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Thompson, M.M. 1993. Exploration and exploitation of new fruit and nut germplasm. p. 155-160. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Exploration and Exploitation of New Fruit and Nut Germplasm

Maxine M. Thompson


  1. FACTORS LIMITING RAPID PROGRESS IN FRUIT BREEDING
    1. Exploration in Central Asia
    2. Plant Exploration in China
    3. Plant Exploration in Eastern Europe
  2. CONCLUSION
  3. REFERENCES

There is a critical need for exploration, introduction and exploitation of new and more diverse fruit and nut germplasm because of the extremely narrow genetic base that exists in our major crop species. Some United States fruit and nut industries are based primarily on one or two major cultivars, e.g. 'Montmorency sour cherry', 'Bing' and 'Royal Ann' sweet cherry, 'Tilton' and 'Blenheim' apricot, 'Bartlett' pear, 'Barcelona' hazelnut, 'Kerman' pistachio, and 'Hayward' kiwifruit. In other crops, such as blueberry, strawberry and peach, where breeding efforts have produced a greater number of commercial cultivars, levels of inbreeding, which have been documented recently, were found to be unacceptably high (Hancock and Siefker 1982; Sjulin 1987; and Scorza et al. 1985). Many breeding programs have reached the point where infusion of new and more diverse germplasm is essential for achieving modern objectives for new cultivars with more efficient fruit production and environmentally safer orchard operations and which will satisfy future consumer demands.

FACTORS LIMITING RAPID PROGRESS IN FRUIT BREEDING

In contrast to the marvelous progress achieved through breeding in major field crops over the past several decades, fruit breeding efforts have resulted in relatively few success stories. First, it is obvious that the length of each generation along with the large space and intensive labor necessary to grow even a limited number of progenies to reproductive age requires a huge investment of both time and money. Evaluation of replicated trial plots of advanced selections further extends by several years the time before a new cultivar can be released to the public. Thus, the minimum time from making the cross-pollination between the ideal parents until the release of a new cultivar is at least 20 years. It is not uncommon for fruit breeders to spend their entire careers making crosses and evaluating seedling populations only to have the next generation breeder release new cultivars resulting from crosses made 20 to 30, or more, years previously. The financial burden of such prolonged selection progress is a major factor in the decline of the number of publicly-funded fruit breeding programs (Brooks and Vest 1985). The paucity of short-term, publishable results may not only delay professional advancement of faculty but is a major obstacle to attracting would-be plant breeding graduate students.

Secondly, the lengthy quarantine process required for many fruit species has discouraged breeders from introducing new germplasm. Clones are usually retained for virus testing at the Quarantine Center at Glenn Dale, Maryland for 6 to 10 years. Many do not survive at the Center due either to propagation failures, cultural problems or, diseases such as fireblight on pears and apples. However, progress in facilitating movement of plants through quarantine is promised now that this facility is being managed jointly by the National Germplasm Resources Laboratory (NGRL) and the Animal and Plant Health Inspection Service of the USDA (APHIS) and more funds and staff are being allocated for this important facet of plant introduction.

A third major limiting factor for breeding progress, one not so obvious to the non-fruit scientist, is the limited sampling of the potential genetic diversity that has been available to American breeders. For historical reasons, much of the fruit and nut germplasm existing in the United States has been derived from introductions from Western Europe which, itself, is relatively poor in native populations of these species. Lack of access to the three major Asian centers of origin and diversity for temperate fruit and nut crop species has presented a formidable obstacle to obtaining and utilizing the great wealth of unexploited fruit germplasm found there. In the Soviet Union, the Caucasus Mountain region is one of the centers of diversity for apple, European pear, quince, sweet cherry, cherry plum, fig, pomegranate, walnut, hazelnut, chestnut, pistachio, and grape. The Central Asian region, including the Republics of Turkmenistan, Uzbekistan, Tadjikistan, and Kazakhstan, is another center for apple, apricot, cherry, plum, fig, pomegranate, almond, pistachio, walnut, and grape. China is well known as the center of diversity for peach, apricot, oriental plum, Asian pear, persimmon, citrus, litchi, Chinese chestnut, and kiwifruit. The multitude of diverse local cultivars, wild forms of cultivated species and many related species in the Soviet Union and China have been "off-limits" for about 70 years for political reasons. The recent opening of these two countries provides the heretofore unavailable opportunity to collect exciting new genetic traits unknown, or rare, in western collections. Exploration for fruit and nut germplasm in these newly opened regions is of primary importance in order to provide the much-needed genetic diversity. In this paper, I shall focus mainly on the potential value of new germplasm acquired through recent plant explorations for a few fruit and nut crops whose breeding progress has been particularly restricted by inadequate germplasm.

