Growers have selected for desirable traits in Jerusalem artichokes since the early days of its cultivation, with the result that a large number of cultivars and clones have been described. The tubers have been the main focus of selection, with substantial variation occurring in size, shape, color, and yield (see Chapter 4). The first tubers taken to Europe were larger than wild tubers and have been continuously selected by growers since the 17th century. However, the first systematic breeding program for Jerusalem artichoke probably dates from the early 1900s, when it was realized that the tubers could be utilized for industrial products such as ethanol.
Considerable selection of Jerusalem artichoke has occurred since it was first brought to Europe from North America in 1607. Nevertheless, for much of the 20th century, the crop was considered only of value as animal feed, a food in time of scarcity, and a relatively minor industrial crop. However, after many years of neglect, breeding programs for Jerusalem artichoke intensified during the 1980s (van Soest, 1993). This was primarily due to increased demand for inulin and fructose by the food industry, although research was also aimed at maximizing biomass productivity and improving other traits. However, breeding as well as research on the crop remains highly cyclic. Increased fuel costs or demand for low-priced fructose or inulin triggers a new round of interest that to date has ebbed with changes in status of the initial stimuli. Each new cycle of interest invariably results in some repetition of previous research and genetic improvements, decreasing the overall efficiency of progress.
Jerusalem artichokes are largely bred by public service institutions. Clones are reproduced asexually, and so once released they are readily multiplied. Hence, commercial plant breeders have no way to enforce plant breeders' rights, minimizing the chances of recouping their breeding program investment. As tubers are so readily multiplied, there is no need for farmers to regularly buy seed, while limited markets for Jerusalem artichoke products have in the past deterred commercial interest in improving the crop. Public institutions today breed Jerusalem artichokes to suit local climatic and photoperiodic conditions, and for particular applications (e.g., high inulin content for food industry applications).
In Canada, Jerusalem artichoke research has centered on the Agriculture Canada Research Station in Morden, Manitoba. Research has been aimed at increasing tuber yields, and the fructose and inulin content of tubers, in accessions adapted to the conditions of western Canada (Chubey and Dorrell, 1974). A number of Morden (M) accessions have been bred and selected in experimental trials (e.g., 'M5,' 'M6,' and 'M7'). Some of these have gone on to become commercially grown cultivars, including 'Columbia' (Chubey and Dorrell, 1982).
In the U.S., a number of small-scale breeding programs have aimed to improve Jerusalem artichoke for industrial applications, including one at the USDA-ARS, Northern Crop Science Laboratory, Fargo, ND, where research has focused on enhancing the crop's value for forage and silage (Seiler and Campbell, 2004).
In the European Union, support has been forthcoming for a number of initiatives on industrial crops, including Jerusalem artichoke; for example, the agroindustrial program of 1990 was established and co-financed by the European Community (van Soest, 1993). At the national level, several countries have institutions involved in the preservation of genetic resources and breeding of Jerusalem artichoke. European breeding programs have at one time or another been conducted in Austria, Denmark, France, Germany, Hungary, Italy, the Netherlands, Russia, Sweden, Ukraine, and other former USSR countries.
The Federal Centre for Breeding Research on Cultivated Plants, in Braunschweig, Germany, for instance, has been a major European center for Jerusalem artichoke genetic resource conservation and plant breeding since the 1980s (Kuppers-Sonnenberg, 1952; Schittenhelm, 1987). Recent research has focused on breeding to maximize inulin yields from the tubers (e.g., Schittenhelm, 1999). In Hungary, research on Jerusalem artichoke was conducted in the 1950s at the Martonvasar Institute of the Hungarian Academy of Sciences (Patzold, 1955, 1957). The main emphasis was on improved cultivation and tuber processing methods, for the production of alcohol and fructose concentrates. This research was suspended when the focus shifted to maize. However, research on Jerusalem artichoke has been revived in Hungary, particularly at the University of Horticulture and Food Industry, Budapest (Barta, 1993).
