Evaluation, Conservation And Domestication Of Genetic Resources Of Sorbus Pohuashanensis

The natural distribution, habitat conditions and natural regeneration of indigenous tree species, Sorbus pohuashanensis (Hance) Hedl. were studied based on literature, specimen and field investigations. 8 populations were sampled in Shandong, Shanxi, Hebei and Liaoning provinces. 7 populations (171 families in total) of them were used for analysis of geographical variation in 10 morphological and growth traits of 1-year-old seedlings. Genetic structure and diversity of all the 8 populations and mating system of 6 populations were aasayed using isozyme markers. A domestication trial was established in Changping Experiment Nursery of Beijing Municapl Forestry Bureau. Propagation by softwood cutting were also conducted to analyze effects of auxins (types and concentrations), position of cuttings on trees, and rooting conditions. This study is intended to provide guidelines for evaluation, domestication and conservation of the genetic resources of the species. Major results are described as follows:1. Most S. pohuashanensis naturally occur in mixed broad-leaved warm temperate forest, and some occur in mixed coniferous and broad-leaved temperate forest, belonging to humid mountain vegetation zone. The habitats can be grouped into 4 vegetation types (including 12 community types): mixed broad-leaved mountain forest, mixed coniferous and broad-leaved mountain forest, pure larch mountain forest and mountain top shrub forest. The altitudinal distribution of the species ranges from 500 to 2200 m with majority occuring between 1200 and 2000m. The species grows mostly in semi-sunny, semi-shade, shade slopes and gullies, however, it grows in sunny slopes in Wulingshan, Hebei province. The brown mountain soil and dark brown soil are the main soil types in the whole natural range of the species. Altitude, temperature and air humidity are likely the main factors affecting the distribution of the species. The species can naturally regenerate through seed and through sprout from roots and stumps. The stump-sprouting is the main pattern of natural regeneration.2. Analysis of variance showed that there are significant differences between population and between families within popuation for morphological and growth traits. The mean phenotypic differentiation coefficient for the 10 traits is 34.38%, indicating that the variance from families within population is the main part of the phenotypic variation. According to the result of cluster analysis based on several phenotypic diversity indices, the 7 populations can be divided into two groups. Correlation between the growth of seedlings and the geographical and climatic factors was calculated. A significant negative correlation was found between the height growth and the latitude. Temperature and mean annual rainfall are the main factors that affected the population variation. However, a significant positive correlation was found between the ground diameter and the longitude. A primary opinion is that the provenance variance of the species is influenced by both the latitude and the longitude, and the latitude plays the major role.3. Genetic structure and diversity of the 8 populations of the species were studied at 10 loci encoding 4 enzymes using horizontal starch gel electrophoresis, Rare alleles were found in each of the 8 populations sampled. The levels of genetic diversity were high at species level with the mean number of alleles per locus Na=2.2, the percentage of polymorphic loci P=100%, and the mean expected heterozygosity He=0.4240. The 8 populations were ranked as HBT>HBW>SXP>HBB>SDL>HBS>SDT>LNL according to He. And same order applies to Ne and Shannon′s information index. A low Level of genetic differentiation among populations was detected (Fst=0.0758) and a higher estimate of gene flow (Nm=3.0472) coincided with a high level of genetic identity (I) among the populations (from 0.8585 to 0.9872). Wright's F-statistics analysis indicated an excessive of heterozygotes both at population level (Fis=﹣0.4180) and at total population level (Fit=﹣0.3105). The results ofχ2-test (P<0.05) indicated that 73.2% of population-loci departed the Hardy-Weinberg equilibrium. The UPGMA cluster analysis suggested that the genetic distances among population were weakly correlated with their geographic distances.4. The horizontal starch gel electrophoresis for polymorphic loci of isozymes was used to exame the genotypes in 6 populations, sampled in Laoshan and Taishan, in Shandong province, Pangquangou, in Shanxi province, Tuoliangshan and Baishishan, in Hebei province, Laotuding, in Liaoning province. Mating system was also analysed using MLT software. The 8 polymorphic loci selected for analysis were Pgm-1,Pgm-2,Pgm-3,Pgi-1,Pgi-2,Pgd-1,Pgd-2,Pgd-3. The results indicated that the gametes ratio of each locus in pollen and ovule pools was imbalance in the 6 populations. The male to female gametes ratio of the six populations was not significantly different shown t-test. The outcrossing rates of polymorphic loci were above 0.981. The margin of multi-locus and single-locus outcrossing rate (tm-ts) showed that slight inbreeding existed in the each population of the species. In addition, the correlation of outcrossed paternity within progeny arrays (rp=0.420) of the Tuoliangshan population was the highest and the outcrossing rate of female parents (rt) of all the 6 populations ranged from 0.083 to 0.108. Therefore, S. pohuashanensis, having a mixed mating model, is a highly outcrossing species. The inferred inbreeding depression (δ= 1) from the electrophoresis data indicated that the natural populations of the species suffered serious inbreeding depression.5. The phenology and growth pattern on 2-year-old seedlings of S. pohuashanensis were studied. The results showed that S. pohuashanensis could grow well in shaded conditions and its phenology was normal in the introductional experimental site during the 2 years of observation. In open field, the leaves of the seedlings are subject to sunburn in hot summer, mainly in 6-7 month, and shading can be used to avoid sunburn. Under the shade of big trees, the peak shoot growth was in late April. The diameter growth peaked in May, June and August respectively. The leaf color gradually turns into red or yellow in autumn, selection can be made to develop varibilities with different leaf colors. High temperature and sunborn are probably the main factors restraining the growth in summer in the low altitudes.6. Using different types of auxins (IBA, NAA, IBA+NAA (10:1), IBA+NAA (5:1), IBA+NAA (2:1) and IBA+NAA (1:1)), with different concentrations (500, 1000, 1500, 2000, 2500 and 3000 mg?kg-1), and different types of rooting media (vermiculite, peat+perlite (3:1) and sand), the rooting ability of cuttings collected from field-grown and greenhouse-grown S. pohuashanensis was studied. Rooting rate, length and number of adventitious roots were measured. Only callus but no adventitious roots were found for the cuttings collected from lateral branches at the top of adult tree. However, the adventitious roots were visible for the cuttings collected from 1-2-year-old saplings, with larger rooting rate (above 70.37%), length (above 4.85 cm) and number (above 19) of adventitious roots. The softwood cuttings collected from the lower part of stems was obvious better than that from the upper parts. Cuttings should be taken early in the season before they were not completedly lignified, about 1.5-2 months after bud shooting. The rooting of softwood cutting was improved by a basal dip in IBA+NAA (10:1) and IBA+NAA (5:1). The vermiculite or peat+perlite (3:1) was the appropriate rooting media. The best rooting rate (above 90%), the largest length (above 6.67cm) and numbers of adventious roots (above 34) were obtained following the 1500 mg?kg-1 treatments.7. The results indicate that S. pohuashanensis is a predominantly outcrossing species. For any genetic resource conservation programe, sampling of seeds (or propagation materials) from more populations with a smaller number of trees at each site is preferable except for in situ conservation. Conservation stands/gene banks, provenance/families trials, clone banks and seed banks shoud be established. While some botanical gardens and arboretums are also proposed to conserve the genetic resources of the species.