Malus sieversii (Ledeb.) Roem. is a wild apple species native to Central Asia. Its provenance includes the region of the Tienshan Mountains, extending from China to Kyrgyzstan and Kazakhstan. It has been recognized as one of the major progenitors of Malus ×domestica, the domesticated apple (Forsline and Aldwinckle, 2004). Malus sieversii is a very diverse species, exhibiting many of the qualities of M. ×domestica (Geibel et al., 2000; Volk et al., 2009). As a diploid primary crop wild relative with desirable traits as a scion and as a rootstock, M. sieversii is a valuable resource for plant breeders (Yan et al., 2008). In China, M. sieversii is distributed mainly in Xinjiang Uygur Autonomous Region in the western Tienshan Mountains.
Wild populations produce many fruits as a result of insect pollination. Preliminary studies revealed that pollen vitality is high and the average pollen vigor is as high as 64%. However, the self-pollination levels are low (Liu et al., 2008). The exact pollen germination percentage in different populations of M. sieversii remains unclear. Seed germination is another major aspect for plant reproduction. Relieving seed dormancy successfully is critical for fruit breeding. Evidence showed that seeds of apple trees do not germinate without stratification (Sińska, 1989). Naked stratification and sand stratification were used to remove seed dormancy and to accelerate germination (Sińska, 1989). This stratification method takes a long time, usually 2–3 months, to relieve seed dormancy. We sought to shorten the time for seed germination.
Due to interference of human activities and natural disasters, the natural population and distribution area of M. sieversii is reduced, and it is critical to protect this precious genetic resource (Volk et al., 2005). Conservation of plant genetic resources is achieved by protection of populations in nature (in situ) or by preservation of samples in gene banks (ex situ) (Cohen et al., 1991). The latter are essential for users of germplasm who need ready access, although it costs more money and time. Ex situ conservation of M. sieversii has been established in the United States using M. sieversii seeds collected from the wild in Kazakhstan and the Kyrgyz Republic (Volk et al., 2005). In situ conservation, which can be more economical, should also be considered as a conservation strategy.
To determine the distribution of geographical populations of M. sieversii and its growth status in Xinjiang, we made field practice for six times. After recording and analyzing the data, we drew a map for distribution of M. sieversii in Xinjiang. This map is the firsthand data for its detailed distribution, which is important for geographic research of the wild apple trees. We also set a garden for in situ conservation and renewal of M. sieversii. This is one of the few gardens for wild apple tree protection and utilization in the world. Moreover, we investigated reproductive characters of M. sieversii, including vigor of pollen and seed. These experimental data and observation of growth status in M. sieversii suggest that injurious insects and human activities, rather than reproductive characters, limit the renewal of M. sieversii.
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