Exploration in Central Asia

In 1930, Nicolai Vavilov informed the western world about the wild populations of fruit species in the Soviet Union in a paper presented at the International Horticulture Congress in London entitled "Wild progenitors of the fruit trees of Turkistan and the Caucasus and the problem of the origin of fruit trees" (Vavilov 1930). Since that time, Soviet scientists have studied and written many publications describing the variability and distribution of various wild populations of fruit and nut species. They have established extensive collections of local cultivars and species, but these plant materials remained inaccessible to the West. Fortunately, in recent years, exchange of scientists and germplasm between the Soviet Union and Western countries, including the United States, has been escalating. In fact, a 5-year reciprocal exchange program for plant exploration is currently underway between the Vavilov Institute for Plant Industry (VIR) in St. Petersburg and the U.S. National Plant Germplasm System.

Apple exploration in the Soviet Union. In 1989, Calvin Sperling and Herbert Aldwinkle visited the wild apple forests near Alma Ata in Central Asia and observed an enormous range of diversity in such traits as fruit size, color, shape, and ripening time, from small wild-type to large-fruited types comparable to commercial cultivars. Prof. A.D. Djangaliev of the Kazakh Academy of Sciences has studied these wild apples and has propagated selections from the forests at the Botanical Garden in Alma Ata. There is sufficient regional variation in botanical characteristics that Soviet scientists have classified the apples there into three species: Malus sierversii (Ldb.) M. Roem, M. kirghisorum, Al. & An. Theod. and M. niedzwetzkyana Dieck. Among several hundreds of young seedlings already growing in Geneva, N.Y. from seeds collected in the mountains near Alma Ata, genetic resistance to three major apple diseases, apple scab [Venturia inaequalis (Cooke) Wint.], cedar apple rust (Gymnosporangium juniperi virginianea Schweim), and fireblight [Erwinia amylovora (Burr.) Winslow et al.], has been identified (H. Aldwinkle pers. commun.). These plants may provide distinctly different genetic sources of resistance from those currently available in the United States. Fruit of these seedlings and of the clonal selections introduced will be evaluated when trees reach reproductive age, and when the clonal germplasm is released from quarantine, it will become available for utilization.

Apricot exploration in the Soviet Union. In 1990, I was fortunate to join Calvin Sperling and David Ramming on a second official fruit exploration to Central Asia, this time for apricots. That year, among 633 apricot cultivars in the collection at the VIR Station in Tashkent, Uzbekistan, about 95% had lost their crop due to a late spring frost. Although we were disappointed not to see more fruit variability, this climatic event provided a natural screen for spring frost tolerance. We collected seeds and scions of several cultivars that bore some fruit in spite of adverse spring temperatures. The trait of late bloom, or frost tolerance, provides a genetic solution to the most limiting factor for expanding domestic apricot production beyond the very restricted climatic zone where our cultivars are adapted. At a period when most cultivars, if they had a crop, would have already been harvested, there was 'Zima Stoiki' (= winter hardy) with a heavy crop of very immature fruit, obviously late blooming as well as very late maturing. Late fruit maturity, also seen in the cultivar 'Oktoberski', will contribute to new cultivars with ripening periods extending far beyond the 5-week span in early summer of current American cultivars.

Several Central Asian cultivars, collectively called "luchak," have glabrous skins, a trait previously unobserved in American cultivars or germplasm collections. This glossy appearance, sometimes with a bright red blush, represents an entirely new, and especially attractive fruit type. Should this trait be determined by a single gene, as is the case for glabrous peaches (nectarines) it should be possible to repeat within a fairly short time period the successes achieved in nectarine breeding, i.e., the creation of a whole series of large-fruited, glabrous apricot cultivars with a wide span in ripening periods.

Another trait of interest was high soluble solids, a trait that greatly enhances the quality of both the fresh and dried product. By contrast to American cultivars whose soluble solids register about 12deg. Brix, Central Asian cultivars range above 20deg.. Historically, in Central Asia there has been strong selection pressure for very sweet fruit because there was no other source of sugar.

