Cucurbits

Todd C Wehner, Rachel P. Naegele, James R. Myers, Narinder P S Dhillon, Kevin Crosby

• Publisher: CAB International
• Year: 2020
• ISBN:
  9781786392930
• Edition: 2
• Topic: Scienze biologiche
• Subtopic: Orticoltura

Completely updated with new content and full-colour figures throughout, the second edition of this successful book continues to provide complete coverage relating to the production of cucurbits, including cucumbers, gourds, muskmelons, pumpkins, squashes and watermelons. These crops are grown worldwide and represent one of the largest and most important groups of horticultural food plants. This second edition of Cucurbits provides up-to-date, succinct and authoritative knowledge on this variety of crops and reflects on significant advances in the areas of production, breeding and evolution.This new edition: - Contains new chapters on abiotic stresses and cucurbits for health- Includes major updates in research on the evolution, movement and distribution of species- Explores new genetic resources and breeding advancements- Delivers current information on methods of improving yield (e.g. grafting) and the management and resistance for pests and diseases- Has an updated list of the most recent taxonomic namesThis book represents a current and comprehensive guide to cucurbits, is highly illustrated and written in an accessible style. It is an essential resource for students, growers and researchers.

1

WHAT ARE CUCURBITS?

DISTRIBUTION AND ECOLOGY

The Cucurbitaceae is a family of frost-sensitive and predominantly tendril-bearing vining plants that are found in subtropical and tropical regions around the globe. There are only a few species that are native to temperate climates; they are either prolific seed-producing annuals, perennials that live for one season until killed by frost, or xerophytic perennials whose succulent underground parts survive the winter. Ecologically, the family is dichotomous; many genera flourish in the humid tropics, particularly in southeastern Asia and the neotropics, whereas other genera are native to the arid regions of Africa, Madagascar and North America. Members of the latter group, the xerophytes, usually have large perennial roots and succulent stems that are clambering and creeping and at least partially subterranean; in some cases, tendrils or leaves are lacking or greatly modified.

Although most crops in the Cucurbitaceae have been selected from the mesophytic annuals, concern over famine and fuel sources in arid countries has led to interest in turning some of the xerophytes into agricultural crops.

NOMENCLATURE

‘Cucurbits’ is a term coined by Liberty Hyde Bailey for cultivated species of the family Cucurbitaceae. Beginning in the early 20th century, the term has been used not only for cultivated forms, but also for any species of the Cucurbitaceae, and it will be so used in this book.

Other vernaculars applied to the family and various of its members are ‘gourd’, ‘melon’, ‘cucumber’, ‘squash’ and ‘pumpkin’. Of these, squash and pumpkin are the most straightforward, almost always referring to species of Cucurbita. An exception is the fluted pumpkin, which is Telfairia occidentalis. The unqualified terms melon and cucumber usually define Cucumis melo and Cucumis sativus, respectively. However, confusion develops when modifiers are added to these terms. Whereas muskmelon or sprite melon refers to a specific type of C. melo, watermelon is Citrullus lanatus, wintermelon is Benincasa hispida and bitter melon is Momordica charantia. Gourd generally is used to describe a cucurbit fruit with a hard, durable rind; usually it refers to bottle gourd (Lagenaria siceraria), or a wild species of Cucurbita, or an ornamental form of Cucurbita pepo. However, various other cucurbits also are called gourds, such as luffa sponge gourd (Luffa aegyptiaca), ridge gourd (Luffa acutangula) and some that do not have hard rinds, such as bitter gourd or bitter melon (M. charantia) and ivy gourd (Coccinia grandis). Sometimes the term refers to tough-rinded species of other plant families, such as the tree gourd (Crescentia cujete L., Bignoniaceae).

Complicating matters further, more than one common name is often applied to a single species. For example, names for B. hispida include ash gourd, white pumpkin, wax gourd and winter melon. Sometimes different names refer to distinct crops within a species, such as pumpkin, zucchini and acorn squash in Cucurbita pepo. In other cases, the different common names for the same species are used interchangeably. Frequently used common names for cucurbit crops are given in the Appendix.

