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ОглавлениеCUCUMBER, MELON AND THEIR RELATIVES
Introduction
A biosystematic monograph of the genus Cucumis (Kirkbride, 1993) recognized 32 species, including two major crops, cucumber (C. sativus) and melon (C. melo), and two minor crops, West Indian gherkin (C. anguria) and African horned melon (C. metuliferus). Other species are sometimes cultivated (e.g. C. dipsaceus) or collected wild and used for food, water, or medicine (e.g. C. africanus L.). Molecular phylogenetic studies have placed additional genera into Cucumis based on chloroplast and nuclear DNA sequence similarities. A recent analysis of 100 Cucumis species from Africa to Australasia (Sebastian et al., 2010) provided a better picture of species diversity and relatedness and added 18 close relatives of cucumber and melon not previously included by Kirkbride. Of the 66 Cucumis species, 25 are Asian and Australian and 41 are African.
Cucumis is divided into subgenus Cucumis, composed of C. sativus and C. hystrix Chakravarty, and subgenus Melo (Mill.) C. Jeffrey, containing the remaining species. Phylogenetic investigations of Cucumis species have been based on crossing relationships, karyotypes, flavonoids, isozymes, chloroplast DNA and molecular markers. All of these studies agree that the subgenera are widely separated, to the point that it has been proposed they be in distinct genera. Puchalski and Robinson (1990) proposed seven groups of Cucumis species according to isozyme patterns. The groupings were generally similar to those above, except that the anguria group was separated into three isozyme groups and C. sagittatus was placed in a different isozyme group than C. melo.
Interspecific crossing relationships (see Chapter 3, Fig. 3.2) (Deakin et al., 1971) and molecular phylogenies (Garcia-Mas et al., 2004) suggest four groups of Cucumis species. Crossing relationship groups are: (i) the anguria group, composed of C. anguria and seven other intercompatible species (marked ‘2’ on Fig. 3.2) with softly spiny fruit; (ii) C. metuliferus; (iii) C. sativus, including the fully compatible C. sativus var. hardwickii (Royle) Gabaev; and (iv) the melo group, which includes C. melo, C. humifructus and C. sagittatus Peyritsch in Wawra & Peyritsch. Species of the melo group have hairy immature fruit, but, unlike other Cucumis species, their mature fruit lack spines. Interspecific hybrids have not been obtained in this group, although fruit with partly developed seeds were obtained in the cross C. melo × C. sagittatus.
Botany
Cucumber, Cucumis sativus, is the fourth most important vegetable crop in the world and the most important cucurbit. Cucumbers, like most cucurbits, are indeterminate, vining (1–3 m), frost-sensitive annual plants that produce cylindrical or round fruit. Determinate cultivars, generated from a naturally occurring mutation for gibberellin sensitivity, were first introduced with ‘Midget’ in 1940 and have been used in home gardens and patio containers.
Usually, a single unbranched tendril develops at each leaf axil, although a tendril-less (tl) mutant has been described. The stems, leaves and young fruit of the plants are covered in stiff, multicellular, unbranched trichomes (Zhao et al., 2015). The leaves of this species are triangularly ovate, with three to five lobes. The first cucumber cultivars were monoecious, but gynoecious, androecious, hermaphroditic, gynomonoecious, andromonoecious and trimonoecious forms are also present in this species. Current cultivars are gynoecious or predominantly gynoecious, except for monoecious garden cucumbers that produce fewer fruit per harvest, but more harvests.
Little variation in petal colour or shape has been observed in cucumber. Flowers are yellow with five or six basally fused petals (Fig. 4.1). Plants often produce male flowers initially (first three nodes) followed by alternating male and female flowers and finishing with mostly female flowers. Flowers remain open for a single day and are pollinated by bees and other insects. Immature fruit are green at the edible stage, except in a few cultivars, where they are white or yellow. Fruit are round to oblong or narrowly cylindrical, with small tubercles (warts) and spines of trichome origin on the rind. Spine colour is associated with mature fruit colour and fruit netting. Fruit of white-spined cultivars are greenish-white to yellow at maturity and not netted. Black-spined fruit become orange or red (brown) when mature and may be netted. Fruit flesh is crisp and usually white, but is pale orange in a few cultivars. Seeds are small, white and flat.
Fig. 4.1. Cucumber flower (left) and melon flower (right). Notice the size variation, and basal fusing of the six petals in the cucumber flower.
Origin and history
Cucumber is of Asiatic origin along with the closely related wild Cucumis sativus var. hardwickii, which was first found in the Himalayan foothills of Nepal. Plants of this botanical variety are highly branched, daylength sensitive and prodigious producers of bitter fruit. The most closely related species to cucumber (2n = 2 x = 14) is Cucumis hystrix (2n = 2x = 24). Both species are more closely related to the species of Australia and New Guinea, such as C. umbellatus, than to the African species, such as C. hirsutus, C. metuliferus, C. myriocarpus, C. anguria and C. sagittatus (Sebastian et al., 2010). Cucumber diverged from C. hystrix 4.6 million years ago, and the cucumber–melon relative diverged from its African relative 11.9 million years ago. Based on the recent sequencing of the Lagenaria genome, Lagenaria diverged from Citrullus 10.4–14.6 million years ago, from Cucumis 17.3–24.3 million years ago and from Momordica 29.2–41.0 million years ago.
The remains of Cucumis crops (melon or cucumber) in eastern Iran have been dated to the third millennium bc. Cucumber cultivation goes back perhaps 3000 years in India and 2000 years in China. China is considered a secondary centre of genetic diversification. Today, cucumber is one of the most important vegetable crops in that country, second only to Chinese cabbage in the total area that is cultivated.
Cucumber was probably not known to the ancient Egyptians, Greeks or Romans. The Latin cucumis, Greek sikyos and Hebrew qishu’im refers to the snake melon, Cucumis melo var. flexuosus (Paris, 2012). In English translations of the Bible, cucumber and melon are referred to in Numbers 11:5, but that probably should have been translated as snake melon and watermelon. Snake melon fruit can be distinguished by small hairs (fuzz) on the surface, whereas cucumber fruit are smooth, with ridges, warts, or spines.
Early travellers brought cucumber to Mediterranean countries from India through Iran, Iraq and Turkey in the 6th or 7th centuries (Paris, 2012). Cucumber probably reached Spain in the 9th century and Tunisia in the 10th century. In the early 14th century, cucumber plants were cultivated in England. There, the fruit were known as ‘cowcumbers’. Portuguese explorers subsequently carried cucumber to West Africa. Columbus introduced the species to the New World, planting it in Haiti in 1494. Today, cucumber is grown throughout the world, in containers on patios in urban areas, in gardens often using trellises, on large commercial farms, in unheated high tunnels and in heated greenhouses (glasshouses or poly houses).
