Who, as a child, didn't enjoy spitting watermelon seeds into a cup? (Naturally, you were not spitting them at your parents, siblings, or friends.) Nowadays, it is more difficult to find a watermelon with seeds on the grocery store shelves, and shoppers can also find varieties of seedless oranges, grapes, and cucumbers. This is due to a genetic mutation, either purposely induced or not, called parthenocarpy.
What is parthenocarpy?Parthenocarpy (Latin for “maiden fruit”) refers to the development of fruit without fertilization. The process produces a sterile fruit which lacks seeds. Parthenocarpy is not to be confused with parthenogenesis, which is a method of asexual reproduction; or apomixis, which is the botanical term for asexual reproduction of plants without the use of fertilization.
StenospermocarpyFruit produced through a related process called stenospermocarpy contain seeds that die at an early stage, causing the fruit to appear seedless. The ovules or embryos abort without producing mature seeds. Pollination and fertilization occur in stenospermocarpy, but not in parthenocarpy.
Parthenocarpy and Plant ProtectionParthenocarpy and stenospermocarpy would not generally be thought of as desirable mutations in plants, as the lack of reproductive fitness would preclude the survival of the plant species. However, in some cases, the parthenocarpic process occurs as a protective mechanism. Wild parsnips develop sets of parthenocarpic fruit which are more desirable for feeding by the parsnip webworm, due to the lower toxin content of the seedless parsnip fruit.
Therefore, the parthenocarpic fruits are sacrificed and the fertile fruits are left intact.
Induction of Parthenocarpy and StenospermocarpySeedless fruit is desired by many consumers of fruits and vegetables such as watermelons, bananas, grapes, and cucumbers. For this purpose, artificial methods of inducing parthenocarpy have been developed. The use of plant growth hormones such as gibberellin or auxin can lead to parthenocarpic induction within plants. The application of exogenous (applied from outside rather than synthesized inside) gibberellin to a plant causes the flowers to react as though fertilization takes place; therefore, fruits are produced. The application of auxin can stimulate the growth of fruit containing either no seeds or unfertilized seeds.
In stenospermocarpy, pollination triggers the development of the fruit but the embryo is aborted before developing into a full seed. Some plants, such as oranges, are self-incompatible (sterile, when pollinated by the same plant variety). Therefore, they must receive pollen from a different variety in order to fruit. Seedless oranges are only produced when the same variety of oranges is grown together in the same orchard, via the use of vegetative propagation. Grafting is used in fruit trees, whereas seedless cucumbers and bananas can be replicated via tissue culture. Fruiting then occurs without pollination.
Stenospermocarpy can be induced in watermelon through crossing. The fruits are produced on triploid plants (plants with two sets of chromosomes from one parent, and one set of chromosomes from the other), and meiosis is prevented from taking place. The seeds are produced by crossing diploid and tetraploid lines of watermelon, and sterile triploid plants are produced. The fruits are produced via pollination by a nearby diploid plant.