Effects of exogenous selenium application on selenium distribution, absorption-translocation in different organs of Ruixue apple

【Objective】The study aimed to explore the absorption and translocation patterns of exogenous selenium in different organs of apple trees. After applying selenium-enriched organic fertilizer and an equal amount of non-selenium-enriched organic fertilizer to Ruixue apple trees, we measured the total selenium, organic selenium, and inorganic selenium contents in various organs of the apple trees in order to provide a theoretical basis for the scientific production of selenium- enriched apples.【Methods】Taking Ruixue apple trees (with T337 as the rootstock) as experimental materials, a controlled pot experiment was conducted. During the fruit enlargement period, apply 0.05 kg per plant of selenium-enriched organic fertilizer evenly in the ring-shaped groove. An equal amount of non-selenium-enriched organic fertilizer was used as the control (CK). At the fruit maturity stage, the whole fruits, leaves, later-al branches, main trunk, underground stem segments of the rootstock, and underground fibrous roots of the trees were collected via destructive sampling. Each organ was excised, brought back to the laboratory, rinsed with deionized water, dried in an oven at 105 ℃ for 30 minutes, and then baked at 80 ℃ for 8 hours. After that, the dry mass of each organ was weighed using a balance. Each organ was crushed using a grinder and sealed in a bag for the determination of total selenium and inorganic selenium content using fluorescence spectrophotometry. After the soil samples were naturally air- dried, they were crushed using a grinder, passed through a 100-mesh sieve, and then placed in sealed bags for the determination of total selenium and available selenium content by fluorescence spectrophotometry. By comparing indicators such as total selenium, organic selenium, and inorganic selenium content in various organs, the absorption, translocation, and distribution patterns of exogenous selenium in different tree organs were determined.【Results】After the application of exogenous selenium, the total selenium and organic selenium contents in all organs of Ruixue apples gradually decreased from the underground fibrous roots to the fruits (i.e., from the bottom to the top of the plant). Specifically, the order of total selenium content from high to low was: underground fibrous roots ＞ the underground stem segment of the rootstock ＞ main trunk ＞ lateral branches ＞ leaves ＞ fruits. The order of organic selenium content from high to low followed the same pattern: underground fibrous roots ＞ the underground stem segment of the rootstock ＞ main trunk ＞ lateral branches ＞ leaves ＞ fruits. By contrast, the order of inorganic selenium content from highest to lowest was: leaves ＞ underground fibrous roots ＞ lateral branches ＞ main trunk ＞ the underground stem segment of the rootstock ＞ fruits. The percentage of organic selenium in each organ reached over 60%. The total selenium accumulation, ranked from highest to lowest, was as follows: main trunk ＞ underground fibrous roots ＞ the underground stem segment of the rootstock ＞ fruits ＞ leaves ＞ lateral branches. Sorted by enrichment coefficient from largest to smallest, the order was as follows: underground fibrous roots ＞ the underground stem segment of the rootstock ＞ main trunk ＞ lateral branches ＞ leaves ＞ fruits. Sorted by distribution rate from largest to smallest, the order was as follows: main trunk ＞ underground fibrous roots＞the underground stem segment of the rootstock ＞ fruits ＞leaves＞lateral branches. The transport coefficients sorted from largest to smallest were as follows: TF Soil-Underground fibrous roots ＞ TF Main trunk-Lateral branches ＞ TF Lateral branches-Leaves ＞ TF Underground fibrous roots-Main trunk ＞ TF Leaves-Fruits ＞ TF Underground fibrous roots-The underground stem segment of the rootstock. Among all organs of Ruixue apples in the CK treatment, total selenium content was the highest in underground fibrous roots. The total selenium content from high to low was as follows: Underground fibrous roots ＞ lateral branches ＞ main trunk ＞ the underground stem segment of the rootstock ＞ leaves ＞ fruits, and the organic selenium content, from highest to lowest, was as follows: underground fibrous roots ＞ main trunk ＞ lateral branches ＞ fruits ＞ leaves ＞ the underground stem segment of the rootstock. The inorganic selenium content, ranked from highest to lowest, was as follows: underground fibrous roots ＞ the underground stem segments of the rootstock ＞ lateral branches ＞ leaves ＞ main trunk ＞ fruits. The percentage of organic selenium in each organ was significantly lower than that of CK. The total selenium accumulation, ranked from highest to lowest, was as follows: underground fibrous roots ＞ main trunk ＞ fruits ＞ leaves ＞ the underground stem segment of the rootstock ＞ lateral branches. The enrichment coefficients, sorted from largest to smallest, were as follows: underground fibrous roots ＞ lateral branches ＞ main trunk ＞ the underground stem segment of the rootstock ＞ leaves ＞ fruits, and the distribution rate from largest to smallest was as follows: underground fibrous roots ＞ fruits ＞ leaves ＞ the underground stem segment of the rootstock ＞ lateral branches. The transport coefficients sorted from largest to smallest were as follows: TF Soil- Underground fibrous roots ＞ TF The underground stem segment of the rootstock- Main trunk ＞TF Main trunk-Lateral branches ＞ TF Leaves- Fruits ＞ TF Lateral branches-Leaves ＞ TF Underground fibrous roots-The underground stem segment of the rootstock. Except that the inorganic selenium content in the underground stem segment of the rootstock under exogenous selenium treatment was lower than that of CK, exogenous selenium treatment significantly increased the total selenium content, organic selenium content, inorganic selenium content, and the percentage of organic selenium in each organ of Ruixue apples compared with CK. Under selenium treatments, total selenium accumulation, enrichment coefficient, distribution rate and translocation coefficient were all higher in fibrous roots, reaching 19.31 µg, 236.44%, 35.99% and 236.46%, respectively for the CK group, fibrous roots also exhibited the highest total selenium accumulation, enrichment coefficient, and translocation coefficient, which were 6.78 µg, 172.75%, and 172.74%, respectively. Exogenous selenium application significantly increased both total soil selenium content and available soil selenium content.【Conclusion】Compared with the application of selenium- organic fertilizer, exogenous selenium application increased the selenium content in different organs of the tree. The selenium content in fruits reached 60.67 µg · kg- 1 , meeting the standard for selenium-enriched apples (30 µg · kg- 1 ). Therefore, selenium-enriched apples could be produced by properly applying exogenous selenium.