Exploration for Fruit and Nut Genetic Resources in Central Asia
Author:Maxine M. Thompson,
Professor Emerita, Department of Horticulture
E-mail: [email protected]
Oregon State University, Corvallis, OR, 97330, U.S.A.
Luby, J., P. Forsline, H. Aldwinkle, V. Bus, and M. Geibel. 2001. Silk road apples-collection, evaluation, and utilization of Malus sieversii from Central Asia. Proc. Workshop ll, “Collection, utilization, and preservation of fruit crop genetic resources-=some case studies”. HortScience (in press)
Vavilov, N. I. l930. Wild progenitors of the fruit trees of Turkistan and the Caucasus and the problem of the origin of fruit trees. Proc. IX International Horticulture Congress, London. pp. 271-286
The concept of centers of origins of crop plants as the source of the greatest genetic diversity was developed by the great Russian agricultural geneticist, plant explorer, and visionary, Nicolai Vavilov. He and his colleagues searched the world for plant genetic resources of crop plants and related species, collected them, and established extensive collections of seeds and plants at Institutes located in different ecological regions in the former Soviet Union. This, the first major national program to collect and preserve plant genetic resources, continues today under the name "The N.I. Vavilov Institute of Plant Industry" (VIR). One of Vavilov 's trips took him to Central Asia where he encountered vast natural populations of apples, apricots, walnuts, pistachio, almond, plums, and vinifera grapes with fruits similar to those of cultivated forms. He reported this discovery to the Western world in l930 at the International Horticultural Congress in London (Vavilov, l930). Although this information stimulated much interest among Western plant breeders, the political situation prevented access to this region for the next 60 years. It was not until l989 that the first American plant explorers were allowed into the former Soviet Central Asia.
In 1988, a year before American plant explorers gained access to the Soviet Union, I participated in a fruit and nut exploration trip to Northern Pakistan, a mountainous region adjacent to the southern borders of these closed regions wherein lay the fruit and nut centers of diversity. As we traveled about there, it was difficult to decide which was the most interesting; the overwhelming grandeur of the mountain scenery, the several fascinating, distinct human cultures, or the objective of the trip-studying and collecting the great diversity of fruits and nuts. Four great mountain systems converge here; the Himalayas, the Karakorum, the Hindu Kush, and the Pamirs, altogether forming a formidable barrier to human penetration. At elevations where most temperate zone fruit and nut trees are found (1,200 to 2,900 m), there are three distinct ecological regions in this mountain system; extremely arid, semi-arid, and moist temperate.
The driest regions, with about 150 mm annual precipitation, occur on the northern slopes, as in Hunza and Baltistan. There, the landscape is essentially a desert and all crop plants, including fruit trees, were introduced and require irrigation. The only natural fruit or nut tree is Pistacia khinjuk, a species with inedible nuts, but possibly of some value as a pistachio rootstock for extreme drought resistance. The second region is semi-arid, as in Chitral which is adjacent to Afghanistan. Here there are scattered populations of oak (Quercus baloot), Pistacia atlantica, and a wild almond species (Prunus kuramica). Also, seen were deodar cedar (Cedrus deodora), juniper (Juniperus excelsa), and an interesting pine (Pinus gerardiana) that has large seeds with shells so thin they can be cracked in one 's hand. In both of these inner mountain regions, towering peaks (many over 7,500 m) alternate with highly unstable, steep slopes plunging into deep river valleys.
Apparently, through the centuries, small groups of people have sought haven in these remote rugged, mountain valleys and therein created mini kingdoms, each consisting of a few tiny villages perched on riverine terraces and alluvial fans, and irrigated by glacier melt water conducted from high above in extensive man-made canals . The diversity of the cultures and languages (we encountered 10 distinct languages) indicates that the people must have come from several different neighboring, or more distant, regions. Through the centuries, they have remained distinct due to physical isolation and the need to defend their minimal resources from other marauding and impoverished tribes.
