Variations in Nutritional Composition of Walnut Kernels Across Different Elevations in Chongqing Region, China

Walnut (Juglans regia L.) is an important economic and oil-bearing tree species, and the nutritional quality of its kernels is influenced by multiple environmental factors. Elevation is an ecological gradient that integratively reflects variations in environmental conditions such as temperature and light availability and shows a certain degree of correlation with kernel nutritional quality. The aim of this study was to clarify the regulatory effect of elevation on the nutritional quality of walnut kernels in Chongqing and to optimize the layout of high-quality walnut production areas. This study used 181 walnut germplasm resources collected from 16 natural populations (production areas) in Chongqing. Six elevation ranges were defined (I: 200–600 m, II: 600–900 m, III: 900–1200 m, IV: 1200–1400 m, V: 1400–1600 m, VI: 1600–1800 m), and twelve nutritional traits of walnut kernels were systematically analyzed, including total fat, protein, soluble sugar, tannin, saturated fatty acids (stearic acid, palmitic acid, arachidic acid), and unsaturated fatty acids (oleic acid, palmitoleic acid, cis-11-eicosenoic acid, linoleic acid, α-linolenic acid). The results showed that the fat content of walnut kernels was generally higher than 60%, with the highest value in zone VI (62.93%). The protein content was the highest in zone III (17.71%) and the lowest in zone VI (16.06%). Soluble sugar and tannin contents were relatively low, both peaking in zone II (3.10% and 10.85%, respectively). The overall content of saturated fatty acids was low, being slightly higher in zone II, with little variation among components across elevations. Among monounsaturated fatty acids, oleic acid was dominant, showing a decreasing–increasing trend with rising elevation, with the lowest value in zone II (20.98%) and the highest in zone VI (26.93%), while palmitoleic acid and cis-11-eicosenoic acid were maintained at low levels. Polyunsaturated fatty acids were dominated by linoleic acid, ranging from 51.22% to 61.04%, with the highest content in zone II and the lowest in zone VI. Comprehensive evaluation and cluster analysis grouped the six elevation zones into three categories, with zone II showing the best nutritional quality, particularly in terms of soluble sugar, stearic acid, and linoleic acid, while zone I had the lowest score. These findings provide a theoretical basis for the selection of high-quality walnut production areas and the precision cultivation of nutrient-rich walnut fruits, as well as important data support for the scientific planning and high-quality development of the walnut industry in Chongqing.