Popular terms for the cucurbits may be confused in other languages as well. Cucumber and melon are sometimes considered the same in India. Older terminology in Chinese included ‘guo-kua’, which is translated into English as ‘melon’, but it includes watermelon (C. lanatus) as well as melon (C. melo). Also, ‘tsaikua’, which usually translates as ‘gourd’, refers to B. hispida, C. sativus, M. charantia and species of Cucurbita and Luffa. This is no longer a problem in China, since the names in Mandarin are now precise.

TAXONOMY

The family Cucurbitaceae, which is not closely related to any other plant family, consists of two well-defined subfamilies, eight tribes (representing varying degrees of circumscriptive cohesiveness) and about 118 genera and 825 species (Jeffrey, 1990). The four major cucurbit crops (watermelon, cucumber, melon, squash) and five other important crops (luffa, bottle gourd, chayote, wax gourd, bitter gourd) in the family belong to the Cucurbitoideae subfamily. Four of these – watermelon, luffa, bottle gourd and wax gourd – belong to the tribe Benincaseae. The classification of these and other cultivated species is given in Table 1.1. Many more wild taxa have actual or potential economic value, making Cucurbitaceae one of the most important plant families for human exploitation.

Table 1.1. Taxonomy of cultivated cucurbit species.

Taxonomic studies of cucurbits at all hierarchical levels have been done. They include comparative analyses of morphology (including specialized studies on trichomes, stomata, palynology and seed coat anatomy), cytology, DNA, isozymes, flavonoids, cucurbitacins, amino acids and fatty acids in seeds, biogeography and coevolving insects. A monograph has been written on Cucumis (Kirkbride, 1993), and the taxonomic relationships within the Cucurbitaceae are being improved using molecular markers (Schaefer et al., 2009; Chomicki and Renner, 2014). These new studies have been useful to the areas of crop improvement and germplasm conservation.

MORPHOLOGY AND ANATOMY

Seedlings

Most cucurbit seedlings are epigean, germinating with the tips of the cotyledons initially inverted but later erect. As it emerges, the hypocotyl straightens and the cotyledons ascend as the seed coat is dislodged by the peg, an outgrowth on one side of the hypocotyl. The function of the peg is to open the seed coat and permit the cotyledons to emerge. The photosynthetic cotyledons of most cucurbit seedlings are more or less oblong in shape. Between them lies the inconspicuous developing epicotyl.

Roots

Cucurbits generally have a strong taproot, which may penetrate the soil to a depth of more than 2 m, as in the case of squash. Even in cucumber, the tap root can extend 1 m into the soil. Cucurbits also have many secondary roots occurring near the soil surface. In fact, most roots are in the upper 60 cm of the soil. Lateral roots extend out as far as, or farther than, the above-ground stems. The cortex of the primary root is apparently involved in the development of secondary roots in cucurbits. Adventitious roots may arise from stem nodes in squash, luffa, bitter gourd and other cucurbits, sometimes without the stem having contact with the soil or other substrate.

Some xerophytic species have massive storage roots that enable the plant to survive severe drought. Those of Acanthosicyos can reach up to 15 m in length. The central taproot on one buffalo gourd plant weighed 72 kg (Dittmer and Talley, 1964). The above-ground parts of this species may die from lack of water or in response to freezing temperatures, but the plant regenerates from surviving stem tissue at the root–shoot transition area when favourable conditions resume.

Older vessels in the secondary xylem are often plugged with tyloses (extensions of the parenchyma cells), especially in watermelon, which can contribute to drought resistance. The sieve tubes in the secondary phloem are among the largest found in angiosperm plants.

Stems

The herbaceous or sometimes slightly woody stems are typically prostrate, trailing, or climbing, angled in cross-section, centrally hollow, sap-filled and branched. Primary and secondary branches can reach 15 m in length. Bush forms of cucurbits have much shorter internodes as well as total stem lengths than vining cultivars.