In France, research has been concentrated at Institut National de la Recherches Agronomique (INRA) institutions in Rennes, Clermont-Ferrand, and, more recently, Montpellier (Chabbert et al., 1983). Increased carbohydrate content for ethanol production has been one of the aims, and numerous crosses between cultivars held in the national germplasm collection have generated novel material for selection (Le Cochec and de Barreda, 1990). In Italy, breeding and field trials to select for enhanced tuber yields and inulin content have been conducted at ERSA (Entre Regionale di Svilippo Agricola della Regione Abruzzo). Clones selected to produce high yields in poor soils have been cultivated in Bari (Faget, 1993; De Mastro et al., 2004).
In the Netherlands, Jerusalem artichoke breeding research is conducted at the Centre for Plant Breeding and Reproduction Research-Dienst Landbouwkunding Onderzoek (CPRO-DLO) in Wageningen, where clones have been bred for increased inulin yields (Meijer et al., 1993; Mesken, 1988; Toxopeus et al., 1994; van Soest et al., 1993). In Sweden, Hilleshog AB Plant Breeding and the Swedish University initiated breeding programs to improve Jerusalem artichoke for bioenergy (Gunnarson et al., 1985).
Plant breeding in the Russian Federation, aimed at producing new cultivars and hybrids, has focused on clone selection from crosses between local populations (landraces), and between imported cultivars and local populations. Free pollination between clones has resulted in wide variation, while selection from the achenes (seed) of promising seedlings has enabled clonal lines to be established. Between 1966 and 1972, for example, at the Maikop Experimental Station, of the N.I. Vavilov Institute of Plant Industry (VIR), a program of intravarietal hybridization was carried out, which demonstrated favorable prospects for Jerusalem artichoke breeding. Established cultivars, including 'Blanc précoce' and 'Waldspindel,' were crossed with wild Helianthus material (e.g., H. macrophyllus Willd.) and other H. tuberosus material held in the VIR germplasm collection, with the aim of producing high-yielding cultivars adapted to local conditions. Among the seedlings produced from this breeding program, a large diversity of hybrid forms were noted with promising characteristics (Pas'ko, 1974). One of the cultivars bred as a result was the drought-resistant and disease-tolerant 'Vostorg' (Pas'ko, 1976).
A breeding program for Jerusalem artichoke at the Institute of Biology, Ural Division of RAS, Syktyvkar, Komi Republic, Russian Federation, has selected tall, cold-resistant, locally adapted clones with high green biomass productivity (Kosmortov, 1966; Lapshina, 1983; Lapshina et al., 1980; Mishurov and Lapshina, 1993). A new cultivar with these traits ('Vylgortski') was released by the institute in 1999 and included in the state seed list (http://ib.komisc.ru/t/en/ir/in/11-nov.html). Jerusalem artichoke breeding has also been conducted at Odessa and Kharkov, in Ukraine. For example, in Kiev (SSR Nauk) during the 1960s, a group of high-yielding hybrid clones were obtained from Jerusalem artichoke x sunflower crosses (Marcenko, 1969).
The Institute of Field and Vegetable Crops, Novi Sad, Serbia and Montenegro, maintains many wild Jerusalem artichoke accessions, collected in the U.S. and Montenegro, as part of a sunflower breeding program. Research has focused on population variability and the analysis of meiosis and pollen viability for wild accessions. A number of interspecific hybrid lines have been produced by crossing wild H. tuberosus with cultivated sunflower (Atlagic et al., 1993, 2006; Dozet et al., 1993; Vasic et al., 2002). A breeding program at the Bulgarian Institute of Plant Industry has also investigated interspecific crosses with Jerusalem artichoke (Kalloo, 1993).
