless Watermelons Are Made
A seedless watermelon plant has three sets of chromosomes instead of the usual two.17cucurbitbreeding.wordpress.ncsu.edu. Seedless Watermelon Breeding With three, an odd number, the chromosomes can’t be shared out evenly when the plant makes pollen and egg cells, so those cells don’t work even though the flowers look normal.18cucurbitbreeding.wordpress.ncsu.edu. Seedless Watermelon Breeding When pollen from another plant triggers the fruit, it grows without forming mature seeds.19cucurbitbreeding.wordpress.ncsu.edu. Seedless Watermelon Breeding
The seed a farmer plants comes from a fresh cross each season. Breeders treat a normal watermelon plant with a chemical that doubles its chromosomes to four sets. They then pollinate that plant with pollen from a normal two-set plant, and the seeds that form carry three sets. The triploid plants can’t make seed that would grow the next generation, so the cross must be repeated for every batch. Yields from it are low, which is why seedless melon seed costs more, and why home gardeners can buy it but can’t save seed from the fruit.
The sterile plant’s own pollen is useless, so fields still need regular plants. Farmers plant normal seeded watermelons among them, and bees carry that pollen over to start fruit growth. Without pollination the seedless melons don’t develop.
Is Seedless Fruit Less Nutritious or Genetically Modified
Seeded and seedless watermelons have the same sugar content and juiciness, because sugar builds up in the flesh independently of the seeds.20HortScience. No Effect of Seed Presence or Absence on Sugar Content and Water Status of Seeded and Seedless Watermelon Fruits Many seedless watermelons are high in lycopene, the red pigment, but some seeded varieties match them.21HortScience. Screening Carotenoid Content in Seeded and Seedless Watermelon Fruit Choosing seedless is about convenience, not health: no spitting or picking out seeds, which suits children and cut fruit trays. The one thing given up is the seeds themselves, which some people like to roast and eat.
Seedless does not mean genetically modified. Seedless watermelons come from conventional breeding and chromosome doubling, and the fruit contains no added genes. Seedless bananas and figs arose through ancient selection long before modern genetics,22Office of the Gene Technology Regulator. The Biology of Musa L. (banana) and seedless grapes were chosen over many generations by growers who kept plants with the useful trait. Scientists have used gene editing to make seedless tomatoes by raising auxin levels in the fruit.23New Scientist. Gene editing opens doors to seedless fruit with no need for bees Those tomatoes can set fruit without bees, which could help where pollinators are scarce, but it is a research option, not how the seedless fruit in a typical store was made. The gibberellin used on grapes is a hormone plants make naturally, and it is a routine approved treatment in grape growing.
The real cost of seedless crops is on the farm. Because every plant is a copy, a disease that hits one can hit all. That is a farming risk, not a safety risk to people eating the fruit.
Can You Grow Seedless Fruit at Home
Seedless grapes, figs and bananas are sold as nursery plants grown from cuttings or shoots. Seedless watermelon needs purchased triploid seed and a seeded variety planted nearby for pollen.24cucurbitbreeding.wordpress.ncsu.edu. Seedless Watermelon Breeding A pineapple top from a store fruit can be rooted to grow a new plant.
To root a pineapple top, twist the leafy crown off a ripe store pineapple with a firm turn and pull away the lowest leaves to expose about an inch of stem. Leave it to dry for a few days so the cut end seals, which helps stop rot. Then stand it in a jar of water or press it into well-draining potting mix, and keep it warm and in bright light until roots form. Fruit takes about two years, and it will usually be seedless unless another pineapple variety pollinates it.
Seedless watermelon at home takes some care. Triploid seed sprouts best in warm soil, so sowing too early in cool conditions often leads to poor germination. Plant a seeded watermelon nearby as the pollen source and mark it, so you can tell the seeded melons from the seedless ones at harvest.