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Published in Issue No. 126, page 41 to 45 - (19631) characters
Characterization of the Cucurbita pepo collection at the Newe Ya’ar Research Center, IsraelH. S. Paris
Cucurbita pepo is perhaps the most variable species for fruit characteristics in the plant kingdom (Duchesne 1786; Naudin 1856). It is native to North America , where it has been cultivated for at least 10 000 years (Smith 1997), but was introduced to Europe only about 500 years ago (Whitaker 1947). This species includes edible-fruited forms, known as pumpkins and squash, and small-fruited, often bitter, non-edible forms, known as gourds. Much of the variability in fruit characteristics among cultivated C. pepo can be attributed to the different quality characteristics needed for the culinary use of the mature fruit flesh and seeds as opposed to the use of young fruits. Today, C. pepo is among the economically most important vegetable crops worldwide and is grown in almost all temperate and subtropical regions ( Paris 1996).
The Cucurbita pepo collection at the Newe Ya’ar Research Center was begun in 1978 as part of a breeding programme. Initially, samples were collected on the basis of horticulturally desirable characteristics. Later, emphasis was placed on obtaining samples from a wide variety of locations and sources, especially of named open-pollinated cultivars, so as to understand better the history, variability and potential of C. pepo germplasm.
Many of the original seed samples were small and were maintained in the collection through self- and sib-pollination. Although such inbreeding results in a narrowing of the genetic base of the original material, it was the only practicable method available for reproducing and maintaining the genetic material. This method was also employed for commercial hybrids, as this was the only way of maintaining their genetic constitution. At present seeds of some 320 accessions are available for distribution.
This paper describes the breakdown of these 320 accessions into subspecies, cultivar-groups and, for the open-pollinated sorts among them, continents of origin, and in so doing aims at contributing to better understanding of how this crop species has developed.
Materials and methods
Seed samples were obtained over the past 22 years from commercial sources, seed cooperatives, genebanks, geneticists, botanists, breeders, plant introduction organizations, extension agents, family members, friends and travellers, almost entirely in North America , Europe and Asia . The samples obtained from commercial sources included cultivars described and illustrated in catalogues of seed companies. Mostly, these were modern hybrids. Information was scanty for most of the old cultivars and landraces. Some samples had no name. For many, the location from which it was collected, other than the country, was not known.
Nearly all the samples have been grown out in the field at least twice. Although only a small part of the collection could be grown out in a single season, the conditions at Newe Ya’ar allowed for two seasons in the field per year, a spring–summer season (sowing in March or April) and a summer–autumn season (sowing in late July or early August). Cultural practices included direct sowing in the field, drip irrigation, preplant and drip fertilization, and either bare ground (spring–summer seasons) or silver plastic mulch (summer–autumn seasons). Based on information available before growing out the samples, similar accessions were grouped together in the same season and next to or near one another in the field for the purpose of comparison. Plants were observed for stem colour, leaf size and shape, presence or absence of silver mottling of the leaves, vine or bush growth habit, presence or absence of branching, relatively closed or open growth habit, fruit shape, developmental fruit colour and other characteristics.
There is much confusion regarding names of cultigens in many crop species, Cucurbita pepo among them. Often, different names have been used for the same cultivar and the same name for different cultivars. For the purpose of this study, two samples were considered to be from the same cultivar if their plants did not differ phenotypically from one another in at least one trait, as expressed by the majority of the plants grown out from each sample, even if the two samples bore different names. If the plants of two samples differed in one or more phenotypic traits (as expressed by the majority of the plants of each sample), the samples were considered to be different cultivars, even if bearing the same name. For example, the ‘Grey Zucchini’ samples I obtained from the U.S.A. had plants that were identical phenotypically to the sample of ‘Faentina’ from Italy and to those of ‘Verte Petite d’Alger’ from France : the three names are used for the same cultivar. On the other hand, the plants of the sample of ‘Verte Non-Coureuse d’Italie’ from one French company were distinct from those grown from a sample of identical name from another French company. These I considered to be separate cultivars.
Another problem of nomenclature is presented by the misleading names of some cultivars. For example, the ‘Grey Zucchini’ of the U.S.A. has short, tapered, cylindrical fruits and thus is a cultivar of vegetable marrow and not a zucchini (Fig. 1). Likewise, ‘Golden Zucchini’ from South Korea is not the same as ‘Golden Zucchini’ from the USA . The latter has uniformly cylindrical fruits and is indeed a zucchini, but the Korean cultivar has long bulbous fruits and is, therefore, a cocozelle cultivar.
