Re: How come there are seedless fruits?

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)