The basic chromosome number in the genus Helianthus is 17. Diploid (2n = 34) species such as H. annuus and H. debilis are found, as are tetraploid (2n = 68) species, such as H. divaricatus, H. eggertii, H. hirsutus, and hexaploid (2n = 102) species, such as H. rigidus, H. macrophyllus, and H. tuberosus (Kihara et al., 1931; Kostoff, 1934, 1939; Wagner, 1932a; Watson, 1928; Whelan, 1978). Meiosis in H. tuberosus is irregular, with the second metaphase often varying in the number of chromosomes from 49 to 53 (Kostoff, 1934). Karyotype analysis of the chromosomes showed the total length of the pairs ranged from 3.90 to 2.05 pm and the arm ratios were from 2.54 to 0.52 (Pushpa et al., 1979). Twelve pairs of the chromosomes had median centromeres, 30 with submedian, and 9 with subterminal. The Xma frequencies per cell and bivalent were found to be 72.46 and 1.42, respectively.
New sources of insect and disease resistance, stress tolerance, and other advantageous traits can often be derived from other species. As a consequence, considerable interest has focused on interspecific crosses among Helianthus species as a means of obtaining such traits, in particular insect and disease resistance. Interspecific hybrids have been produced by crossing H. tuberosus with H. annuus (e.g., Davydovic, 1947; Encheva et al., 2003; Heiser and Smith, 1964; Heiser et al., 1969; Mikhal'tsova, 1985; Pas'ko, 1980; Pustovoit, 1966; Pustovoit et al., 1976; Scibrja, 1938; Stchirzya, 1938), H. hirsutus (Heiser and Smith, 1964), H. divaricatus and H. eggertii (Heiser, 1976; Heiser et al., 1969), H. strumosus (Heiser and Smith, 1964; Heiser, 1965), H. rigidus (Clevenger and Heiser, 1963; Heiser et al., 1969), H. resinosus (Heiser and Smith, 1964), and H. schweinitzii (Heiser and Smith, 1964).
In most instances, the objective has been to move critical genes into H. annuus, the dominant commercial crop of the genus (Sackston, 1992). However, such crosses also provide improved traits for cultivated Jerusalem artichoke. For example, hybrid Fj and F2 plants from crosses of H. tuberosus and the 'Tjumen' sunflower cultivar had low tuber yields, but were more drought resistant than Jerusalem artichoke controls (Murzina, 1971, cited in Kalloo, 1993). Resistance to downy mildew (Plasmopara halstedii (Farl.) Berl. & de Toni) in cultivated sunflower can be traced to H. tuberosus and several additional wild Helianthus species (Miller and Gulya, 1988, 1991; Pustovoit and
Kroknin, 1978; Pustovoit et al., 1976; Tan et al., 1992). Two Russian sunflower cultivars ('Progress' and 'Novinka') developed by Pustovoit are based on wild H. annuus and H. tuberosus crosses (Pustovoit et al., 1976). Jerusalem artichoke also displays some resistance to Alternaria (Lipps and Herr, 1986) and has been a source of resistance to brown stem canker (Phomopsis helianthi Munt.-Cvet. et al.) (Skoric, 1985), head rot (Sclerotinia sclerotiorum (Lib.) de Bary) (Pustovoit and Gubin, 1974; Ronicke et al., 2004), and broomrape (Orobanche cumana Wallr.) (Pogorietsky and Geshele, 1976) in sunflower.
H. annuus and H. tuberosus crosses give hybrids with a chromosome number of 2n = 68 (Marcenko, 1952). H. tuberosus (2) x H. annuus crosses should be made 7 days after stigma extension using fresh pollen (24 to 28°C) (Fedorenko et al., 1982). The pollen mother cells of the hybrid had 16.3% with 2n = 34, 27.2% with n = 33 and 43.6% with n = 32 (Kostoff, 1939). H. tuberosus x H. annuus hybrids have low fertility and are often sterile; pollen viability varied from 12 to 53% (Heiser and Smith, 1964; Le Cochec and de Barreda, 1990). Chromosome bridges have been observed, the frequency of which varies among studies (Kostoff, 1939; Heiser and Smith, 1964). Cauderon (1965) reported strongly asyndetic and weakly asyndetic meiosis, with both showing meiocytes with 34 bivalents. Aneuploid progeny can occur with H. tuberosus x H. annuus hybridization. Trisomic plants (2n + 1) were found in backcrosses that were resistant to downy mildew (Leclercq et al., 1970).
