Parthenocarpy

In botany and horticulture, parthenocarpy (literally meaning virgin fruit) is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Parthenocarpy occasionally occurs as a mutation in nature, but it is usually considered a defect, as the plant can no longer sexually reproduce, but may propagate by asexual means.

However, parthenocarpy of some fruits on a plant may be of value. Up to 20% of the fruits of wild parsnip are parthenocarpic. The seedless wild parsnip fruit are preferred by certain herbivores so serve as a "decoy defense" against seed predation.^[1] Utah juniper has a similar defense against bird feeding.^[2] Being able to produce seedless fruit when pollination is unsuccessful may be an advantage to a plant because it provides food for the plant's seed dispersers. Without a fruit crop, the seed dispersing animals may starve or migrate.

  In some plants, such as seedless watermelon, pollination or other stimulation is required for parthenocarpy. This is termed stimulative parthenocarpy. Banana exhibits stimulative parthenocarpy because it is a triploid -meaning it is the result of a diploid and a tetraploid parent and therefore cannot produce seeds. Plants that do not require pollination or other stimulation to produce parthenocarpic fruit have vegetative parthenocarpy. Cucumber is an example of vegetative parthenocarpy.

Plants moved from one area of the world to another may not always be accompanied by their pollinating partner and the lack of pollinators has spurred human cultivation of parthenocarpic varieties. Some parthenocarpic varieties have been developed as genetically modified organisms.

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Commercial importance

Seedlessness is a very desirable trait in edible fruit with hard seeds such as pineapple, banana, orange and grapefruit. Parthenocarpy is also desirable in fruit crops that may be difficult to pollinate or fertilize, such as tomato and summer squash. In dioecious species, such as persimmon, parthenocarpy increases fruit production because staminate trees do not need to be planted to provide pollen. Parthenocarpy is undesirable in nut crops, such as pistachio, where the seed is the edible part. Horticulturists have selected and propagated parthenocarpic cultivars of many plants, including fig, cactus pear (Opuntia), breadfruit and eggplant. Some plants, such as pineapple, produce seedless fruits when a single cultivar is grown because they are self-infertile. Some cucumbers produce seedless fruit if pollinators are excluded. Strange as it seems, seedless watermelon is propagated by seed. The seeds are produced by crossing a diploid parent with a tetraploid parent to produce triploid seeds.

When sprayed on flowers, any of the plant hormones, gibberellin, auxin and cytokinin, can often stimulate the development of parthenocarpic fruit. This is termed artificial parthenocarpy. Plant hormones are seldom used commercially to produce parthenocarpic fruit. Home gardeners sometimes spray their tomatoes with an auxin to assure fruit production.

Some parthenocarpic cultivars have been developed as genetically modified organisms.^[3]

Some parthenocarpic cultivars are of ancient origin. The oldest known cultivated plant is a parthenocarpic fig first grown at least 11,200 years ago.^[4]

In some climates, normally seeded pear cultivars will produce mainly seedless fruit.

Misconceptions

• Most commercial seedless grape cultivars, such as 'Thompson Seedless' are not seedless because of parthenocarpy, but because of stenospermocarpy.
• Parthenocarpy is sometimes claimed to be the equivalent of parthenogenesis in animals.[4] That is incorrect because parthenogenesis is a method of asexual reproduction, and parthenocarpy is not, except in rare cases such as pineapple. The plant equivalent of parthenogenesis is apomixis.

References

1. ^ Zangerl, A.R., J.K. Nitao, and M.R. Berenbaum. 1991. Parthenocarpic fruits in wild parsnip: decoy defence against a specialist herbivore. Evolutionary Ecology 5:136-145.
2. ^ Fuentes, M. and Schupp, E.W. 1998. Empty seeds reduce seed predation by birds in Juniperus osteosperma. Evolutionary Ecology 12: 823-827.[1]
3. ^ Pandolfini, T., Rotino, G.L., Camerini, S., Defez, R. and Spena, A. 2002. Optimisation of transgene action at the post-transcriptional level: High quality parthenocarpic fruits in industrial tomatoes. BMC Biotechnol. 2: 1. [2]
4. ^ Kislev, M.E., Hartmann, A. and Bar-Yosef, O. 2006. Early domesticated fig in the Jordan Valley. Science 312: 1372-1374. [3]

See also

• Seedless fruit
• Stenospermocarpy