The diversity of cultures is mirrored in the diversity of fruits and nuts, with each former mini-kingdom having its own assemblages of fruit cultivars. Presumably, all food plants were brought with the immigrants from wherever they came. Environmental conditions in the arid and semi-arid regions from about 1,300 m to 2,900 m. elevation are ideal for most temperate fruits and nuts: there are no damaging spring frosts, no severe freezing temperatures in winter, there are sufficient number of chilling hours, summer temperatures are not severely hot, and the very low humidity inhibits common diseases and pests. The principal fruits grown in these drier regions are apricots, grapes, mulberries, apples, pears, peaches, plums, and almonds.
Apricot was traditionally the most important fruit crop in this food-scarce region. It has reliable heavy crops every year; it provides fresh fruit all summer due to the extraordinary wide range of fruit maturity (May to August). The dried fruits are a good food source all winter as are the nutritious, sweet, dried kernels that are eaten either directly or ground into a flour. In fact, it was such an important food source that many years ago the Mir of Hunza decreed that no apricot trees could be cut without his permission. Before the completion of the Karakorum highway in l979, the first paved road to provide access for transport of goods, people survived essentially on subsistence agriculture. Apricots, along with mulberries and grapes, provided the only source of sugar so there has been strong selection for high sugar content in these species.
Common practice is to grow apricot seedlings and evaluate the fruit; if deemed inferior, the tree is grafted over to better a better type; if superior, the tree is named, propagated and distributed to other farmers. For centuries, this on-going selection process has resulted in a few hundreds of named types. These represent a great diversity of cultivars each of which had been chosen for some specific attribute. Many selections have very high sugar content, some up to 34 degrees Brix (a measure of soluble solids), about triple the value in California cultivars.
Some have fruits capable of being stored for extended periods. There is a complete range of skin color at maturity, from green to cream, light orange, and dark orange, with or without a red blush, and flesh color is white, pale orange or deep orange. Seventy percent of the cots that we collected had edible sweet seeds; the others, with bitter seeds are a source of oil for lamps or grooming hair. One important trait that I could not evaluate in Pakistan was revealed in California when seedlings were grown there. All of these introduced plants bloomed later than all of the California cultivars and breeding materials. This trait is particularly valuable in this notoriously early blooming species. Later blooming allows plants to escape spring frost damage and thus, will permit their being grown over a wider geographic range than at present. We collected dormant scions of 102 cultivars and hundreds of seeds from these trees. Unfortunately, our Plant Quarantine Center allowed every one of the clonal introductions to die.
However, at the time of this trip, seeds were not required to pass through the Quarantine Center so these were distributed to breeders. The report from Craig Ledbetter in California is very promising. This year he is evaluating fruits of the second generation trees from crosses of the Pakistan seedlings and his breeding materials. The threat to the apricot diversity in these remote regions of Pakistan began with the completion of the highway and the consequent opening up of trade with the outside world. Dried apricots, now a surplus food, has become a valuable export item. As commercialization requires uniformity, there is increasing emphasis on future planting of only the few cultivars deemed most suitable for drying, with subsequent disappearance of the many forms that have been selected through the ages for multiple other purposes.
Grapes (Vitis vinifera) are another fruit species that is widely grown in these dry regions. Since this grape species does not grow wild in Pakistan and we never heard of grapes being grown from seed, we assumed that all grapes were brought here as clones. There is a wide diversity of types, including those with, green-white, pink, red, and dark purple-red to black fruits, some with soft juicy flesh and some firm and crisp, some very desirable for table use, others more suitable for wine, and a few dried for raisins. Special cultivars had been selected for producing good wine In certain localities where wine-making had long been a traditional practice. However, since the Islamic government has prohibited, under severe penalties for transgressions, manufacture or selling of alcoholic drinks since l972, these cultivars are in grave danger of being lost as they are being replaced by table grapes.
We estimated that there are at least a few hundreds of different grape cultivars there. In Baltistan, a collection of local grapes was being established and they had 70 different types of table grapes from just this small region. We collected cuttings of 69 cultivars and heard of many more. A more thorough study of grapes should be done in northern Pakistan and some attempts be made especially to evaluate and save the best wine type cultivars. Depending on the cultivar and elevation, grapes ripen from July to early October so one needs to be there during an extended period for a thorough analysis of their genetic diversity.