Inversions and other disturbances appeared to have resulted in the structural deviations. Wagner (1932a) crossed three hexaploid species (H. rigidus, H. macrophyllus, and H. tuberosus) with the diploid H. cucumerifolius as the female parent and obtained seed sets from 8 to 37%. Reciprocal crosses were unproductive. Hybrids between H. rigidus and H. tuberosus are common and fertile, and therefore allow gene flow between the two species (Pas'ko, 1975). Tuber formation was a dominant trait with 96% of the hybrids having tubers, though of 87 crosses, only 25 had Jerusalem artichoke-like tubers. The hybrids between H. annuus and H. tuberosus had lower tuber yields but higher drought tolerance than Jerusalem artichokes (Kalloo, 1993).
Large variations in pollen viability have been observed in F1 hybrids; for instance, in a study with 15 Fj hybrids it varied between 12.4 and 57.1%. Likewise, pollen viability of 180 H. tuberosus x H. annuus F1 hybrids and 170 backcrosses (BC1) was 17.2% (1.2 to 34.2% range) and 3% (0 to 11.6%), respectively (Cedendo et al., 1985). The maximum number of chromosome bridges was 6. The mean number of bivalents of the BC1 progeny was 15.2 and trivalents 1.57 per cell. Heiser and Smith (1964) found pollen stainability (viability) ranged from 12 to 53%, though most of the F1 hybrids were female sterile. Low pollen viability can be accounted for by irregularities in meiosis in the pollen of F1 hybrids (Atlagic et al., 1993).
Measures to reduce the incompatibility between Jerusalem artichoke and sunflower have been investigated at the Bulgarian Institute of Plant Industry. F1 hybrids from crosses showed great variability (e.g., in mode of branching), with some plants showing resistance to Orobanche, and others having high seed oil content. The best crossing results were obtained by pollinating with fresh sunflower pollen, at an air temperature of 24 to 28°C and 70% relative humidity (Encheva et al., 2004; Fedorenko et al., 1982; Georgieva-Todorova, 1957; Kalloo, 1993).
In many instances, the success of interspecific hybrid crosses is limited, although with embryo culture it can often be improved. In a range of crosses, the success rate was increased using a two-stage technique (Chandler and Beard, 1983). The embryos were initially developed on a solid medium, for germination, and then transferred to a liquid medium. Embryos were excised and cultured for 3 to 7 days.
A regenerating tissue culture system also facilitates somatic hybridization and somaclonal variation, expanding the potential for incorporating genetic variation. Immature embryos, approximately 12 mm2 in area, have the capacity to regenerate (Witrzens et al., 1988). Initial culture used Murashige-Skoog medium salts and organics and 30 gl-1 sucrose and 1 mgl-1 6-benzylaminopu-rine. After the third week, 0.5 mgl-1 of indoleacetic acid was added. Problems encountered included the premature initiation of flowering and the occurrence of "vitreous" plantlets that could not be successfully transplanted to a potting media. The addition of phloridzin (10 pM), esculin (30 pM), or naringin (100 pM) to the culture medium improved success. See Section 9.3 for additional information.
Selfing is seldom successful (Marcenko, 1939; Scibrja, 1937; Wagner, 1932b). Of 1028 selfed flowers, only three fertile egg cells were formed (0.29%) (Wagner, 1932b). The high self-incompatibility, however, means that emasculation is not necessary unless crosses are made using alternative species as the female parent (e.g., H. annuus). Is such cases, the tip of the flower is opened and the anthers very carefully excised using tweezers before the styles have opened (Oliver, 1910, as cited by Wagner, 1932b). Pollen grains adhering are gently removed using a fine spray of water. Four days after anther removal, the styles were dusted with fresh pollen, yielding from a 22 to 90% seed set.
