Effects of nitrogen and potassium supply on fruit yield and nutritional quality of aeroponically grown tomato cultivars

Tomato (Lycopersicon esculentum Mill.) fruit is one of the most consumed vegetables in the world. This vegetable is rich in minerals and human health-benefiting compounds, and thus long-term consumption of tomato fruits facilitates to reduce the incidences of cardiovascular disease and prostate cancer. Therefore, developing strategies to improve the tomato production should have great potential applications and significance in human health. Numerous studies have provided evidences that nutrition management is a key agronomic strategy to improve the yield and nutritional quality of vegetables. Accordingly, previous studies have investigated the effects of application of nitrogen (N) or potassium (K) alone on the yield and the quality of tomato fruits in hydroponics systems. However, the effects of combined application of the above two nutrients still remain largely unknown up to now. Furthermore, conventional hydroponic cultivation often results in oxygen deficiency of roots which would cause anoxia of the roots. Aeroponics is a system of advanced soilless culture technologies. It not only avoids the oxygen deficiency of roots, but also can efficiently meet the nutrient requirement of crops. The objective of this study was to investigate the combined effects of N and K on the yield and nutritional quality of tomato fruits in aeroponic cultivation.Two tomato cultivars, dwarf-red (D-red) and dwarf-yellow (D-yellow), were used three levels of N (2, 12, 22 mmol/L) and two levels of K (4, 8 mmol/L) were applied to the plants. After about 5-month growth, the tomato fruits were harvested, and the yield and several nutritional quality indexes including nitrate content, sugar acid content, antioxidant contents and antioxidant activity were analyzed.The fruiting rate, the size of fruit, the fruit yield, the contents of soluble sugar, soluble solids, carotenoids, β- carotene and lycopene, the DPPH radical scavenging rates and the ABTS value (both hydro-and lipo-antioxidant activity) of fruits were all increased when the N level was increased from 2 mmol/L to 12 mmol/L, but all of these were tended to decrease when the N level were further increased to 22 mmol/L. The titratable acidity and the nitrate content were both increased along with the increase of N level. In contrast, the contents of ascorbic acid, flavonoids and total phenolic were declined along with the increase of N level. In addition, increasing the K level from 4 mmol/L to 8 mmol/L could also increase the fruit yield, the contents of soluble sugar, soluble solids, ascorbic acid, flavonoids, total phenolic, carotenoids, β-carotene and lycopene of fruit, but decrease the nitrate content. In all N and K combination treatments, the D-red cultivar showed greater fruit yield, higher contents of antioxidant compounds, and stronger antioxidant activity than the D-yellow cultivar.In conclusion, 12 mmol/L N and 8 mmol/L K might be suitable for tomato cultivation in aeroponic system, which not only increased the fruit yield, but also improved the nutrition quality. The D-red should be a better cultivar for aeroponic cultivation compared with the D-yellow.