Effects of spraying selenium in different forms and at different stages on selenium absorption and accumulation and main quality indexes of ‘Jintao' kiwifruit

Abstract: 【Objective】Selenium (Se) is recognized as an essential micronutrient for humans and animals as a crucial component of glutathione peroxidase (GPX) . As a new type of fruit in the twenty-first century, kiwifruit is more and more favored by the majority of residents. Consuming Se-rich fruit may serve the dual function of supplementing selenium nutrition and regulating diet structure. However, there are rare reports about the Se enrichment capacity of the kiwifruit. Therefore, this study aims to: 1) determine the maximum Se tolerability of the kiwifruit tree by foliar spraying different concentrations of Se, and 2) further explore the effects of spraying stage and Se source on the Se concentration, cumulative distribution and conversion, as well as on fruit quality.【Methods】Here, ‘Jintao'kiwifruit (Actinidia L.) was used for the experiments. The experiments were performed in the villages of Changliang, Jianshi county, Hubei, China. The soil was yellow brown, with pH 5.20, organic matter 12.17 g· kg^-1, alkali-hydrolyzable nitrogen 75.30 mg · kg^-1, available phosphorus 10.40 mg·kg^-1, available potassium110.54 mg· kg^-1, and total Se 22.35 μg· kg^-1. The planting density of kiwifruit trees was 1 200 plants· hm-2.In Experiment Ⅰ, Na2SeO3 and Na2SeO4 were selected as Se sources. There were four Se levels of 50, 100, 200 and 300 mg· L-1 and spray volume was 1 000 L· hm-2. Spray was conducted in the young fruit period (June 23, 2016) , and spraying clear water was used as the control group. In Experiment Ⅱ, in2017, based on the results of Experiment Ⅰ, in the same orchard, Na2SeO3 and Na2SeO4 were sprayed at100 mg· L-1 separately in the young fruit stage (June 25) , the expanding stage (July 31) and sugar accumulation stage (September 16) . The equal amount of clear water was applied at different stages as control. The experiment consisted of a total of 9 treatments, which were randomly arranged with three replicates. Kiwifruit samples were divided into peel and flesh for analyses of Se content, accumulation and distribution and quality parameters (moisture content, vitamin C, soluble solids and titratable acids) . Values obtained from different treatments were subjected to ANOVA. Separation of means was performed on multiple range tests using the SPSS 20.0. Origin 2017 and Excel 2016 were used to generate graphs and tables, respectively.【Results】Spraying Na2SeO3 and Na2SeO4 significantly increased Se concentration in the leaves and branches and the increment increased with the increase of Se concentration. When the concentration of was over 100 mg· L-1, leaves turned yellow with withered edge withered and other toxic symptoms. In both Se sources, Se concentration in the leaves and branches increased as the spraying was delayed. Se concentration in the leaves and branches in the treatment of spraying at sugar accumulation stage was about 3 to 4 times that in the treatment at the young fruit stage. At the same spraying stages, the Se concentration in the leaves and branches sprayed with Na2SeO4 was 1.7-2.3 and 1.2-1.4 folds that with Na2SeO3, respectively. The Se concentration in fruit flesh decreased significantly with the delaying of the spray, but Se concentration in the peel showed an opposite trend. Se concentration in each part of the fruit sprayed by Na2SeO4 was 1.1-1.8 times higher than that by Na2SeO3. Se source had no significant effect on the accumulation and distribution of Se within fruit, but spraying stage had significant effects on the accumulation and distribution of Se in the fruit. Se accumulation reached the highest (52%-58%) when Se was sprayed at young fruit stage, but it decreased by 16%-19% when sprayed in the expanding stage. Selenium was mainly accumulated and distributed in the peel when the spray was conducted at sugar accumulation stage, which accounted for 76%-78%. Spraying stage and Se source had no significant effect on the proportion of organic Se in different parts of fruit. Regardless of Se forms or spraying stages, the organic Se in the flesh and peel could be up to over 60% and 75%, respectively. The foliar application of Se increased the concentration of soluble solids and Vitamin C, reduced the titratable acids and improved the quality of kiwifruit.【Conclusion】For kiwifruit, at a spraying volume of 1 000 L·hm-2, the highest safe concentration of Se is 100 mg· L-1. The utilization efficiency of Se in kiwifruit is higher when Se is supplied in Na2SeO4 form than in Na2SeO3 fruit. Se sprayed at early stages is readily transported to the flesh, while after the spraying stage, more Se sprayed is distributed in the peel. Therefore, advancing the spraying with Na2SeO4 is recommended for producing Se-enriched kiwifruit.