Click here for the 'Seeds of Eaden' seed shop Seedless fruits have been cultivated for many years as there is a demand for fruit without those pesky seeds. Cross breeding of different varieties and genetic sports have meant that a wide range of seedless varieties of fruit are available. These include banana, cucumbers, grapes, lemons, limes, melon, oranges and pineapple. So how do fruits that do not contain seeds propagate?
Traditionally, plants produce seed in order to reproduce. Seedless varieties of fruit are produced when normal fertilisation of plants does not occur. Some plants produce fruit without fertilisation in a process called pathenocarpy, translated literally as virgin fruit. This means that fruit is produced without the fertilisation of ovules and therefore the fruit is seedless. Other 'seedless' fruit may be produced after pollination occurs in a process called stenospermocarpy, where the ovules or embryos abort before they can produce mature seed. Small seeds are produced in this case, but are immature and lack a hard seed coating.
Naturally occurring mutations, genetic selection and manipulation has led to an increased variety of seedless fruit being produced as there is a high commercial demand for these fruits. Botanists and scientists hunt out valuable seedlings that may have a commercial value, looking for disease resistance, taste quality and plants that are easy to grow. Plant hormones can stimulate plants to produce seedless fruit. Auxins, gibberellins and cytokins will stimulate flowers to produce sterile fruit. These can be naturally occurring or artificially applied.
Without the aid of seeded fruit to reproduce, plants can be propagated through vegative cuttings, grafts and tissue cultures. This creates a cloned replica of the parent plant. However, having cloned plants results in a reduced genetic diversity of species which may have a detrimental effect in the future.
Seedless watermelon are grown from seed and are a result of hybridisation of two different types of melon. A seedless fruit arises from fertilisation between a diploid parent (2 sets of chromosomes) and triploid parent (3 sets of chromosomes). The fruit is sterile (without seed) because meiosis is prevented. In order to produce a fruit, pollen from the diploid parent plant must fertilise the flower of the triploid parent plant. Therefore both varieties of watermelon must flower at the same time.
Seedless bananas can produce fruit without fertilisation and are produced on triploid parents. Most commercially grown bananas are a single clone of the Cavendish cultivar. Bananas propagate from suckers around the base of a parent plant. Tissue culture is used to propagate these plants on a commercial scale.
Seedless grapes do contain small, immature seeds. Most grapes are propagated through vine cuttings, which root and form new (identical) plants. Alternatively, embryo rescue tissue culture may be used where these young seeds are grown on to produce a mature plant.
Pineapples are grown commercially in fields of a single variety as this means there is no cross pollination, which is required to produce seed. If cross pollination does occur between two varieties then the fruit will contain seed. The pineapple propagates from the leafy top part of the fruit.
Some guava cultivars has naturally occurring sterile fruit. Persimmon fruit has separate male and female individuals. If the female plant is not near the male plant and cannot be pollinated a seedless fruit may be produced. The seedless naval orange was discovered in the 19th century and all grafts date back to a single mutant tree in Brazil. Cuttings from this parent plant are grafted on to citrus rootstocks.