Many of the xerophytic cucurbits are true caudiciforms; that is, the lower part of the perennial stem, which is usually subterranean or at ground level, is thickened, succulent and drought resistant. In Marah, the large underground tubers originate from the hypocotyl and stem base (Stocking, 1955). The succulent stems of Ibervillea sonora (S. Wats.) Greene can continue to sprout new growth annually during periods of drought lasting 8 years or more (Macdougal and Spalding, 1910).

The vascular bundles of cucurbit stems are bicollateral (phloem to the inside and outside of the xylem), discrete, usually ten, and arranged in two rings around the pith cavity. The relatively large sieve tubes are also scattered in the cortex in some cucurbits (e.g. squash), serving to join all phloem elements together. The anomalous stem anatomy of cucurbits and other vines may serve to increase stem flexibility, to facilitate nutrient transport, to promote healing of injuries, or to provide protection against stem destruction via redundancy (Fisher and Ewers, 1991).

Many cucurbits have soft to rough hairs (trichomes) on their stems and foliage, whereas chayote, smooth luffa, stuffing cucumber and some other cucurbits are glabrous or nearly so. Trichome morphology is quite variable: hairs are glandular or eglandular, unicellular to multicellular, and simple or branched.

Leaves

Cucurbit leaves are usually simple (i.e. not divided into leaflets), palmately veined and shallowly to deeply three- to seven-lobed. There is usually one leaf per stem node. Along the stem, leaves are helically arranged with a phyllotaxy of 2/5; in other words, there are two twists of the stem, which segment contains five leaves, before one leaf is directly above another. This means that the angle of divergence between neighbouring leaves is 144° (2/5 of 360°).

Leaf stomata are mostly anomocytic, lacking subsidiary cells. The petiole in cross-section often has a crescent or ring of unequal vascular bundles, the larger ones bicollateral. Stipules at the base of the petiole are typically absent, but have been transformed into photosynthetic thorns in Acanthosicyos. Extrafloral nectaries, which frequently attract ants, occur on some cucurbit leaves (e.g. ivy gourd).

Succulent leaves are rare, even among the xerophytic cucurbits. Those of Xerosicyos have large water-storage cells in the inner mesophyll and perform crassulacean acid metabolism (CAM). However, the deciduous leaves of Seyrigia only perform C₃ photosynthesis even though the succulent stem performs CAM. The large ephemeral leaves of most cucurbits adapted to an arid environment avoid heat damage by maintaining high levels of transpirational cooling (Rundel and Franklin, 1991).

Tendrils

Most cucurbits have solitary tendrils at their leaf axils. Tendrils are unbranched in species such as cucumber and branched in luffa and other taxa. They are often coiled, helping plants to cling to trellises and other supports. Terminal adhesive pads develop on the tendrils of several species, allowing attachment to tree trunks and other large textured objects. Some cucurbits lack tendrils, e.g. squirting cucumber and bush cultivars of summer squash, while other cucurbits may have more than one tendril per node.

Tendrils in most of the cucurbit crops are interpreted as modified shoots. However, in luffa and other species, they are considered a stipule–stem complex. There are still other interpretations concerning the anatomical origins of cucurbit tendrils and research is ongoing. In cucumber, a single nucleotide polymorphism (SNP) resulted in the transition from tendrils to tendril-less, removing the plant’s ability to climb (Wang et al., 2015).

Flowers

Many cucurbits have large showy flowers that attract pollinating insects, but Echinocystis, Sechium and some other genera have small, rather inconspicuous flowers. The typically unisexual flowers occur in leaf axils, either alone or in inflorescences. They are often white or yellow, but may be red (e.g. Gurania) or other colours. The hypanthium is cup- or bell-shaped. The sepals or sepal lobes, typically numbering five, and the corolla, which is usually five-lobed and more or less fused, extend beyond the hypanthium. Flowers have radially symmetrical, bell-shaped corollas that may differ between male (staminate) and female (pistillate) flowers.

Staminate and pistillate flowers on monoecious cucumber and squash plants are originally bisexual, with both stamen and pistil primordia initiated. During ontogeny, depending on the hormonal status of the tissue near the flo...