Pollen collection and application methods are the same as for the sunflower. Generally, pollen from flowers that have been bagged to prevent contamination is collected using a cloth or cotton swab. Collection early in the day is preferred in that direct sunlight reduces the pollen viability (Gundaev, 1971). Fresh pollen gives the highest percentage seed set; however, it can be successfully stored for varying periods depending upon the conditions (i.e., 2 weeks at room temperature in stoppered vials (Putt, 1941); up to 4 weeks at 4 to 6°C and a humidity of <40 gkg-1 (Frank et al., 1982; Miller, 1987); 4 years at -76°C (Frank et al., 1982); 6 years in liquid N (Roath, 1993)).
There is considerable genetic variability within the Jerusalem artichoke gene pool, such that a number of desirable traits can be potentially obtained from within the species. In a study of 63 populations of Jerusalem artichokes collected in Montenegro, the clones were evaluated for 31 morphological characteristics: leaf size, plant height, uniformity of flowering, flower head inclination, head size, head shape, branching, branching type, leaf shape, leaf color, anthocyanin in leaves, leaf glossiness, leaf margin, leaf cross-sectional shape, leaf base shape, angle of leaf lateral nerves, leaf length, petiole length, pubescence, internode length, bud openness, length of bracts, pubescence of bracts, bract shape, bract size, length of bract, number of ray flowers, shape of ray flowers, color of ray flowers, color of disk flowers, and stigma anthocyanin intensity (Dozet et al., 1993). The populations represent escapes from cultivation of material largely introduced during or after World War II and displayed considerable variation with geographical location. Similar divergence was found within 19 wild populations collected within the U.S. (Dozet et al., 1994). Clones can be separated based upon branching type into monopodial, sympodial, and intermediate forms. Sym-podial branching was associated with earliness and seed production, while monopodial type gives earlier clones (Pas'ko, 1982). Similarly, crosses among 35 clones of H. tuberosus resulted in some Fjs with higher tuber yields and total fermentable sugars than controls (Frese et al., 1987), indicating the potential for genetic improvement.
Jerusalem artichoke is very easy to multiply vegetatively. Plants can have up to 50 tubers, while tuber pieces can be used for planting. However, although propagation is successfully achieved by planting tubers, clones produce little diversity for improving key traits. To produce improved lines, it is therefore necessary to propagate via sexual reproduction: by crossing to produce seed. A number of difficulties have to be overcome in crossing, including incompatibility and a reluctance to flower in the long-day photoperiods that occur in the more northern latitudes, where much Jerusalem artichoke is grown.
Seed sterility is a general problem in plants that are usually multiplied vegetatively. Past selection has focused on the vegetative organs, at the expense of the organs of sexual reproduction. Seed sterility has hampered crossing and hybridization. A high incidence of irregular chromosomes has been noted, which interferes with the efficiency of meiosis in the germ cells, resulting in sterile pollen (Kostoff, 1934).
There are three general approaches used in Jerusalem artichoke breeding: (1) controlled crosses conducted under greenhouse conditions, (2) natural open-pollinated crosses using polycross nurseries, and (3) a variation of the latter where isolated pairs are allowed to cross in the field. Each method has its advantages and disadvantages.