Classification of samples
Beginning with Duchesne (1786), there have been a number of attempts at subspecific classification of Cucurbita pepo. I prefer to use a two-tiered, botanical-horticultural approach ( Paris 1996). Botanically, the species is divided into three subspecies, based on allozyme variation, seed morphology and various phenotypic characteristics (Decker 1988): subsp. pepo, subsp. ovifera and subsp. fraterna, the last of these representing wild Mexican C. pepo gourds. The first two include gourds as well as edible forms. Horticulturally, the edible-fruited forms are divided into cultivar-groups (Trehane et al. 1995), based on the highly polygenic characteristic of fruit shape. There are eight such groups: Cocozelle, Pumpkin, Vegetable Marrow, Zucchini, Acorn, Crookneck, Scallop and Straightneck (Paris 1986; Figure 1). The first four groups belong in subsp. pepo and the latter four in subsp. ovifera. Of the non-edible forms, the ball, orange and warted gourds are more closely allied with subsp. pepo whilst the egg and pear gourds, as well as wild gourds of the United States , belong in subsp. ovifera (Decker 1988). Recent results, obtained using cluster analysis of an inter-simple sequence repeat multilocus marker system of the DNA (Katzir et al. 2000), are consistent with this classification.
The 320 accessions were observed and classified according to subspecies, cultivar-group and whether they are open-pollinated or hybrid. No attempt was made to determine whether the purported hybrids were indeed such, or were merely said to be so by the seed company. The open-pollinated forms were further classified according to continent of origin. The hybrid sorts were not classified as to geographic origin because for many of them that is uncertain; many have been developed by multinational companies and their subsidiaries and are commercially available in many countries, but the parents and their countries of origin are kept secret.
Of the 320 samples in the C. pepo collection, 241 (75%) are C. pepo subsp. pepo (Table 1; Fig. 2). Of these 241, 217 (90%) are classified into one of the four edible-fruited cultivar-groups of this subspecies. The remaining 24 are gourds (non-edible fruited) or classified as ‘mixed/intermediate’ or as ‘unique’. The Zucchini Group has by far the largest number of cultivars. Each of the other edible-fruited groups of C. pepo subsp. pepo has over 40 representatives, more than twice the number of any of the cultivar-groups of C. pepo subsp. ovifera. Each of the C. pepo subsp. pepo cultivar-groups has representatives from three or more continents. The mixed/intermediate C. pepo subsp. pepo accessions are not uniform or they have intermediate fruit shape; most are from Yugoslavia and Turkey . The unique forms are the two edible-fruited but small, gourd-size ‘Little Gem’ and ‘Rolet’ from South Africa .
There are 69 samples of C. pepo subsp. ovifera, 51 (74%) of which are classified into the four edible-fruited cultivar groups of this subspecies (Table 1). The remainder of the C. pepo subsp. ovifera are gourds or classified as ‘unique’. The unique forms are ‘Delicata’ and similar forms from the northern U.S.A. and ‘Jack Be-Little’. C. pepo subsp. ovifera contains a greater proportion of gourds (19%) than does C. pepo subsp. pepo (2%). Of the C. pepo subsp. ovifera gourds in the collection, six are cultivated whilst the other seven are wild forms. The collection also contains two samples of wild gourds from Mexico (C. pepo subsp. fraterna), two commercial hybrids of C. pepo subsp. pepo x C. pepo subsp. ovifera and six ornamental mixtures of gourds from the two cultivated subspecies.
The Zucchini Group contains far more cultivars than any other group, but 70 of its 81 cultivars are hybrids (Table 1). Of the C. pepo subsp. pepo cultivar-groups, the zucchini contains the fewest number of open-pollinated cultivars, 11, but this number is nevertheless greater than the average number of such sorts possessed by the cultivar-groups of C. pepo subsp. ovifera. The Zucchini and the Straightneck Groups have the highest proportion of hybrid cultivars: 86 and 78%, respectively. For the other groups, the proportion of hybrid cultivars is distinctly lower. Only four of the 43 pumpkins are hybrids and none of the mixed, unique and gourd forms is.
The Pumpkin Group contains the most open-pollinated cultivars (39), followed by the Cocozelle Group (31). The majority of the open-pollinated forms of C. pepo subsp. pepo are European. The cocozelles and vegetable marrows have mainly an Old World distribution whilst the pumpkin and zucchini cultivars have approximately equal numbers of cultigens in the Old and New Worlds. In contrast, the majority of the open-pollinated sorts of C. pepo subsp. ovifera are North American.
Before attempting to interpret what the content of the Cucurbita pepo collection at Newe Ya’ar might mean historically and with regard to crop development, the biases that may have affected the content of the collection need to be considered. The collection began with the goal of breeding improved forms of C. pepo, especially zucchini, pumpkins, acorn squash, scallop squash and, later, cocozelle and vegetable marrow squash. Initially, emphasis was placed on gathering cultivars from American seed companies, whose catalogues were easily available. Later, the collection was supplemented by germplasm ordered from the United States Plant Introduction System and other collections, as well as Italian seed companies and cooperatives and colleagues from North America, Europe and Asia, and by myself and friends who had visited various localities and purchased seed packets.
