Effects of different nitrogen and potassium fertilizing quantities on the yield and nutritional quality of melon (Cucumis melo L.)

Abstract:【Objective】Agricultural production continues to be constrained by a number of biotic and abiotic factors that can reduce crop yield quantity and quality. Nitrogen (N) and potassium (K) are essential nutrients that affect most of the biochemical and physiological processes that influence plant growth and metabolism. It also contributes to the survival of plants exposed to various biotic and abiotic stresses. N is the main component of protein and enzyme and plays an important role in crop growth. N is an important component of chlorophyll, and chlorophyll is the key to the production of organic matter through photosynthesis. Potassium is a catalyst for various enzymes in plants, which can promote photosynthesis, enhance plant resistance to disease, and increase the sugar content and quality of the fruit. To provide a theoretical basis for the rational application of nitrogen and potassium fertilizers with plastic film mulching in a high yield and quality cultivation environment, the effects of different nitrogen and potassium fertilizers on yield and nutritional quality of melon were studied.【Methods】An early maturing and thick-skinned melon variety‘RX8' (‘TC620-8-56'בTA11-1') with plastic film mulching cultivation in greenhouse was used for our study. In this study, five treatments (N2 K2, N2 K1, N1 K2, N1 K1 and NK) were performed to investigate the influence of different fertilizing quantities of nitrogen and potassium on chlorophyll content, root activity, yield and nutritional quality of melon. The amount of fertilizer applied to medium nitrogen and potassium (NK) treatment was determined according to the amount of nitrogen and potassium fertilizer applied locally in the past 5 years along with the soil fertili-ty of the experimental plot. The amount of nitrogen and potassium applied for the treatment of heavy nitrogen (N2) , heavy potassium (K2) , light nitrogen (N1) and light potassium (K1) were increased and decreased by 60% respectively on the basis of NK treatment.【Results】The effects of different nitrogen and potassium fertilizers on the SPAD value of nodal leaves, root activity, yield and the nutritional quality of melon were significantly different. Physiologically, the SPAD-value first increased and then decreased with the increase in the nitrogen application rate, which was not obvious with potassium, however, when the supplication of the nitrogen fertilizer was insufficient while applying the potassium fertilizer, the rate of chlorophyll decomposition could be reduced. The root activity of melon exhibited a downward trend with growth during fruit setting. The root activity of NK was the highest in the late development stage (49 days after planting) , and the difference was significantly higher than that of other treatments at 56 and 63 days after planting (p < 0.05) . From the yield and yield components, the differences of the fruit vertical and transverse diameter, shape index, average weight and yield of high potassium and low potassium treatments did not reach the 5% significant level under the same nitrogen level.The fruit vertical and transverse diameter, average weight and yield from high nitrogen-treatment were higher than those from low-nitrogen treatment, with an average increase of 4.81%, 6.04%, 19.8% and20.5% respectively under the same potassium supply. And the differences of these indexes, except for vertical diameter, reached a significant level of 5%. This study also shows that the fruit vertical and transverse diameter, average weight and yield of melon treated with NK were the highest, and the average weight and yield were significantly higher than those of other treatments (p < 0.05) , with an average increase of 21.6% and 22.1% respectively. In terms of nutritional quality of the fruit of melon, the center and marginal soluble solid contents, solid/acid, soluble protein content and vitamin C content from NK treatment were all significantly higher than those in the other treatments by 9.09%, 14.3% and32.9%, 12.4%, 16.4% (p<0.05) , respectively. With an increase of the nitrogen application rate, it first decreased and then increased. The content of titratable acidity of melon tended to decrease first and then increased with the increase of nitrogen and potassium supplied. The titratable acidity decreased at first and then increased with the increase in the nitrogen and potassium application rates. The titratable acidity of N2 K1 was the highest, followed by N2 K2, and NK was the lowest, which showed that the titratable acidity increased as the nitrogen fertilizer was increased and the differences between the treatments were significant, especially in the soil potassium deficiency conditions. However, a reasonable amount of nitrogen and potassium can reduce the titratable acid content and improve taste.【Conclusion】On the whole, the soluble solid content, solid/acid, soluble protein content, vitamin C and other beneficial taste substance contents and the production of the fruit of melon treated by NK were the highest, with the contents of unfavorable taste flavor substances such as titratable acid being the lowest, and 30 of the NK treated melon flesh texture scores were the highest, and 30 individuals had the highest score on the flavor quality of melons treated with NK. Therefore, under the high-yielding and high-quality cultivation conditions with continuous mulching and plastic film mulching, the amount of nitrogen and potassium fertilization of the NK treatment was 200 kg∙hm^-2 and 300 kg∙hm^-2 respectively which was conducive to the improvement in the yield, while taking into account the nutritional quality, and improved economic efficiency.