A major problem with using open pollination in the field is that the genetic variability that can be generated is quite restricted due to distinct differences in flowering dates. Thus, early clones have completed flowering before late-season clones have begun, preventing crossing. As a consequence, breeding for certain traits (e.g., maturity date) generally necessitates some controlled greenhouse crosses. Pollen parents are grown in growth chambers under short-day conditions (14 h artificial light). Schittenhelm (1987) found that 10 m2 of chamber area was sufficient to produce pollen for 600 to 700 crosses. In addition to expanding the genetic range of crosses that can be made, controlled crosses in the greenhouse produce substantially more seed per flower pollinated. This ranged from 0 to 5.7 seed per flower pollinated, with an average seed set of 2.68. A seed set with open-pollinated plants is much lower, owing in part to the more extreme conditions in the field. The total number of seeds that can be obtained, however, is generally far greater, and the cost per seed is substantially lower.
8.6.2 Natural Open-Pollinated Crosses Using Polycross Nurseries
Polycross nurseries involve placing selected parent clones in an isolated area with the plants positioned in a manner that facilitates all possible combinations. Crossing is by way of natural pollinators, and while the female parent is known, the pollen source is not. The primary advantage of the technique is that it requires minimal time and labor. Large numbers of seed can be produced, and the cost per seed is very low.
Open pollination produces seedlings with a great deal of genetic variation. Around 14,000 open-pollinated seeds were obtained from four early and medium-late flowering cultivars ('Columbia,' 'Bianka,' 'Précoce,' and 'Yellow Perfect') to obtain visually selected clones from over 8,000 seedlings, in a study in the Netherlands (van Soest et al., 1993). Eighty third-year clones were selected, on the basis of tuber yield and tuber composition, and from these four superior clones were selected for further field study. The tuber yields and inulin content of these clones were superior to those of the commercial cultivar 'Columbia,' demonstrating the potential for genetic improvement. The exceedingly low success rate (~0.02%) underscores the numerical advantage of breeding methods that produce large amounts of seed.
Open-pollinated flower heads from several clones were harvested and threshed to obtain seed in another Dutch study. Seed was obtained from the mainly early flowering cultivars 'Columbia,' 'Topinsol,' 'Bianka,' 'Topianka,' 'Yellow Perfect,' 'Rozo,' 'Cabo Hoog,' 'Précoce,' 'Sukossdi/Nosszu,' and 'D-2120.' Abundant yields (over 20,000 seeds in total) were recorded, so despite potential problems of meiotic disturbance, partial male sterility, and incompatibility, open pollination can produce high seed yields (Mesken, 1988). Seed dormancy was broken by storing for about 4 weeks at 2°C, and then treating with a 0.2% solution of KNO3 for 1 week at 10°C. The seeds were then kept in boxes in a soil-sand mixture under a 28°C day and 18°C night regime. A germination rate of 60% was recorded for seed from open pollination, compared to 70% from controlled greenhouse crosses (Mesken, 1988).
Crosses between isolated clones can be made in southern production locations (e.g., Spain) where the plants are under natural short-day conditions (Le Cochec, 1988). Isolation distance is the same as for sunflower, 800 m (FAO, 1961). In crossing experiments with four established clones ('K8,' 'Nahodka,' 'Fuseau 60,' and 'Violet de Rennes'), sufficient seed was obtained for a subsequent experimental program. With a total of 13,663 plants in three locations over 3 years, 5,372 achenes in total were produced. Each seed has a unique genetic composition, unlike tubers produced via vegetative propagation, and can give rise to a new and distinctive clone (Le Cochec and de Barreda, 1990). The material has been tested in breeding programs in Germany, France, and Spain, with a view to raising inulin yields and enhancing disease resistance.
For cross-pollination between clones, synchronous flowering is essential. Kays and Kultur (2005) assessed the flowering date and duration of 190 clones (Figure 8.1). Substantial genetic variation was observed, with the onset of flowering ranging from 69 to 174 days after planting, and flowering duration ranging from 21 to 126 days. The results suggested that flowering could be manipulated to some extent by planting date at lower latitudes. However, at higher latitudes growth under controlled conditions may be required to synchronize flowering.
