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).