Effects of foliar application of selenium on selenium and heavy metal contents and fruit quality in sweet cherry

Abstract: 【Objective】Selenium is one of the essential trace elements in the human body. The deficiency of selenium could be harmful to human health. Although sweet cherries are tasty, beautiful and nutritious, which gets a very popular appeal to people, there is a lack of selenium in sweet cherries. In this experiment, the effects of different selenium concentrations were explored on the main biochemical indexes of leaves and fruits, the fruit quality and heavy metal content, so as to provide a theoretical basis for the production of selenium-rich sweet cherries.【Methods】Sweet cherry‘Tieton'was used as the experimental material, and the sodium selenite (Na2 SeO3) solution was selected as selenium source. The experiment was conducted in Cherry Valley, Houzhai Township, Erqi District, Zhengzhou City, Henan province. The plants were 6 years old with moderate tree vigor at a spacing of 2.5 m×3 m. Sizes of the selected plants as well as the quantity of fruits for every single plant were basically identical. The soil of the experiment area was light sandy soil with pH 7.5. On April 16, after the flower dropping, sodium selenite (Na2 SeO3) solution was sprayed on the surface of the leaves. One week later, the second time spraying was conducted. The concentrations for treatment included 5, 10, 20 and 40 mg · L-1. All of the spraying was completely randomized with three repetitions. Meanwhile, the control was treated with water. During the fruit ripening period (May 27) , appropriate amount of fruits and leaves were taken from the treated plants, which were distributed on the canopy in different directions. Some of them were placed in liquid nitrogen, while the others were brought back in self-sealing bags. The former were stored in an ultra-low temperature refrigerator of-80 ℃. Measurement included fruit hardness, single fruit weight, contents of titratable acid, soluble solids, vitamin C, selenium, cadmium, lead, mercury, MDA and Pro, as well as SOD activity.【Results】After spraying different concentrations of Na2 SeO3 on the leaves, the selenium in fruits and leaves increased as the increase of selenium concentration. With the concentrations of 5, 10, 20 and 40 mg · L^-1, respectively, selenium contents in leaves were 3.06, 7.01, 8.61 and 15.06 times than that of the control, reaching a significant level. Furthermore, with the concentrations of 5, 10, 20 and 40 mg · L^-1, selenium contents in fruits were 1.09, 5.74, 8.76 and 9.36 times higher than that of the control, reaching a significant level. After spraying different concentrations of selenium, the contents of heavy metals in leaves, including cadmium and lead decreased by different degrees. The selenium concentration of 20 mg · L^-1 had the best inhibitive effect on cadmium and lead contents in leaves. The selenium concentration of 5 mg·L^-1 had the most obvious inhibitive effect on cadmium and lead contents in fruits. In the control, the contents of mercury in leaves and fruits were 0.002 8 mg · kg^-1 and 0.000 7 mg·kg^-1, respectively. With the selenium treatment, there was no mercury detected in leaves or fruits. After spraying selenium on the leaves, SOD activity in leaves and fruits was higher than that of the control, reaching a significant level. SOD activity in leaves reached the highest level when selenium concentration was 20 mg·L^-1, while SOD activity in fruits reached the highest level with selenium concentration of 5 mg·L^-1. However, SOD activity in leaves and fruits decreased with the increase of selenium concentration. At the selenium concentrations of 10 mg·L^-1 and 20 mg·L^-1, the content of MDA in leaves was significant. The content of MDA in fruits reached a significant level when selenium concentration was 5 mg·L^-1. However, medium and low selenium concentrations had greater effects on reducing MDA than high selenium concentration. After treatment at different concentrations, the contents of proline in leaves and fruits decreased significantly. After spraying with different concentrations of selenium, the average single fruit weight increased, and the fruit hardness increased by different levels. The average single fruit weight increased most significantly when the selenium concentration was 10 mg · L^-1, and the hardness of fruit increased significantly when the selenium concentration was 5 mg · L^-1. The AsA content in fruits increased significantly, and titratable acid decreased significantly. At the selenium concentration of 5 mg·L^-1, the contents of AsA and titratable acid both reached a significant level.【Conclusion】The selenium sprayed on leaves of sweet cherries can increase the selenium contents in leaves and fruits. The selenium contents in leaves and fruits increased as the selenium concentrations increased. The heavy metal contents in leaves and fruits, including cadmium and lead, decreased significantly, while the content of mercury dropped to an undetected level. In leaves and fruits, SOD activity increased while MDA and Pro contents decreased and lower concentration of selenium had greater effects than higher one. The average single fruit weight and hardness increased. The AsA contents increased at different levels but the titratable acid contents decreased, and the fruit quality was improved.