For many of the accessions, only a small quantity of seeds was supplied and these had to be reproduced in order to assure their being maintained. Some, most notably plant introductions from Mexico , were very late to flower and set fruit and therefore could not be reproduced and maintained. Thus, it could be expected that this collection would be biased against crooknecks, straightnecks and gourds, local cultivars from areas to which I have not had access, and late Mexican forms. On the other hand, this inherent bias is tempered by the fact that for the past few years I have made a concerted effort to obtain seeds of all named open-pollinated C. pepo cultivars. Another tempering of the bias is that some common cultivars are grown in various, perhaps unexpected places, under a local name or no name at all. For example, I received an unnamed seed sample from Nepal in 1982. This was a cocozelle cultivar then unknown to me, and I listed it as ‘Cocozelle Nepal ’. However, in 1989, I received a sample of ‘Romanesco’ from Italy and when I compared it with the Nepalese sample, I discovered that they had identical plants and fruits. Yet another tempering factor is the fact that many hybrids are short-lived and cannot be reproduced because their parents are not publicly available. Thus, the collection contains several crookneck and straightneck hybrids that no longer exist. However, newer ones have replaced them and the number of extant cultivars in these groups appears to have changed little in 20 or 30 years.
C. pepo hybrids, first promulgated by Curtis (1940), have been available commercially for half a century. The number of hybrid cultivars and the proportion they form of the total number of cultivars provides an indication of how heavily a given cultivar-group has been bred since the commercial introduction of hybrid cultivars in the 1950s. From Table 1, it is clear that the zucchini, with dozens of hybrids, is far and away the most intensively bred C. pepo. Hybrid zucchini squash are offered in catalogues of seed companies from North America , Europe and Asia . The straightneck squash is a small group of cultivars but, nonetheless, most straightnecks are hybrids, as are a fair proportion of crooknecks; these necked squash hybrids have been developed and commercialized by American seed companies.
Most of the open-pollinated sorts pre-date the hybrids and therefore give a better indication of the possible geographic origins of the cultivar-groups of this species and of their state of development before the 1950s. The Pumpkin, Cocozelle and Vegetable Marrow groups have by far the greatest number of open-pollinated sorts. I have observed great genetic variation, expressed in both vegetative and reproductive characteristics, among the open-pollinated cultivars in each of these groups (Paris 1996), suggesting that each has been cultivated for a considerable length of time. The majority of cocozelles and vegetable marrows are from Europe , suggesting that these groups developed there between 50 and 500 years ago. There are nearly equal numbers of pumpkins on the two sides of the Atlantic Ocean . Those of Mexico are often grey–green or black–green and grey–green striped with thick lignified rinds; those of the U.S.A. are orange at maturity, grooved and not lignified. Those from Europe are slightly ribbed, often black-green and orange striped, with thin lignified rinds; others have bush plants and are small, black-green or orange, with thin lignified rinds. Thus, distinct kinds of pumpkins have been developed in both North America and Europe . The zucchini group contains fewer open-pollinated cultivars than any of the other C. pepo subsp. pepo groups. Perhaps other open-pollinated zucchinis had existed formerly but became obsolete with the advent of many improved hybrids. I have not observed as much variation among the extant open-pollinated zucchini cultivars as I have observed for the other C. pepo subsp. pepo groups. Thus, it appears that the zucchini group is a relatively recent development.
The cultivar-groups of C. pepo subsp. ovifera contain fewer open-pollinated cultivars than their counterparts of C. pepo subsp. pepo. Again, it is possible that more cultivars had existed but were replaced by modern hybrids. On the other hand, except for the scallop group, these are almost entirely North American in their distribution; only the scallops have any commercial importance elsewhere. The small number of open-pollinated cultivars of these groups can be attributed to their limited geographical range of popularity.
The collection is maintained at the Newe Ya’ar Research Center in a cool (10°C), relatively dry (50% R.H.) seed storage chamber. Periodically, the collection has been and will continue to be renewed by self- and sib-pollination. The collection, in addition to being a germplasm source for breeders, is expected to be invaluable for obtaining a better understanding of relationships within C. pepo, especially using the latest techniques of DNA analysis.
The list of the 320 accessions in the collection is maintained in a database using Excel ® for Windows ™ format. The list includes the name of each cultivar and its synonyms, seed source, whether the existing seed stock is original or from an increase, whether it is open-pollinated or hybrid and cultivar-group affiliation. This list can be obtained in printed or electronic form from the author. Small seed samples are available to all interested in the genetics, breeding and history and development of C. pepo, on formal written request.
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