Plant seeds are an important source of oil, which serves as a staple agricultural food product and an important industrial raw material. Moreover, plant seed oils are an increasingly important renewable and environmentally safe bioenergy resource. However, increasing demand for seed oil has resulted in increasingly prevalent and serious oil shortages. To address this problem, efforts have been focused on the development of woody oil trees, which have recently become a major edible oil source in some western European countries. Woody crop oils with a large share of the edible oil market include coconut, olive, palm, almond, walnut, and tea oils. However, there are many potentially excellent woody edible oils in nature that have not been exploited, including those coming from maple tree species.

Acer truncatum, a member of the Aceraceae family, and is a forest tree species native to northern China, Korea and Japan, but can also be found in Europe and North America (Guo et al., 2014). It is a deciduous tree species native to northern and western China, and it is mainly distributed in the Yellow River basin, the northeast, Inner Mongolia, Jiangsu, Sichuan and other regions. This species of maple has been commonly chosen as a landscape tree for its brilliant autumn leaf color (Zhao et al., 2007; Li et al., 2015). A. truncatum is also an ideal source of timber, protein, medicinal substances, and chemical raw materials due to its sturdy texture and high levels of protein, chlorogenic acid, tannin, and flavonoids (Ma et al., 2005). In addition, its abundant oil seeds have been roasted for many years for use as a food source in northeastern China, but have not yet been used on a large scale. Previous studies (Wang et al., 2006; Hu et al., 2017; Sun et al., 2018) have suggested that maple seed oil would be an excellent edible oil because it contains a large percentage of unsaturated fatty acids (92%) and nervonic acid (6.22%). Notably, nervonic acid, an important fatty acid for brain growth and maintenance, has been shown to prevent neural deficits and shows promise for the treatment of neurological disorders such as schizophrenia and psychosis (Akoh et al., 2001; Barcarolo et al., 2003; Chen et al., 2017). It has a high content of vitamin E (125.23mg/100g) (Wang et al., 2006). Therefore, A. truncatum is regarded as a potential species for use in the development of valuable nutritional and medicinal substances.

A. truncatum is also a renewable biomass energy tree species which produces approximately 30 kg of fruit per tree after 20 years and its seed ratio is up to 66.4% (Wang et al., 2006). Currently, it is receiving much attention from researchers in China due to its great potential value for energy and other numerous applications. Consequently, the seed oil was approved as a new food resource by the Chinese Ministry of Health in 2011. A. truncatum seed oil (ATO) is usually obtained by cold pressing and commercialized as a crude oil in China (Hu et al., 2017). At the present time, the A. truncatum artificial cultivation area encompasses 4 × 10⁴ ha and should continue to expand rapidly due to this tree’s numerous industrial prospects. However, as most previous studies have focused on the edible, medicinal, and health care uses of its seed oil, no studies have investigated variations in oil content or fatty acid composition among natural stands. Moreover, only limited information is available regarding the effects of environmental factors on seed oil yield and quality.

In this study, we selected 138 accessions from 14 A. truncatum tree populations growing across the known natural distribution area of this species in China. The objectives were to analyze seed oil variation and investigate the influence of environmental factors on seed oil content and fatty acid composition. The information obtained from these studies should guide breeding programs, aid in selection of the most adaptable lines for large-scale cultivation, and ultimately stimulate further development of the ATO industry in China.

Yield is the trait most predictive of whether or not an oil is suitable for large-scale production and adaptability to multiple industrial uses. Consequently, crops that contain high oil content at harvest ultimately reduce overall final production costs (Kumar and Sharma, 2011). This mean content (28.57%) value for A. truncatum greatly exceeded values for Glycine max (17%), Olea europaea (20%), and most results for Sapium sebiferum (12–29%) (USDA, 2012; Karmakar et al., 2010), and was comparable to values for Jatropha curcas (20.05%-38.33%) (Kaushik and Bhardwaj, 2013) and wild manihot (17%-31%) (Alvesa et al., 2014). This proves that it is feasible to extract oil from the seed of A. truncatum for industrial purposes.

Seed oil quality and uses are determined mainly by fatty acid composition, with rich variety in fatty acid compositions and content observed among the various species (and even among varieties). Therefore, fatty acid composition and content can be used as fingerprints to identify biological oil sources, as well as to test the authenticity of oils (Li et al., 2011). At the species level, a total of 14 distinct fatty acid components were detected in this study, which is similar to that reported by Hu et al., (2017), and a number similar to that obtained for Xanthoceras sorbifolia by Yu et al., (2017), although their oil content differed from the values reported here. The degree of unsaturation (89.60%–92.90%) of ATO is higher than peanut oil (83.1%), soybean oil (84.4%), olive oil (86.2%), cotton seed oil (74.1%) and other edible oil (USDA, 2012). C18:2 (27.08%–36.71%) is essential for human metabolism due to the lack of enzymes responsible for its biosynthesis (Hanganu et al., 2012), and it contains a special substance named nervonic acid that is an essential fatty acid which helps maintain brain health by its participation in biosynthesis and maintenance of nerve cell myelin (Akoh et al., 2001; Barcarolo et al., 2003). Due to its scarcity, C24:1 draws a high purchase price. Therefore, it is of great significance to find alternative plant sources rich in C24:1. A. truncatum is one of a handful of known species that contains C24:1 in seed oil, including Brassicaceae, Lunaria annua, Borago officinalis, Cannabis sativa, Tropaeolum speciosum, Cardamine graeca, Malania oleifera, and X. sorbifolia (Katavic et al., 2010; Chen et al., 2017; Yu et al., 2017). Therefore, the C24:1 content is an important indicator of ATO value. The content ranged from 3.90% to 7.85%, with a mean value of 5.76% in this study, which is much higher than the content of the current source of medicinal nervonic acid, X. sorbifolia (1.52%–3.04%) (Yu et al., 2017). And the yield per plant is rich (about 30 kg of fruit after 20 years). So, A. truncatum seeds will become a promising raw material to extract nervonic acid.