Also, sweet, edible seeds is a common trait which has been selected through the centuries by local people whose goal has been to maximize food production on a limited amount of tillable land. A unique trait that was said to occur in a local cultivar, one which we did not see or collect, is an endocarp (the hard part of the pit) that is so thin that it can be cracked with one's teeth. Incorporation of edible kernels in new duo-purpose domestic cultivars would certainly enhance orchard profits and provide a new nutritional product for American consumers.

In the Zailinsky mountains near Alma Ata, we had the opportunity to visit the wild apricot forests with Tatanya Nicolaievna Sulova, a botanist who has studied variation and made selections from among these trees. Wild apricot forests grow in some of the same general regions as the wild apples but at a somewhat lower elevation. At about 1,060 m, apricot trees merge into the lower limits of the range of apples. These wild populations occur at the northernmost range of this species and are clearly more cold hardy than Central Asian cultivars which can be grown only in milder climates farther south. In one extremely cold winter in Alma Ata, wild trees survived mid-winter temperature of -43deg.C, whereas trees of cultivars perished. Most of the wild fruit have bitter seeds and is small, but size varies considerably as do all other traits such as color, time of ripening, and quality. This year when most trees were severely attacked by Coryneum blight [Stigmina carpophila (Lev.) M.B. Ellis], some of Tanya's selections were practically free of symptoms, a trait she has observed consistently over several years. This may be a valuable source of genetic disease resistance to incorporate in new domestic cultivars.

Walnut germplasm in the Soviet Union. There are vast forests of wild walnuts in the Tien Shan mountains in the Soviet Union which are located west of the province of Xinziang in China. While on the apricot expedition last year, we received information about the walnut collections and selection programs underway at various institutes in Kirghizia, Tajikistan, and Uzbekistan. Clones having clusters with 10 to 20 nuts, precocious trees, dwarf trees, and paper-shell nuts were mentioned. However, at that time, as we were unable to visit these Institutes, we obtained only some market samples of diverse, unknown origin. Hopefully, an expedition can be arranged soon to explore further this exceptionally rich Central Asian center of diversity for walnuts.

Apricot exploration in Northern Pakistan. In 1987, David Brenner and I studied and collected Central Asian apricots in another geographic region--the Karakoram and Himalayan mountains of Northern Pakistan. Among these extremely high, precipitous mountains, in very small river valleys, lie several former mini-kingdoms, each isolated from the other and from the outside world until very recently. One of these, Hunza, is famous for supposedly having exceptionally long-lived people. Through centuries, as peoples filtered into these mountain valleys from various regions in Central Asia they brought apricots which, for them, was a main staple food, providing fresh fruit throughout the summer, dried fruit and edible kernels for winter, oil from bitter seeds for lamps, and firewood in this relatively treeless land. The practice of planting seeds from the best trees over an extended period of time has resulted in an incredible amount of variation. Many years ago, they learned to graft so now, in each village, in addition to seedling trees, one finds many favorite local cultivars. While some exist in only one village, others are distributed more widely, but primarily within the confines of each former kingdom. In the several villages we visited, we encountered 180 different named cultivars which is only a sample of the variation that exists in the region as a whole. Overall, fruits are characterized by very high soluble solids, sweet seeds, and relatively small size. Fruit size is not important there. Selection has been for quality and total productivity.

In Hunza, one of the larger-fruited cultivars, 'Habiju', has outstanding quality for both fresh and dried usage, with high soluble solids (averaging about 23deg. Brix), pronounced aroma, and rich flavor. 'Alishah Kakas' was another favorite in Hunza because of its exceptionally high soluble solids (31deg. Brix), fine quality, and firm texture making it suitable for shipping fresh, as well as excellent for drying.

In Baltistan, 'Margulam' was prized as a fresh fruit for its juiciness, sweetness, and fine flavor, whereas 'Halmon' was the best for drying due to its high soluble solids (30deg. Brix) and rapid drying characteristic. 'Kachachuli' was unique in that, although the fruit has relatively high soluble solids (22deg. Brix), the flesh reaches a moderate degree of firmness but does not soften further with age. In fact, the local name actually means "apricot that doesn't ripen." It reminded me of the non-ripening trait in our shipping tomatoes. Locally, 'Kachachuli' is grown mainly for its large edible seed, but I envision this non-ripening trait incorporated, through breeding, into larger, high quality cultivars with remarkable storage and shipping characteristics. A local storage cultivar, 'Sharappa Margulam', was said to hold its quality until March if stored underground (the only method of temperature control available). The introduction of a longer storage life into new cultivars could greatly extend the apricot marketing period, as could a series of cultivars ripening successively. As in the Soviet Union, we found cultivars ripening over a 3-month period, from late June until late September. A useful trait for a home-garden fruit tree was found in 'Rangbuon' whose fruit ripens over a 5-week period on the same tree. Although a record of bloom time was not available, we were told that some were two weeks later than others and it is most likely that the late maturing cultivars do bloom considerable later.