Latitude has a considerable influence on the flowering time of particular Jerusalem artichoke cultivars. For 'Violet de Rennes,' for example, flowering date varied from June 20 to September 5 to September 30 for plants growing in Tenerife (28°N), Valencia (39°N), and Rennes (48°N), respectively, with the plants in Rennes failing to produce seed (Le Cochec, 1988). Valencia lies at the same latitude as the crop's center of origin in North America. In fact, most cultivars (excepting very early maturing ones) do not flower or set seed in Northern Europe. Therefore, flowering must be artificially induced in order to cross using these cultivars in Northern European countries.
Research in the Netherlands has involved exposing numerous clones to varying photoperiods and temperature conditions. Artificially shortening day length can induce flowering and therefore the production of seed. Mesken (1988) reported that:
• Short-day treatments of 11 hours induced flowering in most genotypes tested.
• For some clones a duration of 2 weeks was most effective; for others it was 4 weeks.
• For late-flowering genotypes given short-day treatments and grown in greenhouses, flowering was advanced by 3 weeks.
It was concluded that late-maturing clones should be given an 11-hour short-day treatment for 4 weeks. This induced nearly all the clones tested to flower in time to produce adequate pollen for crossing purposes (Mesken, 1988). To make crosses, pollen from the male parent is collected in a paper bag and pollination is carried out by hand using a small brush (Mesken, 1988).
Irradiation is used in plant breeding programs to induce mutations, whereby increasing the genetic variability with which to work. The primary disadvantage of this technique is that the percentage of beneficial mutations is generally exceedingly low.
The influence of radiation on Jerusalem artichoke was first studied during the 1950s, to assess its effects on tuber composition (Patzold and Kolb, 1957). In the 1980s, radiation was assessed as a breeding tool. Tubers irradiated with 3 krad of gamma rays produced offspring showing abnormal leaf shapes and sizes. White-skinned tubers were obtained, whereas the parent cultivar ('Violet de Rennes') had red tubers. Four plants were developed from the white-skinned tuber material, having
0 50 100 150 200 250
Beginning and end of flowering in days after planting
FIGURE 8.1 Timing of the onset and duration of flowering for individual Jerusalem artichoke clones. (From Kays, S.J. and Kultur, F., HortScience, 40, 1675-1678, 2005.) Individual clones are: 1. NC10-85, 2. NC10-8, 3. NC10-9, 4. NC10-15, 5. NC10-16, 6. NC10-18, 7. NC10-24, 8. NC10-25, 9. NC10-28, 10. NC10-32, 11. NC10-34, 12. NC10-35, 13. NC10-48, 14. NC10-88, 15. BBG 2, 16. 'Gute Gelbe,' 17. 'Waldspindel,' 18. 'Waldoboro Gold,' 19. 'Fuseau,' 20. 'Magenta Purple',z 21. 'Waldspindel',z 22. 'Jack's Copperskin,' 23. PI 547227, 24. NC10-5, 25. NC10-7, 26. NC10-14, 27. NC10-26, 28. NC10-41, 29. NC10-62, 30. NC10-78, 31. 'Mahlow rot,' 32. 'Bela,' 33. 2327, 34. 