Mulberries are another very important food source in the northern areas. White mulberry (Morus alba) trees are very common throughout the region. They appear to thrive with limited irrigation, and always bear heavy crops of very sweet fruits. Color of fruits may be white (the most prized for drying), red, or black. Otherwise, there appears to be limited variability. The sweet, white fruits are commonly dried and stored for winter consumption. At lest one form is seedless and is propagated by cuttings, which accounts for the uniformity across different villages and regions. A delectable cultivar of black mulberry (Morus nigra) has also been introduced and is clonally propagated, but is much less common. Fruits have a more sprightly flavor than M. alba. They are very juicy, and therefore are not suitable for drying, so remain strictly a seasonal fruit.
Other fruits grown, but to a lesser extent, in the dry, inner mountain valleys are apples, pears, peaches, and plums. There are several different apple cultivars but, except for a few with firm flesh, most of them are not the type preferred by Western consumers. Fruits were rather soft and bland tasting due to low acid content.
Scattered throughout, there is a variety of pear cultivars, derived from both European (Pyrus communis) and Asian (Pyrus pyrifolia) species, but all different from those in the U. S. Peaches were less common and similar to those found in Central Asia (sometimes called Prunus ferganensis or P. persica var. ferganensis). They are small sizes (20-60 gms) compared to Western cultivars; mostly late maturing (September-October); very sweet and soft and easily bruised; juicy, so not suitable for drying; most often white-fleshed; and both cling and freestone. One useful attribute observed was tolerance to high pH soils. This was evident in a collection where local peach trees had dark green leaves whereas leaves of introduced cultivars expressed pronounced chlorosis.
Plums are relatively common and several different types were seen, all derived from one of two species; Prunus salicina or Prunus cerasifera. These were mostly grown as seedlings. Local plums had smaller fruits than California cultivars (a few of which had been introduced recently) and they exhibited the full color range usually seen in Oriental plum species. We did not see any European plums (Prunus domestica), a species that has forms suitable for drying.
Limited amounts of almonds are grown in the dry regions and there is much genetic variability, as seen in nut markets. These nuts are much esteemed and much more expensive than walnuts. Although the environment is ideal for this crop, there are relatively few trees. When questioned why there were not more almonds grown, we were told that the trees donÕt produce enough. This was quite understandable since they traditionally have planted seedlings that are delayed in bearing crops, generally non-productive, variable in kernel quality and shell thickness, and often segregate for bitter and sweet kernels. As people there prefer the long shapes, there has been selection for this trait. Whereas people have grafted apricots, apples, and pears for a very long time, somehow they have done almost no grafting of almonds, although this is not a difficult process. In only one village did we discover a few grafted trees of a local thin-shelled almond selection.
The third ecological region studied comprises the mountain slopes and valleys facing south, where there is higher rainfall than the inner mountains due to the influence of summer monsoons originating in southeast India. At mid elevations, there are mixed forests of typical moist, temperate zone conifers (pines, spruce and firs) and broadleaved species (maples, elms and horse chestnut), as in the Kaghan and upper Swat Valleys. Both wild and cultivated walnuts (Juglans regia), as well as an inedible wild relative (sometimes called subspecies fallax) are widely distributed in this region. This wild form has very small nuts with shells so thick that the very small kernel cannot be extracted. Its potential as a rootstock has not been investigated because the Pakistanis grow all their walnut trees from seeds. Walnut trees with edible nuts grow to a very advanced age and some trees are enormous; one measured 6.8 m diameter at breast height. There is a great deal of nut variability in size, shape, shell thickness, color of kernels, and taste that needs further study and collection on a future trip devoted to these objectives. We were told that one tree had nuts with essentially no shell, but fallen boulders on the road prevented us from going to see it. We did, however, see others that had only partial shells. There were some very desirable nuts of good size, with thin shells and attractive light kernels.
