3.1. Variation in Oil Content and Fatty Acids Composition of E. ulmoides Seeds
The oil content and fatty acids composition of
E. ulmoides seeds are shown in
Table 2. For seed oil, the average amount of 240 individuals was 29.97% (
Figure 1), the lowest value was found in Guilin (28.54%) and Lueyang (28.54%), which was significantly lower than that in Chaoyang (31.35%), Haidian (30.80%), and Luoyang (30.92%) populations. Tang et al. [
13] revealed a low amount of seed oil content (24.18–28.01%) of
E. ulmoides seed collected from different production areas, which may be related to the number of samples. High oil content populations can be applied to food industries.
The composition of fatty acids in this study consisted of linolenic acid, oleic acid, linoleic acid, palmitic acid, and stearic acid, which are similar to those reported by Shu et al. [
14] and Zhang et al. [
15]. By multiple comparisons, significant differences in fatty acids composition of the seed oil existed among most regions. Among all fatty acids, linolenic acid was the most abundant, followed by oleic acid and linoleic acid. For linolenic acid, the lowest and the highest content were found in Guilin (56.68%) and Luoyang (60.70%), respectively; the content from the Guilin population was significantly lower than those from all other populations except Cili and Shapingba. The second most abundant fatty acid, oleic acid, varied from 16.31% in the Lueyang population to 17.80% in Cili. As the third most abundant fatty acid, linoleic acid ranged from 11.02% in Luoyang to 13.32% in Guilin. Palmitic acid, the fourth most abundant fatty acid, varied from 6.16% in Haidian to 7.03% in the Cili population; the contents in the Cili and Guilin populations were significantly higher than those from the other 10 populations. For stearic acid, the highest and the lowest values were found in Shapingba (2.77%) and Xiangshui (2.07%), respectively. The ratio of UFA/SFA ranged from 8.98 to 10.63 a higher ratio of UFA/SFA in human diets is considered beneficial for health.
E. ulmoides seed oils contained high concentrations of UFA (88.51%), especially linolenic acid (59.28%) (
Figure 1), a known characteristic of
E. ulmoides seed oil, and these results are similar to those reported by Jiao et al. [
16]. The total amounts of polyunsaturated fatty acid (PUFA) and monounsaturated fatty acid (MUFA) were 71.34% and 17.17%, respectively (
Figure 1). α-linolenic acid is an essential fatty acid, which can be metabolized into DHA and EPA [
17]. A diet rich in PUFA is important for the structure and function of many membrane proteins, and MUFA has been shown to lower blood cholesterol levels [
8]. These results suggest that
E. ulmoides seed oils could be a better material for food and health care industries.
3.3. Principal Component Analysis of Oil and Fatty Acids of E. ulmoides Seeds
Principal component analysis (PCA) was applied to determine the most significant characteristics of the data set. Two principal components were extracted by the PCA and explained the highest variation in the data set (
Table 4). The first PC (PC1) explained 58.983% of the total variation and showed a positive correlation with linoleic acid, palmitic acid, oleic acid, and stearic acid, and a strong negative correlation with linolenic acid. The second PC (PC2) explained 22.988% of the total variation and was positively correlated with oil content.
According to the scatter plots constructed by PC1 and PC2 (
Figure 2), the oil content and fatty acids composition of
E. ulmoides seeds of different groups were greatly affected by geographical distribution, especially the Cili, Guilin, Lueyang, Shapingba, and Zunyi populations. The Chaoyang, Haidian, and Luoyang populations were grouped together, with all of them showing high linolenic acid content and oil content. The Guangyuan, Yuncheng, Anguo, and Xiangshui populations were grouped together, with all of them having medium linolenic acid content and oil content.
3.4. Effect of Geographical and Climatic Factors on Seed Oil and Fatty Acid Variation
The fatty acids composition of plant seed oil can be affected by genetic, geographical, and climatic factors, as well as their interactions [
19]. Many studies have shown that environmental factors, such as precipitation, sunshine duration, elevation, and temperature, strongly affect fatty acid content [
20,
21]. Yao et al. [
20] reported that in
Camellia meiocarpa Hu. (
Camellia L.), palmitic acid content was positively correlated with annual sunshine duration, and stearic acid content was significantly positively correlated with annual precipitation. In soybean seeds, the linoleic and linolenic acid contents decreased as the temperature increased, whereas the oleic acid content increased [
22]. In
Sapindus spp. (Sapindaceae) seed oil, C16:0 was positively correlated with maximum temperature, C20:1 was significantly correlated with elevation, and C18:2 was sensitive to the interaction between maximum temperature and elevation. Precipitation was another important factor that was significantly correlated with C20:0 [
21]. To date, how the fatty acids composition of
E. ulmoides seeds is affected by geographical and climatic factors has not been clearly studied. Hence, CCA analysis was employed in this study for the first time to assess to what extent environmental factors influence the variation of fatty acids of
E. ulmoides seed oils.
As shown in
Table 5, all geographical and climatic factors were incorporated in the CCA analysis. The canonical axes explained 89.33% of the total variation in oil content and fatty acids composition. The first two axes explained 78.20% of the total variance (89.33%). The first axis of the CCA was predominantly positively correlated with latitude, annual average sunshine duration, and longitude, and negatively associated with mean annual precipitation, average annual temperature, and annual average relative humidity. The second axis was positively correlated with elevation.
Next, forward direction selection was carried out to remove redundant variables, and the CCA model was simplified for all factors and only climate factors and only key environmental factors in the CCA model were selected by ANOVA analysis (
Table 6). In the simplified ‘all factors’ model, latitude significantly affected variation in fatty acids composition, explaining 52.00% of the total variance (
p < 0.001). Elevation explained 26.69% of the total variance and was significant (
p < 0.05). In the simplified ‘climate factors’ model, mean annual precipitation explained 51.88% of the total variance and was very highly significant (
p < 0.001). No significant effects of the interactions between environmental factors were found.
A triplot reflecting oil content and fatty acids variation in relation to environmental factors was constructed based on the CCA analysis (
Figure 3). Latitude was positively correlated with linolenic acid and precipitation was another important factor that was significantly correlated with oleic acid. Oil content and other fatty acids showed general sensitivity to the selected environmental factors. Yu [
23] found that the epoxyeicosatrienoic acid content of
E. ulmoides seeds was positively correlated with annual temperature and was negatively correlated with longitude; no other correlation between environmental factors and other fatty acids was observed. In other high linolenic acid plants, such as
Perilla frutescens and
Linum usitatissimum, linolenic acid content was significantly correlated with cold environment by comparing different planting areas [
13].
The variation of fatty acids composition in E. ulmoides seed is largely controlled by environmental and genetic factors, but it has not been ascertained which have a greater effect. Additional studies to investigate the effects of genotype and the interactions between environmental and genetic factors on the fatty acids composition of E. ulmoides seeds would be helpful for the selection of genotypes containing useful fatty acids.