Discussion
By screening the germplasm resources and selecting the appropriate variants, the selection and breeding of good varieties of A. truncatum can be initially realized. Earlier studies on phenotypic traits of A. truncatumproved that it has abundant variation, which provides a theoretical basis for screening the genes for good traits in ornamental trees, and this experiment proved that the oil content and fatty acid content ofA. truncatum seed kernels also have a high degree of variation. One of the important reference indexes for screening A. truncatumgermplasm with high oil content and high quality is the oil content. From the results, it can be seen that the oil content of the seed kernels of the 70 test materials ranged from 14.43% to 50.11%, with an increase of 247%, and the average oil content of the seed kernels was 33.11%; the average oil content of the seed kernels of the interspecific populations was 33.26%, with a variation range of 28.35% to 46.77%, with an increase of 65%, compared with the average oil content of the seed kernels of the populations of 33.26%. with an increase of 65%, which is more stable compared to the results of the individual samples. The results of this measurement are more similar to the results of other studies that have been seen[12, 22]. The coefficient of variation of seed kernel oil content was 21.64%, and the coefficients of variation of major fatty acid fractions ranged from 24.13% to 38.20%, with larger coefficients of variation for each index, which shows that there is abundant variation among the populations of A. truncatum , which is of great significance for the screening of high-oil A. truncatum single plants and their use in breeding of good varieties, so as to increase the A. truncatum oil production and oil value, and lay an important theoretical foundation for the improvement of the benefits of A. truncatum economic forests in the future. It is of great significance to improve the yield and oil value of maple oil, and to lay an important theoretical foundation for the future improvement of economic benefits of maple forest. A. truncatum oil can not only be used as high-quality health care edible oil, but also has high medicinal value. In this experiment, the fatty acid fractions with contents above 1% were tested and analyzed, and the measured average content results were as follows: linoleic acid (21.11%) > oleic acid (17.45%) > erucic acid (14.28%) > eicosapentaenoic acid (6.36%) > neuronic acid (5.54%) > palmitic acid (3.44%) > stearic acid (1.70%) > α-linolenic acid ( 1.13%), in which the content of neuronic acid was similar to the determination of neuronic acid content of A. truncatum in Yunnan region by Liu Xiangyi[23] et al. (5.25%) and the content of neuronic acid in the seed oil of A. truncatum in the whole country by Qiaoqian [12] et al. (5.76%), but was lower than that of the content of neuronic acid in Shandong province by Wang Yinhua[22] et al. (6.78%), which was more similar to the results of the present experiment It can be seen that the neuronic acid content of A. truncatum is higher in Shandong Province, which is more valuable for selection and breeding.
Differences in yield and quality traits of crops are the result of gene-environment interactions, meaning that different environments can lead to differences among seed sources[24, 25]. In the correlation analysis, there were mostly significant or highly significant negative correlations between the oil content of A. truncatum seed kernels and the major fatty acids, indicating that the fatty acid fractions of A. truncatum seed kernels with high oil content were on the contrary lower, combined with the results of the correlation analysis with the geographic factors, it can be seen thatA. truncatum has more seed sources of high oil content in the southern plains where the elevation is low, the air temperature is high, and the precipitation is sufficient; the major fatty acid fractions mainly showed a significant or highly significant positive correlation with the longitude, and significant or highly significant negative correlation with the frost-free period, which is more suitable for the fatty acid accumulation within the seed oil of A. truncatum in the northern part of the country where the longitude is high and the air temperature is low. The oil content of seed kernel and fatty acid fraction showed different correlations with geographic factors, which was similar to the results of the correlation analysis between the two, the oil content of A. truncatum seed kernel is mainly affected by altitude, temperature and precipitation, while the fatty acid fraction is mainly affected by longitude, temperature and growth period, and the results of the correlation analysis also reflected the A. truncatum ’s preferences for cool and humid, and the drought-tolerant habit, which is also in line with the current recognized opinion. The correlation results also reflected that A. truncatum is cool, humid and drought-tolerant, and also agreed with the current accepted opinion that the accumulation of seed oil in the seed kernel of A. truncatum is more favorable with the increase of altitude[26, 27]. The A. truncatum samples in this experiment were only collected from the Central Plains, with little variation in latitude, longitude and climatic factors, which led to the limitations of the study. In the subsequent study, it is necessary to further expand the screening range of A. truncatum population collection sites, to eliminate the limitations of the sampling locations, and to draw more accurate and reliable conclusions.
