Most fruits start growing after pollination
, when the pollen
has fertilized the plant's ovular compartment
, in its flower
. Plants which grow fruit without pollination (and thus have no seeds) are said to be parthenocarpic
, from Greek
meaning virgin fruit. Fruit bearing plants all grow some
parthenocarpic fruits, which usually differ in size and appearance than the regular fertilized fruit. Growing edible parthenocarpic
fruit requires a great deal of selection by the grower, both to maximize the number of seed
less fruit and to choose the plants which bear close-to-normal fruit.
Some fruits are naturally parthenocarpic, and don't need any intervention to produce many seedless fruits. Bananas, for instance, are triploid (three chromosomes) and rarely produce eggs or sperm with valid chromosomal layout for reproduction. Seedless watermelons work much the same way, only they aren't naturally triploid and must be bread from a diploid (two chromosomes) and tetraploid (four chromosomes) parent, which itself must be manipulated to be tetraploid.
Clementines and naval oranges have self-incompatibility genes that keep flowers from being pollinated by the same plant. If every tree in the orchard is cloned from one plant, all of the pollen is exactly alike, and appears to the flower to be from the same plant. When that happens, the trees grow their fruit parthenocarpically, and seedless oranges are the result.
Geneticists have also discovered that the hormone auxin can cause normally non-parthenocarpic plants to produce fruit without being fertilized. Auxin usually encourages the plant to grow shoots in one direction or another, but when exposed to the ovary it lets the fruit begin growing without pollination. This discovery hasn't been brought to market yet, but expect to see seedless plums and apples sometime soon!