In Haripur, the lowest elevation site (540 m), low chilling requirement may be found in the popular cultivars 'Nukap', 'Lala', 'Boi', and 'Safeda'.

Walnut exploration in Northern Pakistan. Diversification of walnut germplasm was increased by random sampling collections that we made in the mountains of Northern Pakistan and adjacent regions in Kashmir, India. In this region, there are walnut trees, all seedlings, in every village from about 1,000 to 3,000 m elevation, some of which are very ancient. In this region, both wild (with inedible nuts) and cultivated forms of Juglans regia L. are distinguished. While we did not study walnuts in depth there, we did observe an enormous amount of variation in nut size, shape, and shell thickness. From one tree, we collected nuts which had very thin, porous, incomplete shells and, in two different cases, we heard about a tree with nuts having no shell at all, merely a membranous covering around the shell. Loy Shreve has found resistance to walnut blight and leaf fungal diseases in a clone introduced earlier from Pakistan. The original tree was notable for heavy, regular bearing, freedom from diseases, and for having a large nut with good quality. Grown in southern Texas, it has proven to exhibit these desirable traits as well as the ability to retain an attractive light kernel color in the hot climate there. Several small plantations of this selection, termed 'Abbotabad' (from the city where it was found) are already growing in Texas and appear to show good potential (Loy Shreve pers. commun.).

Plant Exploration in China

Walnut germplasm in China. Three walnut exploration trips to China provide another example of recent success in securing much-needed, greatly expanded germplasm diversity. Abundant wild walnut forests are distributed throughout the Tien Shan mountain range, which lies mostly in the Central Asian Republics of the Soviet Union, but which extends into northwest Xinziang Province in The People's Republic of China. Walnuts are widely cultivated in several provinces, and there are several research programs directed towards improving cultural methods and selection of new cultivars (Ji 1980).

In 1986, Loy Shreve was the first horticulturist to visit Chinese walnut research stations from which he succeeded in obtaining about 20 selections, mostly from Xinziang, which are now under test in the warm climate of Texas. Two years later, in 1988, Bill Gustafson, Todd Morrissey, and C. Bish went to both Xinziang in Northwestern China and to Jilin and Inner Mongolia in the Northeast with the major goal of locating cold-hardy nut germplasm suitable for Nebraska. They were able to collect 78 seedlots of horticultural/forestry plant materials including walnuts, almonds, hazelnuts, pistachio, and several other genera. Additional scions and seeds were subsequently sent by the Chinese. Representative plants of these accessions are now under test in Nebraska and elsewhere.

In 1990, Gale McGranahan, Charles Leslie, and William Barnett visited Xinziang and the vicinity of Beijing with the objective of collecting more diverse germplasm for the California breeding program. Although several new walnut cultivars have been released from the California program over the past several years, all of these have been derived from a single parent, 'Payne'. Until now, this was the only clone available which has, and transmits to its progeny, the unique trait of bearing nuts on both lateral and terminal positions. This characteristic results in precocious bearing and much heavier yields than other walnuts whose nuts are borne only terminally. As walnut is one of the orchard crops most delayed in coming into full production, one of the most valuable finds in Xinziang was the extreme precocity of many selections. In the vicinity of Aksu, virtually all of the walnuts had the lateral bearing habit and many seedlings are so precocious that they set nuts in, or before, the third growing season, or during the first year after grafting. Another useful reproductive trait in some clones is the occurrence of two or more waves of flowering, which could be a valuable aid in avoiding spring frost damage. The trait of multiple-nut clusters, commonly 8 to 12 nuts on spikes, as compared to domestic cultivars with 2 to 3 per cluster, offers the potential for greatly increased yields. Among these clones bearing several flowers on spikes, some spikes bear only pistillate flowers, some only staminate flowers, and some bear both. Of even greater significance, in some clones, spikes had some perfect flowers. Strong selection pressure for perfect flowers has the potential of eliminating the need for planting two cultivars for cross-pollination in orchards, currently a necessary practice for this normally dichogamous species. Of particular importance, is that many of the Chinese selections are resistant to important diseases, e.g., walnut blight, anthracnose, and various leaf fungal diseases. Extreme early maturity, with nuts harvested in mid-August, was also observed. Scions of several selections with outstanding traits as well as large numbers of seeds representing a wide array of diversity have been introduced from both Xinziang and from the Qiong Long Shan Experiment Station near Beijing and have been propagated in Davis, California for future evaluation and exploitation.