'Stampede',z 35. NC10-73, 36. 'Waldoboro Gold,' 37. PI 503276, 38. PI 503277, 39. 'Fuseau' (Idaho), 40. NC10-4, 41. NC10-6, 42. NC10-11, 43. NC10-13, 44. NC10-46, 45. 'Stampede,' 46. 'Remo,' 47. 'Columbia,' 48. 'Top,' 49. 'Novost,' 50. KWI 204,z 51. 12/84,z 52. 952-63,z 53. 'Dwarf Sunray',z 54. 'Orrington',z 55. 'Miles #1,' 56. 'Urodny,' 57. 'Nora,' 58. C 2071-63, 59. 'Nakhodka,' 60. PI 503269, 61. PI 503274, 62. NC10-52, 63. NC10-83, 64. NC10-84, 65. HEL 63 'Gibrid,' 66. 228-62, 67. 2071-63, 68. 'Leningrad,' 69. 'Dave's Shrine,' 70. 'Long Red McCann,' 71. 'Grem Red,' 72. 9, 73. 'Mari,' 74. PI 503272,z 75. PI 503279, 76. PI 503283, 77. NC10-29, 78. NC10-40, 79. NC10-70, 80. NC10-81, 81. 'Deutsche Waldspindel,' 82. 'Medius,' 83. 'Fuseau 60,' 84. 'Nahodka,' 85. 'Onta,' 86. D19-63-122, 87. D19-63-340, 88. 'Nakhodka,' 89. 'Grem White,' 90. 'Volga 2,' 91. 'Nescopeck,' 92. 'Southington Pink,' 93. 'Austrian Wild Boar',z 94. 'Fuseau',z 95. D 19,z 96. PI 503265, 97. 'Gold Nugget,' 98. 'Clearwater,' 99. NC10-44, 100. NC10-100, 101. BBG 1, 102. 'Mahlow Gelb', 103. 'Tambovski Krasnyi,' 104. 'Sachalinski Krasnyi,' 105. HEL 53,z, 106. 'Rozo',z 107. 'CR Special,'z 108. 'Skorospelka,' 109. 'CR Special,' 110. PI 503262, 111. PI 503280, 112. NC10-75, 113. NC10-82, 114. Unknown, 115. 'Brazilian White,' 116. 'Sunchoke,' 117. D 19, 118. 'Violet de Rennes,' 119. 'Parlow Gelb,' 120. 'Gross Beeren,' 121. 'Neus,' 122. 'Maikopski,' 123. HEL 68, 124. 'Medius,' 125. BT3, 126. 'Castro',z 127. 'Totman',z 128. 'Cowell's Red',z 129. 'Olds',z 130. 'Sko-rospelko',z 131. 'Clearwater',z 132. 'Refla',z 133. 'Vanlig'z, 134. 'Swenson,' 135. Hybrid 120, 136. 'Monteo,' 137. 'Freedom,' 138. 'Mulles Rose,' 139. NC10-22, 140. 'Challenger,' 141. BT4, 142. dwarf, 143. 'Drushba,' 144. 'Sunrise,' 145. 'Susan's Yard,' 146. 'Gurney's Red,' 147. 'Swenson,' 148. 'Wilton Rose',z 149. 'Reka',z 150. 'Coldy Mille,' 151. PI 503266, 152. NC10-94, 153. NC10-99, 154. 'Firehouse,' 155. 'Sodomka,' 156. 'Boston Red,' 157. 'Whitford,' 158. 'Jack's Copperskin,' 159. K 24,z 160. 'Leningrad,' 161. PI 503271, 162. 'Boyard,' 163. 'Jack's White,' 164. 'Silverskin,' 165. NC10-76, 166. 'Kierski Beli,' 167. 'Sugar Ball,' 168. 'Drown's Long Red,' 169. 'Flam,' 170. 'Draga',z 171. 'Sunchoke',z 172. 'Garnet,' 173. PI 503275, 174. NC10-90, 175. NC10-92, 176. 'Challenger,' 177. HEL 69, 178. Hybrid 120, 179. 'Beaula's,' 180. 'Karina,' 181. 'Bianca,' 182. 'Cross Bloomless,' 183. 'Vadim,' 184. 'Schmoll',z 185. J.A. 61z, 186. PI 503264z, 187. 'Roter Topinambur,' 188. 'Lucien's Painting,' 189. 'White Crop,' 190. 'Kiev White.' Superscript z indicates clones in which the duration of flowering is missing, having succumbed to Sclerotium rolfsii Sacc. before flowering was complete.
unbranched stems that were thinner than the controls; one had uniformly white tubers, the other three tubers with some pigmentation. The use of irradiation produced some plants that were capable of sexual reproduction, unlike untreated controls (Coppola, 1986).
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