Effects of different ammonium/nitrate ratios on leaf physiological characteristics, yield and quality in muskmelon

Abstract: 【Objective】Muskmelon belongs to Cucumis in the Cucurbitaceae family. It is an annual vine herb with rich nutrition and aroma, sweet and of refreshing taste. In order to provide theoretical basis for high yield and quality cultivation of muskmelon, the effects of ammonium/nitrate ratios on leaf phys-iological characteristics, yield and quality were studied under the multi span shed and plastic film mulching conditions. Nitrogen is an important component of protein, nucleic acid, hormone and other important organic compounds in plants. It plays an extremely important role in the formation of crop yield and quality, and is closely related to its physiological metabolic activities. The main forms of inor-ganic nitrogen absorbed by plants are nitrate nitrogen and ammonium nitrogen. Different forms of nitro-gen affect the physiological metabolism of crops, and ultimately affect the yield and quality. Under sin-gle nitrate nitrogen nutrition, the growth of underground and aboveground parts of crops is unbalanced, the root system is long and thin, and the aboveground growth is slender. Under the single ammonium ni-trogen applying conditions, the crop leaf area is smaller, the leaf proportion is larger, and the crop growth is inhibited. Moreover, the higher ammonium nitrogen concentration would cause the crop to show green deficiency symptoms, resulting in ammonium toxicity. However, proper application of am-monium nitrogen fertilizer could improve the crop growth. Simultaneously, it could significantly reduce the nitrate content in the crop.【Methods】The experiment was carried out in a multi-span shed in the Modern Experimental Base of Henan Academy of Agricultural Sciences (Yuanyang) in 2016 and 2017.The tested muskmelon variety was Zhongyun 20, which was selected by the Institute of Horticulture,Henan Academy of Agricultural Sciences. Under the condition of the same nitrogen supply level (200kg · hm^-2), five different levels of ammonium/nitrate ratios (NH4+-N/NO3--N) were set up, which were 0∶100, 25∶75, 50∶50, 75∶25 and 100∶0. In the early stage of muskmelon fruit setting, the leaves of 3 representative muskmelon plants were selected from each plot to determine the soluble protein content, malondialdehyde content, superoxide dismutase activity and catalase activity. After the fruit ripened, the yield was calculated according to the plot area. Five representative fruits were selected from each plot, and the weight, vertical and transverse diameters and flesh thickness of each fruit were measured, and the soluble solids contents in center and edge as well as titratable acid, vitamin C and soluble protein contents were measured. The solids-acid ratio and fruit shape index were calculated.【Results】The re- sults from two-year experiments showed that the variation trend of soluble protein content (SP), super-oxide dissolving enzyme (SOD) and catalase (CAT) activities in muskmelon leaves after fruit setting was similar among different ammonium/nitrate ratios. The results showed that the activities of SP, SOD and CAT increased first and then decreased with the increasing application of NH4+-N/NO3--N ratio. The content of SP was the highest when the ammonium/nitrate ratio was 25∶75, while the content of SOD and CAT was the highest when the ammonium/nitrate ratio was 50∶50. The effects of different ammoni-um/nitrate ratios on MDA content were opposite to those of SP, SOD and CAT. The results showed the MDA content decreased first and then increased with the increase of NH4+-N/NO3--N ratio. When the ammonium/nitrate ratio was 50∶50, the MDA content was the lowest, and the pure ammonium treat-ment was the largest, which indicated that the higher proportion of ammonium/nitrate might inhibit the activity of related enzymes in plant nitrogen metabolism, resulting in ammonium toxicity and destroy-ing the membrane system. When the ratio of ammonium to nitrate was 50∶50, the yield of muskmelon was the highest. The soluble solid contents in the center and edge as well as solid/acid ratio, vitamin C and soluble protein contents increased first and then decreased with the increase of ammonium/nitrate ratio. While the contents of soluble solid in the center and edge were the highest when the ratio of am-monium/nitrate was 25∶75, and the lowest was in the pure nitrate treatment. The solid/acid ratio, vita-min C and soluble protein contents were the highest when the ratio of ammonium nitrate to nitrate was 50∶50, the lowest was pure nitrate treatment, and the titratable acid content was the lowest with the in-crease of the ratio of ammonium to nitrate, the changing trend was opposite to the above nutritional quality indicators, which showed the trend of first decreasing and then increasing with the increase of ammonium/nitrate ratio.【Conclusion】Through comprehensive analysis of effects of different ammoni- um/nitrate ratios on leaf physiological characteristics, yield and quality of muskmelon, it was deter-mined that the yield and quality increased by applying reasonable ratio of ammonium/nitrate under the condition of high yield and quality cultivation with plastic film mulching. The optimum ratio of ammo- nium to nitrate was 50∶50 or 25∶75. The ratio of ammonium/nitrate that was too high or too low would reduce the activities of antioxidant protective enzymes and osmotic adjustment substances in the leaves during its reproductive growth of muskmelon, increase the membrane lipid peroxidation products, and aggravate the senescence of membrane lipid peroxidation in leaves, which was not conducive to the im-provement of yield and quality.