Effect of spraying with selenium fertilizer on yield, quality and nutrient absorption of watermelon in gravel-mulched field

Abstract：【Objective】The selenium content of watermelon is higher in gravel-mulched field than in other types of fields because selenium is rich in the watermelon, also called“selenium-melon”. In recent years, selenium-melon has become one of main commercial crops in Ningxia Hui Autonomous Region and Gansu province of China. According to the study, the selenium content of gravel layer decreased with the increase of cultivation time, which also affected the selenium uptake by watermelon. This study examined the effect of suitable exogenous selenium application on the yield, quality and nutrient uptake by watermelon fruit, which would lay a theoretical basis for high quality cultivation and reasonable application of selenium fertilizer for watermelons in middle and old gravel-mulched fields.【Methods】The experiment was carried out at Gaolan County, Gansu province, China, where there has been a long history in watermelon cultivation in gravel- mulched fields. A late- mature watermelon cultivar‘Xisharuibao’ served as materials and Na2SeO3 as selenium source, six concentrations of Na2SeO3 (0, 20, 40, 60, 80, 100 mg ·L^-1 ) were set to spray the watermelon leaves for three times (vine-extending stage, flowering stage and fruit-setting stage). The treatments were arranged in a randomized manner with three replicates, and the area of trial plot was 32 m2 . The effects of selenium nutrient solution on the fruit yield, quality and nu-trient absorption by watermelons in gravel-mulched field were studied.【Results】The yield, quality and a part of nutrient elements of watermelon fruit were promoted at lower selenium concentrations, while they were inhibited at higher selenium concentrations, with the highest happening when the concentration of selenium was 60 mg·L^-1. The yield as well as soluble solids and vitamin C contents increased by 12.60%, 8.03% and 10.51%, respectively than the control, and sugar gradient between fruit core and edge was the lowest. Nitrogen content in fruit showed firstly increased and then decreased with the increasing of the selenium concentrations , the N content of the treatments with 40 mg ·L^-1 , 60 mg ·L^-1 and 80 mg ·L^-1 increased by 13.95%, 14.73% and 12.40%, respectively than the control. No obvious influence was found on phosphorus content in fruit at low selenium concentrations, but it decreased significantly at high selenium concentrations. The potassium content in fruit increased with the increasing of the concentration of selenium, the potassium content of the treatments with 60 mg·L^-1 , 80 mg·L^-1 and 100 mg·L^-1 increased by 120.09%, 127.38% and 229.70%, respectively than the control, and facilitation for the absorption of three elements was in the order of K＞N＞P. The calcium and magnesium contents in fruit firstly increased and then decreased with the increasing of the selenium concentrations , which was also the highest at the selenium concentration of 60 mg·L^-1 and increased by 48.29% and 19.49%, respectively than the control, and facilitation for the absorption of two elements was in the order of Ca＞Mg. The contents of selenium, zinc and iron in fruit increased with the increasing of the selenium concentrations , the zinc, iron and selenium contents in fruit increased by 94.74%-230.70%, 65.20%-126.01% and 209.45%-252.08%, respectively than the control when the concentration of selenium fertilizer was 60-80 mg·L^-1, and facilitation for the absorption of three elements was in the order of Se＞Zn＞Fe. For selenium content in fruit, it increased more quickly when the concentration of selenium fertilizer ranged from zero to 60 mg ·L^-1 and then went stabilized. Correlation analysis showed that the concentration of selenium fertilizer was highly positively correlated with the contents of other nutrient elements except phosphorus when concentration was less than or equal to 60 mg·L^-1 , but it was significantly positively correlated only with K, Zn, Fe and Se, and negatively correlated with N, P, Ca and Mg when concentration was greater than 60 mg·L^-1.【Conclusion】The results showed that suitable concentrations of selenium fertilizer could improve yield, quality and nutrient absorption by watermelon in gravel-mulched field. The selenium content in fruit was between 0.016-0.032 mg·kg^-1 under the condition of spraying with selenium fertilizer，which reached the national standard for selenium- rich food. Therefore, selenium-enriched watermelon could be produced by foliage spray method, and the effect of selenium concentration on yield, quality and nutrient absorption by watermelon was considerable, so we suggest that the optimum concentration of Na2SeO3 for application was 60 mg·L^-1 in gravel-mulched field.