After the oil content and major fatty acid fractions of A. truncatum seed kernels from nine populations with a total of 70 samples were analyzed separately by principal component analysis with dimensionality reduction, the nine indicators were simplified into two representative principal components, the first principal component included eight major fatty acid fractions of palmitic acid, stearic acid, oleic acid, linoleic acid, eicosapentaenoic acid, erucic acid and neuraminic acid, with a contribution rate of 72.884%, and the second principal component only encompassed the oil content, with a contribution of 11.409%. Quality evaluation is an important part of the genetic breeding work of forest trees, and the comprehensive evaluation by the commonly used principal component analysis and affiliation function method can ensure the scientificity and accuracy of the evaluation results[28]. After the comprehensive evaluation of the oil content and major fatty acid fractions of the seed kernels of A. truncatum , the results obtained by the two evaluation methods were basically the same except for individual populations, and their evaluation results were accurate and reliable. Based on the evaluation results, 10 germplasm with high scores in the comprehensive quality evaluation were screened out: STA-7, STA-3, SZB-4, SJA-14, STA-4, STA-5, SJA-9, SZB-6, SZB-7, and SJN-2, and three excellent populations, STA, SZB, and SJN, with high relative contents of major fatty acids were also screened out, especially ST -4 had a neuronic acid content as high as 10.8%, which was in line with the requirements of the selection and breeding objectives of high oil Yuanbao maple.
As a high-value plant, selecting and breeding excellent germplasm with high oil and high yield is the top priority in the future research direction of A. truncatum , so in order to better and more accurately screen the excellent A. truncatum germplasm, further expanding the scope of the collection site, and at the same time combining with molecular marker technology to find the target genes with more precision, is the key to break the obstacles of A. truncatumselection and breeding work in the future. In future studies, components other than fatty acids within the seed kernels of A. truncatum , such as proteins, vitamin E3 and soluble sugars, which are also of high medical value, can also be explored[3, 29]. In addition, emerging technologies such as SSR molecular marker technology can be used for genetic breeding of forest trees. Chun Li[30] and others used SSR molecular markers to correlate the oil content of sesame seed kernels with the protein quality, and detected 19 SSR loci that were significantly correlated with the two traits, which provided an important theoretical basis for molecular marker-assisted breeding of sesame oil, the results provide an important theoretical basis for molecular marker-assisted breeding of sesame oil; Qiuyue Ma [31] and others designed a high-quality draft genome assembly that predicted at least 28,438 genes, providing new insights into the biosynthesis of very long-chain monounsaturated fatty acids, in addition to identifying three KCS genes that may contribute to the regulation of neuronic acid biosynthesis, which advances functional genetics research on Genus metacrisis as well as providing theoretical foundations for the study of the molecular mechanisms affecting the production of neuronic acid. In summary, the identification of high oil A. truncatum asexual lines at the molecular level can effectively avoid individual errors caused by the traditional methods of identifying plant traits, so as to find the key genes more accurately, and to make an important contribution to the work of selecting and breeding high oil and high yielding A. truncatumexcellent germplasm resources.
Conclusions
Compared with other oilseed woody species, A. truncatum has higher seed kernel oil content (33.11%) and neuronic acid content (5.54%), and has rich variability. It is worth noting that the data obtained in this experiment showed opposite trends in the increment of seed kernel oil content and fatty acid fractions; the variations of seed kernel oil content and fatty acid fractions were strongly influenced by geographic and ecological factors: the southern plains with lower elevations, higher temperatures, and sufficient precipitation had higher seed kernel oil content, and the northern part of the country with high longitude and low temperatures was more suitable for the accumulation of fatty acids. Therefore, if the derived research results can be used to guide the introduction and selection of excellent A. truncatum germplasm (e.g., high oil content and high neuronic acid), the development of its oil industry can be further enhanced. Finally, ten high-quality individuals, STA-7, STA-3, SZB-4, SJA-14, STA-4, STA-5, SJA-9, SZB-6, SZB-7, and SJN-2, as well as three high-quality populations, STA, SZB, and SJN, were selected by the principal component analysis and the affiliation function method. This work provides a scientific and theoretical basis for improving the yield and quality of the oil and lays the foundation for its application in edible and medicinal uses.
Acknowledgements Thanks to the technical support provided by Shandong Huabo Genetic Engineering Co.
Conflicts of Interest The authors declare that they have no conflict of interest.
Authorship Conceptualization, M.H. and L.Y.; methodology, M.H.; software, M.H.; validation, L.Y., Y.L. and Y.W.; formal analysis, L.Y.; investigation, Y.L. and Y.W.; resources, F.R.; data curation, M.H.; writing—original draft preparation, M.H.; writing—review and editing, M.H. and L.Y.; visuali-zation, Y.L.; supervision, L.Y. and D.W.; project administration, D.W. and L.Y. ; funding acquisi-tion, D.W. All authors have read and agreed to the published version of the manuscript.
Funding This research was funded by Taishan Scholars Project and Shandong Province Agricul-tural Seed Improvement Project Fund (2020LZGC00903).