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Definition: cucumber from The Columbia Encyclopedia

fruit of Cucumis sativus, a species of gourd whose many varieties are descended from a plant native to Asia and Africa. Cucumber is classified in the division Magnoliophyta, class Magnoliopsida, order Violales, family Curcurbitaceae.


Summary Article: Breeding Cucumber
From Principles of Plant Genetics and Breeding
Taxonomy
Family Cucurbitaceae
Genus Cucumis
Species Sativus
Economic Importance

Cucurbits or vine-crop family (the Cucurbitaceae) comprises a number of very popular crops, including watermelon (Citrullus lunatus), cucumber (Cucumis sativus), and melon (Cucumis melo). Others are the pumpkin, squash and gourds. About 80% of the production occurs in Asia, with China leading the world in production. Other major producers are Turkey, Iran, the United States and Japan. Cucumber is among the top 10 most widely cultivated crops in the world. It is the third most commonly grown vegetable crop in the world.

Origin and History

Cucumber is believed to have originated in India, with secondary centers of origin in China and the Near East. It was domesticated in Asia and later introduced into Europe.

Commercial Market Classes

There are two key market classes of cucumber – pickling cucumbers and slicing cucumbers. Breeding, production, and marketing are designed around these classes.

Pickling Cucumbers

Pickling cucumbers are produced for processing or pickling. The varieties in this class are characterized by thin skins, short and block fruit, and skin color gradient that starts with dark green color at the stem to light green at the blossom end. The part of the fruit that touches the soil in field production is white or lighter green.

Slicing Cucumber

Slicing cucumbers are grown to be sliced and eaten fresh, commonly in salads. They have longer fruits with thick skins and dark green color.

Varieties
  • English cucumbers. These may have fruits as long as 24 inches. They are mostly parthenocarpic and usually grown in greenhouses.

  • East Asian cucumbers. The fruits are slender with deep green skin color. They are ridged and bumpy on the surface. They are grown for slicing or pickling.

  • Lebanese cucumbers. These have small fruits that are smooth. They are almost seedless.

  • Persian cucumbers. These cucumbers are also called mini seedless cucumbers. The vines are parthenocarpic and require no pollinators for fruit setting.

  • Other varieties include apple, Dosakai, Kekiri, and Armenian.

Cytogenetics and Genetics

The genus Cucumis has been partitioned into two subgenera, Cucumis (x = 7) and Melo (x = 12). Cucumber (Cucumis sativus L., 2n = 2x = 14) is the only species with a haploid chromosome number. Over one hundred genes have been identified in cucumber. This includes seedling mutants (e.g., non-lethal color mutants like virescent (v), and non-color traits like bitter-free (bi)); stem mutants (e.g., bush (bu), compact (cp) and determinate (de)); leaf mutants (e.g., blunt leaf apex (bla), crinkled leaf (cr)); flower mutants (e.g., sex expression genes and modifiers (F, a, m), male sterility genes); fruit mutants (e.g., bitter fruit genes (Bt, bi), skin texture genes (Tu, te, P, I, H), spine density (f), fruit length (Fl, fl)); fruit and spine color (e.g., red (R), black or brown spines(B), heavy netting (H)). Many genes have insect resistance and disease resistance, for example, resistance to cucumber mosaic virus (Cmv), scab, bacterial wilt, target leaf spot, downy mildew have been identified.

The narrow genetic base of cucumber limits the wide use of marker technology in crop improvement. The condition impedes the construction of a saturated molecular genetic map. The cucumber genome has been sequenced, thereby affording researchers insight into traits such as sex expression, disease resistance, and the biosynthesis of cucurbitacin.

General Botany

Though most cucurbits are monoecious (have separate pistillate and staminate flowers), there are andromonoecious and gynoecious annuals. They are mostly indeterminate trailing vines, even though determinate types have been developed for home gardens. Prior to 1980, most cucumber cultivars were monoecious in which staminate flowers are produced first. Currently, cultivars of both pickling and slicing cucumber used in the United States, for example, are gynoecious hybrids. When produced in the fields, gynoecious cultivars are blended with a monoecious hybrid (10–15% of the total plants) or inbred to provide adequate amounts of pollen for fruit set. Cucumbers are naturally outcrossing and have a potential for interspecific hybridization.

Common Breeding Methods

Cucumber has a narrow genetic base, thereby limiting the possibility of crop improvement via cross breeding. Little heterosis has been observed in cucumber crosses. Both conventional and molecular methods have been used to successfully improve various traits of cucumber. The use of molecular markers is rather limited because of the challenge of using this approach when the species has a narrow genetic base and the level of polymorphism is low. The sequenced cucumber genome affords breeders insight into traits such as its sex expression, disease resistance, biosynthesis of cucurbitacin, and "fresh green" odor.

Pedigree and backcross methods have been used to incorporate simply-inherited traits like gynocey, disease resistance, and fruit into elite inbreds for release to producers as new cultivars or for use as parents in producing single cross hybrids. Recurrent selection is preferred for quantitative traits such as yield, early maturity, and fruit quality. Some reports indicate the successful use of recurrent selection for breeding increase fruit yield, disease resistance as well as herbicide resistance in the crop. S1, half-sib and full-sib family selection methods have also been used to improve yield in cucumber.

Breeding Objectives

Plant breeders have developed varieties for specific production systems and markets. For the food service industry, the major goal is to develop cultivars that are clean and spineless. Breeders approach this objective by introducing glabrousness, a recessive (gl) trait that was induced via mutagenesis. For greenhouse production, modern cultivars are gynoecious (all pistillate or female flowers), parthenocarpic fruits (seedless – no need for pollination), and high quality (lack of bitter principle – burpless cucumber).

Breeders are also interested in cultivars that are resistant to major diseases, including antrachnose, powdery mildew, downy mildew, angular leaf spot, cucumber mosaic virus, fusarium wilt and scab.

Key Reference and Suggested Reading
  • Pierce, L.K. and Wehner, T.C. (1990). Review of genes and linkage groups in cucumber. HortScience, 25: 605-615.
Internet resource
Copyright © 2013 by John Wiley & Sons, Ltd.

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