In this same region, we also found a tree hazelnut species, Corylus jacquemontii. This species occurs in relatively few scattered locations in the Himalayan mountains and populations are certainly destined to decline due to lack of regeneration. Near villages, a common practice is to cut back the trees each year to stimulate the vigorous annual shoots which are then cut for fodder. Unfortunately, these shoots are the location of next year's nut crop. Although nuts are edible, apparently the fodder is more important than the nuts. Consequently, there is no regeneration and populations of this species are disappearing. The non-suckering trait is of interest for possible development of rootstocks for commercial hazelnut trees, all of which require annual removal of suckers at the base of trees.
The Swat Valley, located in this relatively moist, temperate region, has long been more accessible to the outside world than the inner mountain regions. Therefore, many western cultivars of apples, pears, peaches, plums and apricots have been introduced and, due to the availability of more flat agricultural land, they are grown on commercial-sized farms. The most interesting local fruits were a group of unique pear cultivars. In addition to typical European and Asian pears, several of these distinctly different types of pears were clonally propagated and were found only in the Swat Valley. There were varying sizes of fruits, all of which were much smaller and had a different aspect from the recognizable species. Unfortunately, we were unable to evaluate the fruit quality because they were not quite ripe at the time we were there. A few years later, I discussed these pears with a Chinese pear specialist who was visiting Corvallis. When I showed him photos and herbarium specimens, he suggested that they may be Pyrus xinjiangensis, a species recognized in Xinjiang, China but unknown to us. This is quite likely the origin of these pears because from the 2nd to the 9th century AD, Buddhism flourished in the Swat valley and Chinese pilgrims were known to make the perilous journey from Xinjiang in northwest China through the Karakorum and Himalayan mountains to this religious center. They could well have brought their favorite pears with them. Several of these pears have finally been released from Quarantine and are now being propagated at the Clonal Germplasm Repository in Corvallis. These cultivars offer new, and distinctly different germplasm for pear breeders to evaluate for potentially valuable traits.
Pyrus pashia is the only wild pear species in the moist Swat Valley. Due to intensive use of land for food production, there are relatively few wild trees. However, seedlings are commonly planted around homes and the small fruits are eaten after undergoing a storage period, during which the hard flesh softens, sweetens, and turns dark brown. Of interest was a very large-fruited selection of this species, the only one that is propagated clonally. Fruits are 2-3 times the size of seedlings and trees do not appear to be hybrids with any other pear species. One can only assume that a giant-fruited mutation must have occurred and was selected long ago by local people.
Another interesting fruit in this relatively humid region was the cultivated clone of the local wild grape (Vitis jacquemontii). This is another example of people selecting an outstanding individual plant from a wild species and its subsequent clonal propagation as a cultivated plant. Not only are the clusters and individual berries larger and much higher quality, but the plant obviously has perfect flowers (both male and female parts) because it sets much fruit when grown with no other grape plants nearby. Clusters are 15-20 cm long and contain 50-100 berries. Individual fruits are small (about 1 cm diam.), round, black, with a tasty tart-sweet flavor, and with a few seeds. By contrast the wild species has male and female plants, a common trait in wild grape species, and clusters and fruits are smaller, bitterish and virtually inedible. This grape thrives where vinifera grapes are devastated with mildew, so can be grown satisfactorily in regions with warm, humid summers.
Beginning in l989 when the Soviet Union finally opened the doors to American plant explorers, six fruit and nut exploration trips have been conducted; 4 for apples (1989, 1993, 1995, 1996), one for apricots (1990), and one for walnuts (1994). Today, time constraints allows for only brief mention of these. The repeated visits for apples permitted a thorough analysis and collection of a wide range of new and valuable apple diversity, especially in Kazakhstan. Currently, there are several on-going cooperative evaluation projects with these collections. An overview of this extensive work is presented in a forthcoming article in the journal HortScience (Luby et al, 2001). The apricot and walnut expeditions revealed many interesting new fruit and nut diversity but, due to their short duration, served primarily as fact-finding trips. Repeat visits to Central Asia are needed for a more thorough study and collection of apricots and walnuts, as well as other important species there, namely vinifera grapes, almonds, and pistachios.