Plant Exploration in Eastern Europe

Sour cherry exploration. The weaknesses of 'Montmorency', the 400-year-old French cultivar that comprises 99% of the United States sour cherry industry, and the extremely limited germplasm available in the United States made it imperative to acquire new germplasm in order to make any breeding progress. This moderate-sized tree species, Prunus cerasus L., which occurs wild and has been cultivated for a very long time in Eastern European countries, originated long ago from chance hybridization between the large-statured, cold-sensitive sweet cherry, Prunus avium L., whose native range includes Southeast Europe-Southwest Asia, and the shrubby, cold-hardy species, Prunus fruticosa Pall., whose distribution ranges farther north into the Soviet Union. The known sour cherry diversity found in regions surrounding the Caspian, Black, and Adriatic Seas suggests that this is the center of origin of this species. Since 1984, Amy Iezzoni has made five trips to seven Eastern European countries to study this variability and to collect germplasm (Iezzoni 1984). She found there a huge spectrum of variation ranging from plants that resemble the sweet cherry progenitor to those that are more similar to the other progenitor and all possible combinations of both. A wide variation was found in bloom time, fruit maturity, cold hardiness, important fruit qualities such as color, soluble solids, firmness, flavor, size, plant growth habit (from small dwarf trees to tall trees comparable to sweet cherry), and, of great significance, resistance to one of the most important diseases attacking our orchards, cherry leaf spot (Coccomyces hiemalis Higgins). She collected among the countless local cultivars that have been selected through centuries of cultivation, as well as acquired modern cultivars and selections from various Experiment Station breeding programs.

Iezzoni's methods of approach have greatly hastened the exploitation of this new germplasm and serve as a fine example for what can be done to circumvent the lengthy quarantine period as well as the long generation time of fruit tree crops. In addition to introducing clonal material to the Quarantine system, she has made the best possible use of seeds and pollen and currently has over 10 ha of seedling trees representing new germplasm from elite sources. Not only has she collected and planted open-pollinated seeds from select cultivars in Eastern Europe; she has introduced pollen from abroad and used it for crosses with her local selections. In addition, because of the on-going working relationships she established, her European collaborators have made crosses for her between their selected clones and sent the hybrid seed for growing out in Michigan. Already these seedling trees are being screened for disease resistance, growth characteristics and fruit characteristics (Hillig and Iezzoni 1988; Krahl et al 1991). This rapid progress was made possible by an extremely important component of plant exploration, and one which is too often overlooked; that is, the establishment of friendships and on-going collaboration with individual scientists in the host countries which has the potential for long-term benefits for both parties.

CONCLUSION

The dazzling diversity that has been collected in Central Asian apricots and walnuts, in Chinese walnuts, and in Eastern European sour cherries compared to the limited germplasm available to American breeders, provides examples of what may be in store for other fruit and nut species when explorers are able to search geographic centers of diversity. During the 1980s and 1990s we are experiencing a new "golden age" for fruit plant exploration, wherein we need to take advantage of all possible opportunities. Further, the establishment of the National Clonal Germplasm Repositories during the past decade provides the mechanism for securing long-term maintenance of newly introduced germplasm for future utilization. Because of quarantine and generational time constraints, we will be well into the 21st century before the benefits of this new germplasm will be realized in new cultivars. Only then will we be able to evaluate the full value of exploitation as we begin to see new cultivars which, (1) represent a greatly expanded germplasm base, thus reducing genetic vulnerability, (2) provide a wider range of climatic adaptation, (3) spread the harvest season, (4) have a modified plant architecture (e.g. dwarf, precocious walnut trees), (5) have improved fruit quality, (6) have longer storage life, (7) provide new products, and (8) exhibit greater resistance to pests and diseases.

REFERENCES


Last update April 9, 1997 aw