Variability is the result of different environmental selection forces that are the root cause of phenotypical differentiation among populations. In this study, altitude, temperature, and precipitation are the main environmental factors causing variation. The trend is the same in the study of Dewhurst and King (1998), and Darwish (2014), demonstrating higher oil yields in plants collected from relatively higher altitudes. However, the oleic acid content differed from that of previous studies in that it is dependent on mean temperature and can decrease by up to 2% with each degree (ºC) of increase in temperature (Rondanini et al., 2011). However, no significant relationship between mean temperature and oleic acid content in oils was found in this study, but significant negative correlations were observed between annual average temperature and both PUFA (-0.584) and linoleic acid (-0.601). It has been demonstrated that low temperatures increase the PUFA content of plants to maintain the fluidity of biological membranes (Falcone et al., 2004); in a high temperature environment, the increase in SFA could be explained by lipid peroxidation (Arbaoui and Link, 2007). That also explains the observation that palmitic acid exhibits a significantly positive correlation with annual average temperature (0.666).

Laribi et al., (2009) and Rebey et al., (2011) observed water stress effects on lipid metabolism. In our study, precipitation exhibited a significant negative correlation with C18:2 (-0.790) and a significant positive correlation with C24:1 (0.592). In addition, researchers have noted that an increase in altitude has tended to promote greater UFA content (with subsequently lower SFA content), possibly because altitude increases lead to lower temperatures and higher exposure to light (Dewhurst and King, 1998; Lai et al., 2010). However, this relationship is in contrast to the results of this study, which coincide with Vicia sativa by Mao et al., (2012) who found that other environmental factors besides temperature and light intensity, such as soil and air, may also influence the lipid metabolism of plants. This will be the direction of our future work.

In addition to environmental factors, genetics need to be considered. Rahimmalek et al., (2017) suggested that the thymol content in Iranian Ajowan (Trachyspermum ammi) can be affected by both genetic and environmental factors. It is necessary to consider the differences within and among populations when screening promising germplasm resources for oil use, since the biosynthesis of fatty acids can be regulated by both genetic and environmental factors simultaneously. Thus, further research should be conducted to explore the effects of environment and genotype in order to guide in the selection of genotypes which exhibit high oil yield and useful fatty acids.

Results of HCA and PCA showed that within a certain geographic area, populations with similar physiological characteristics can be clustered into a class which is the same as the result of Rahimmalek et al., (2017) on Iranian Ajowan. Within a class, it was observed that the fatty acid composition of ATO assumed variation patterns which are characteristic of geographic isolation. Fatty acid profiles could be roughly correlated with growth in three major geographic areas of China: (I) Eastern central region (TA, LF, NX, JY, and NJ, with high MUFA especially C18:1); (II) Southwest region (FZ and ABZ, with high SFA); (III) Northeast and northwest regions (DQTL, CF, CC, PQ, YJ, HY, and YS, with high UFA, especially PUFA and C18:2). These findings also indicate that it is feasible to purchase seeds in the region which could guide future development of ATO for industrial production.

A. truncatum is a woody oil tree species with high seed oil content (28.57%), high unsaturated fatty acid content (91.47%), and high nervonic acid content (5.76%) ompared to grain and other seed oil sources. Notably, the data presented in this study show patterns of variability in oil content and fatty acid composition which correlate with geographic location: northern high altitude populations exhibited higher oil content; populations in the eastern central region exhibited high levels of MUFA (especially C18:1); populations from the southwest region were rich in SFA; populations from the northeast and northwest regions had high UFA content (especially PUFA, and C18:2).

Seed oil content and fatty acid composition were found to be influenced by ecological factors, especially altitude, temperature, and precipitation. The results suggest that both fat production and quality of ATO may be improved by planting trees in moderately cold areas. Consequently, further studies will incorporate such results to guide in the selection of excellent individual trees which have high specificity (such as high oil, high C24:1) to improve oil yield and quality. The populations of DQTL and YS were chosen as germplasm resources with high oil content, while JY, NX, and PQ were regarded as excellent germplasm resources with high C24:1. This work should lay the groundwork for improving oil yield and quality and should ensure ATO’s future success as an important industrial crop.