Effects of selenium treatment on the accumulation of mineral elements and nut quality in pecan

Abstract:【Objective】Selenium is a relatively rare but essential micronutrient for maintaining good health and also often referred to as the most important antioxidant food for the human body. Soils from more than 40 countries and regions in the world were reported to be deficient in Se. In China, soils from72% of the sampled areas were found lacking in or containing very low levels of Se, and more than 300 million people were suffered from severe Se deficiency symptoms, with serious adverse effects on their life and health. Recent studies have shown that the safest and most effective way to supplement Se is by means of the soil-plant- (animal) -human food chain. Therefore, artificial selenium supplementation has become an important means of preventing various selenium-deficient diseases, especially by the development and utilization of selenium-enriched products for China and even the whole world. Because pe-can nut contains a small amount of selenium and can be used as a source of selenium, it may be used as an ideal natural selenium supplement. In this experiment, the effects of sodium selenite treatment with different concentrations was studied on fruit development, mineral element accumulation and nut quality, so as to provide technical reference for the scientific use of selenium in pecan cultivation and the development of selenium-enriched nut products.【Methods】The study was carried out at an orchard located in Yangmei mountain, Xinchang county, Zhejiang province, China (30°15′ N, 119°58′ E) , with the altitude of 50 m, the annual mean temperature of 15.6 ℃, January average temperature of -1 ℃, August average temperature of 32 ℃ and the extreme minimum temperature of -10.8 ℃. The annual precipitation is 1 478 mm and the land is yellow-red soil. The materials were four 12-year-old fruit-bearing pecan (‘Weston') trees grafted on Mahan stocks. On July 21 and July 29, 2017, the healthy and disease-free fruits selected from the four directions of the East, South, West and North of the middle canopy were sprayed with different concentrations of sodium selenite solutions. The treatments were as follows: T1:20 mg·kg^-1 Na2 SeO3, T2: 40 mg· kg^-1 Na2 SeO3, T3: 80 mg· kg^-1 Na2 SeO3, and T4: 160 mg· kg^-1 Na2 SeO3 with CK sprayed with purified water. All the above treatments were repeated three times. Four pickings were carried out on August 15 (S1) , August 31 (S2) , September 16 (S3) and September 30 (S4) in accordance with the corresponding labels. Twelve fruit samples (48 in total) with healthy, disease-free, pest-free, and almost uniform size were taken from each direction of the canopy. The samples from each treatment were divided into three equal parts and the fresh fruits were parted into four parts of husk, shell, seed coat and kernel. These materials were placed in an oven at 65 ℃ until constant weight, and then crushed after 60 mesh screen. The fruit, nut and kernel were weighed with an electronic balance. The contents of Zn, Cu, Mn, Fe, K, Ca, and Mg in the four parts of husk, shell, seed coat and kernel were measured by atomic absorption spectrometry. The Se content in the kernels was determined by inductively coupled plasma mass spectrometry. The fat content of the kernels was determined by Soxhlet extractor method. The content of fatty acids in kernels was determined using the internal standard method. The amino acid content of the kernels was determined using an amino acid analyzer.Data were expressed as X±SE (mean±standard error) , data analysis was made using GraphPad Prism 5 software, and MS Excel 2007 software was used to draw the tables. Data variance, significance and correlation analysis were performed using SPSS 19.0, with significant differences by using Duncan's new multiple range test.【Results】High selenium treatment showed a significant effect on the nut quality.When spraying with 160 mg·kg^-1 sodium selenite, the weights of fruit, nut and kernel were 31.20 g, 10.80 g and 5.45 g, respectively, which were significantly higher than the control by 13.91%, 6.73%, and 11.22%, respectively. Selenium spray treatments promoted the accumulation of Se, Zn, Mn, and Mg in kernels and inhibited the accumulation of Cu, Fe, K, and Ca. With the increase of the concentration of selenium treatments, the Se content of the kernels increased significantly. When spraying with160 mg· kg^-1 sodium selenite, the Se content was 0.50 mg· kg^-1, which was a significant increase of 49 times compared to the control. Selenium treatment was significantly positively correlated with the Zn contents in kernels. When spraying with 160 mg· kg^-1 sodium selenite, the content of Zn significantly increased by 11.16% compared with the control. The inhibitory effect of selenium on the content of Cu, K and Ca showed a tendency to increase and then weaken with the increase of selenium concentration. And the inhibitory effect of selenium on the content of Fe gradually declined with the increase of concentration. There was no significant difference in the contents of unsaturated fatty acids among the treatments. The ratio of unsaturated to saturated fatty acids was T4 (11.40) >T3 (11.01) >CK (10.87) >T2 (10.49) > T1 (10.27) . The content of each amino acid in kernels sprayed with selenium was higherthan that of the control. The total amount of amino acids was T4 (12.21 g·100 g^-1) > T3 (11.25 g·100 g^-1) >T1 (11.20 g·100 g^-1) > T2 (11.09 g·100 g^-1) >CK (9.35 g·100 g^-1) . In addition, the contents of amino acid (except lysine and methionine) in the kernels sprayed with 160 mg · kg-1 selenium was significantly higher than that in the control.【Conclusion】The application of sodium selenite to the pecan fruit promoted the growth and development of the fruit, which affected the accumulation of mineral elements in pecan kernel and improved the nut quality to a certain extent. Especially, the spraying with 160 mg· kg-1 of sodium selenite, significantly increased the weights of fruit, nut and kernel, significantly promoted the absorption of selenium and zinc elements, increased the contents of amino acids in kernels, and improved the nut quality. It is suggested that pecan fruits have the ability to accumulate selenium efficiently.