Actinidia germplasm resources and kiwifruit industry in China

Kiwifruit is native to China, and today’s cultivated kiwifruit are the large-fruited and good-fl avored selections of two commercially developed Actinidia species: A. deliciosa C.F. Liang et A.R. Ferguson and A. chinensis Planch. The genus Actinidia Lindl. comprises 66 species and about 118 taxa, with a remarkably wide natural range extending from the tropics (latitude 0°) to cold temperate regions (50° N). With the exception of four species native to neighboring countries (A. strigosa Hook & Thoms. in Nepal, A. petelotii Diels in Vietnam, A. hypoleuca Nakai and A. rufa (Sieb. et Zucc.) Planch ex Miq. in Japan), all other 62 species, about 45 varieties, and 7 forms occur naturally in China (Cui, 1993; Ferguson, 1990; Huang et al., 2000; Liang, 1983). Such an unusually wide range of variation of species diversity and rich genetic resources provides tremendous potential for cultivar improvement and sustainability enhancement of the world kiwifruit industry. It is impossible therefore to ignore the value of Actinidia genetic resources in China, which occurs both naturally in the wild and is conserved in fi eld repositories (Huang, 2003). The economic potential of A. deliciosa was exploited following the fi rst seed introduction from China in 1904 and the fi rst commercial vineyard appeared in New Zealand in 1930 (Ferguson and Bollard, 1990). Since the 1970s, an international kiwifruit industry of >100,000 ha has been developed with annual production of 1 × 10 t (Huang and Ferguson, 2001). The successful development of kiwifruit industries, fi rst in New Zealand and then in other countries, has stimulated China’s interest in the potential of native Actinidia species, although China has a long history of local consumption of the Actinidia as fresh fruit, functional foods, Chinese herbal medicines, and as much as 150,000 t fresh fruit per year were collected from the wild (Cui, 1993). This wild harvesting has occurred for centuries, but until about 20 years ago, only very few sporadic attempts at cultivation have been made (Ferguson, 2001). 0.6 g in A. maloides Li to 240 g in A. deliciosa. The two most commercially important species, A. deliciosa and A. chinensis, have the largest fruit, but signifi cant variation in fruit weight is also observed within both species. A. chinensis fruit weight ranges from 20 to 120 g, while that of A. deliciosa ranges from 30 to 200 g. In general, all fruit are oval and spherical in shape. There are at least 15 different fruit shapes that have been identifi ed: ovate-globose, fl at globose, oblong, near globose, ellipsoid, short cylindric, cylindric, ovoid, long globose, ovatecylindric, long cylindric, ovoid-cylindric, long cone, ovate-cone, and ovoid-globose. One of the main breeding goals has been to develop cultivars with lighter skin than ‘Hayward’. Although intraspecifi c variation of skin color is very limited and hardly genetically manageable, interspecifi c variation is broad and genetically heritable in most species. The interspecifi c variation of the fruit skin color ranges from brown to green, yellow, orange, light red, and even purple (Table 1). Most Actinidia species have fruit skin covered with unpleasant pubescence. The stiff and long hairs of ‘Hayward’, selected from A. deliciosa, are considered undesirable and should be improved by future breeding and selection. A wide range of interspecifi c variation in type and amount of pubescence exists within the genus. This variation provides the breeding potential to improve this trait, and even a smooth skinned apple-like fruit may be developed in the future. There exits a continuous variation in skin pubescence ranging from long, stiff hairs on A. setosa (Li) C.F. Liang et A.R. Ferguson to the smooth skinned applelike fruit of A. arguta. Actinidia arguta has a nice glabrous skin, but is small in size. This species has been successfully used in interspecifi c breeding for creating large sized, smooth skinned fruit. Actinidia macrosperma C.F. Liang has smooth orange-colored skin when ripe, making it a suitable parent for breeding efforts. It has also been used to improve skin color and heat tolerance. Actinidia has a wide range of fl esh color although commercial cultivars are either green or yellow-fl eshed. Flesh color varies from light green to orange, dark red or purple. Flesh color has been considered a trait with strong gene × environment interactions. In most warmer climate regions, red fl esh color (usually red color around fruit core ring) in some selections of both A. deliciosa and A. chinensis can be discovered in late spring to early summer, however, the red color becomes faded in midsummer. At high elevations where large day–night temperature fl uctuations exist, red color can persist until maturity (Huang, personal observation). Most species produce edible fruit, but fruit taste varies considerably. The species with excellent palatable fruit are A. chinensis, A. arguta, A. kolomikta (Maxim. et Rupr.) Maxim. , A. callosa Lindl. , A. chrysantha Liang, and A. hubeiensis Sun et Huang. A. polygama (Sieb. et Zucc.) Maxim. is the least palatable with a mix of bitterness, astringency and a shocking mouth numbing effect. More detailed morphological variation of horticulturally important fruit characteristics are listed in Table 1. A nationwide selection program was initiated by the Chinese government in 1977–78. The program resulted in >1400 superior genotypes being selected from wild populations of A. chinensis, A. deliciosa, and A. arguta (Sieb. et Zucc.) Planch et Miq. Most of these selections were topworked at research stations and evaluated. Subsequently, experimental trials, regional tests, and replicated trials of commercial scale resulted in 57 cultivars and a large number of selections (Cui, 1993). The outcome of this program made a unique situation for the Chinese kiwifruit industry with a different cultivar structure from the rest of the world. Since 1978 when commercial kiwifruit vineyards were fi rst established in Hubei and Sichuan provinces using ‘Hayward’ (developed in New Zealand), commercial plantings have continuously increased to the present 57,396 ha. China now has >40% of all the planted kiwifruit in the world. Production likewise increased rapidly from 5,000 t in 1990 to 291,450 t in 2001. It is estimated that in 2002, the total production of kiwifruit in China was about 340,420 t. China has now surpassed New Zealand and Italy and has become the largest kiwifruit producer in the world (Huang and Ferguson, 2001). The current rapid growth of the world kiwifruit industry and its future sustainability will depend to a large extent on a thorough and continuous exploitation of the rich genetic resources and a sound management and conservation strategy for the germplasm. Since the late 1970s, extensive research has been conducted exploring genetic resources in China and has generated a wealth of information on the extent of genetic variation that exists. Our enhanced understanding of many basic aspects for further germplasm exploitation will lead us toward the development of a sustainable kiwifruit industry in China. Unfortunately, little of this basic information has been published in western languages. In this paper, we provide an overall review of the basic information that has been gathered on Actinidia germplasm and the development of Chinese kiwifruit industry over the past 2 decades.