Effect of exogenous selenium application on selenium content and fruit quality as well as its physiological mechanism in walnut

Abstract:【Objective】In order to clarify the physiological responses of exogenous selenium application to enhancing selenium content and nut quality, the 6-year-old walnut (Zhonghe 1) trees were selected to study the effects of spraying three different selenium sources on the selenium content in leaves, husk and kernel during the fruit swell period. The effects of 3 selenium sources on fruit quality, mineral nutrient content and antioxidant enzyme activity were analyzed. Thus, it may set a perspective and provide a firm theoretical basis for future Se fertilizer application in Serich walnut production.【Methods】In the fruit swell stage, 400 mg ·L^-1 of sodium selenate, selenium yeast and sodium selenite solution were applied on the leaf surface of seedlings for 8 times. Selenium contents in leaves, husk and kernel were determined by hydride- generation atomic fluorescence spectrometry (HG-AFS) (AFS8510; Beijing Haiguang Instrument Co., Beijing, China). Fruit and kernel weights were weighed with an electronic balance. The rate of crude, linoleic, unsaturated fatty acid and saturated fatty acid in the kernel were determined by LC-MS and GC-MS. The concentrations of zinc (Zn), iron (Fe), potassium (K), calcium (Ca) and magnesium (Mg) in the kernel were measured by the inductively complied plasma-atomic emission spectrometry. Kernel samples (0.2 g) from 4 treatments were homogenized in 5 mL of 0.1 mol·L-1 phos-phate buffer (pH 7.8) and centrifuged at 4000 × g for 10 min at 4 ℃. And the supernatant was used to assay the concentration of malondialdehyde (MDA) and the activities of superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) by ELISA kit. The effects of different selenium sources on fruit quality, mineral nutrient content and antioxidant enzyme activity were analyzed.【Results】Compared with the water control, Se yeast treatment significantly increased selenium content by 192, 36 and 68 times in leaves, husk and kernel, respectively. Na2SeO4 and Na2SeO3 treatments also dramatically increased Se levels by 101 and 96 times in leaves, 23 and 24 times in husk, 63 and 61 times in kernel, respectively. Na2SeO4, Se yeast and Na2SeO3 notably increased the weight of fruit and kernel by 14.2%, 25.2% and 9.3%, and 18.1%, 36.2% and 20.6%, respectively, compared to the water control. With regards to the kernel quality, Na2SeO4, Se yeast and Na2SeO3 dramatically decreased the contents of kernel crude fat and kernel saturated fatty acid by 9.2% , 18.5% and 8.1% , and 7.6% , 13.7% and 6.4%, respectively, compared to the water control. However, 3 types of exogenous selenium markedly increased the rate of kernel linoleic acid content at the same level. It was worth noting that there were no significant differences in the rate of kernel unsaturated fatty acid contents among all treatments. The three selenium treatments increased the contents of Zn and K in the kernel to varying degrees, but significantly reduced the level of Ca, while there was no significant effect on Fe and Mg contents. Compared with water control, Na2SeO4, selenium yeast and Na2SeO3 treatments increased Zn and K contents by 13.2%, 22.5% and 12.1%, and 30.2%, 50.9% and 30.4%, respectively. However, the Ca content in the kernel was reduced to 17.1%, 17.4% and 17.3%, respectively. Compared to the appropriate control, Na2SeO4, Se yeast and Na2SeO3 notably increased the activity of SOD, POD and APX by 28.2%, 76.5% and 28.3%, 60.2%, 129.5% and 86.2%, and 9.9%, 24.4% and 17.3%, respectively. However, Se yeast and Na2SeO3 had no significant effects on MDA content, while Na2SeO4 decreased it by 26.8%, compared to the water control.【Conclusion】The exogenous selenium treatments could increase the selenium levels in leaves, husk and kernel, and the selenium yeast treatment had the most significant effect. The 3 selenium treatments significantly increased fruit and kernel weights and kernel linoleic acid content, but significantly decreased kernel crude fat and saturated fatty acid contents. Selenium spraying promoted the absorption of mineral nutrients like potassium and zinc, but inhibited the absorption of calcium. However, there was no significant effect on the absorption of iron and magnesium in the kernel. Spraying sodium selenate, selenium yeast and sodium selenite solution during walnut fruit swell stage significantly increased the activities of superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) in the kernel, among which selenium yeast treatment had the most significant effect. Except sodium selenate treatment significantly reduced malondialdehyde (MDA) content in the kernel, the other 2 selenium treatments had no significant effect on MDA content level. In a word, in the walnut fruit enlargement period, spraying 3 kinds of selenium fertilizer on the leaves (sodium selenate, yeast selenium and sodium selenite) can increase the selenium contents in leaf and fruit, thus affecting the absorption of mineral nutrients, improve the antioxidant capacity of the fruit, reduce the degree of peroxide, and then improve the quality of walnut fruit. Among them, foliar spraying with selenium yeast had the best comprehensive efficacy, so that it could be widely used in the seleniumrich cultivation of walnut.