Screening and comprehensive evaluation of nitrogen-potassium efficient resources of melon

Abstract：【Objective】Nitrogen and potassium affect the growth, yield and quality of crops, and potassium is known as the“quality element”. In order to solve the problems of wasted resources and soil degradation caused by high water and fertilization, the research on“fertilizer reduction and efficiency enhancement”has been launched. As a nitrogen and potassium-loving crop, melon (Cucumis melo L.) is an important crop in the improvement of the efficiency of nitrogen and potassium utilization by tapping its own nitrogen and potassium absorption and utilization potential and by screening and cultivating nitrogen-potassium efficient varieties. The study aimed to investigate the genetic differences of the different melon germplasm resources with the efficiency of nitrogen and potassium, establish the evaluation system of the efficiency of nitrogen and potassium in melon seedlings, screen the germplasm resources with high efficiency of nitrogen and potassium, and provide theoretical basis for melon nitrogen and potassium gene mining and new variety selection.【Methods】The 72 thin- skin melon varieties (lines) were grown in cavity tray substrates with normal nitrogen and potassium supply (CK, N 4.88 mmol·L^-1 , K2O 2.50 mmol·L^{- 1} ), low nitrogen (LN, N 0.01 mmol·L^{- 1} ), low potassium (LK, K2O 0.01 mmol·L^{- 1} ), and low nitrogen and potassium (LNK, N 0.01 mmol·L^-1 , K2O 0.01 mmol·L^-1 ). The net photosynthetic rate, transpiration rate, intercellular CO2 concentration, stomatal conductance, saturated vapor pressuredifference, instantaneous water use efficiency, plant height, stem thickness, leaf area, root volume, root fresh weight, aboveground fresh weight, root dry weight, aboveground dry weight, root to crown ratio, nitrogen content, potassium content, nitrogen accumulation, potassium accumulation, nitrogen use efficiency, potassium use efficiency, nitrogen use index, and potassium use index were collected from the 72 melon varieties (lines). The coefficients of variation, low N tolerance index, low K tolerance index and low N and K tolerance index were calculated for each trait. The correlation analysis, principal component analysis and cluster analysis were conducted using the integrated affiliation function method to comprehensively analyze and evaluate the nitrogen and potassium efficiency types of the different melon varieties (lines).【Results】The net photosynthetic rate, instantaneous water use efficiency, plant height, stem thickness, leaf area, aboveground fresh weight, root dry weight, aboveground dry weight, potassium content, nitrogen and potassium accumulation, nitrogen use index were all significantly lower in the low nitrogen, low potassium, and low nitrogen-potassium treatments than those in the normal nitrogen- potassium treatment. The transpiration rate, stomatal conductance, root volume, root fresh weight and nitrogen content, were significantly lower in the low nitrogen and low nitrogen-potassium treatments than those in the normal nitrogen-potassium and low potassium treatments. The saturated vapor pressure and nitrogen use efficiency were significantly higher in the low nitrogen and low nitrogenpotassium treatments than those in the normal nitrogen-potassium and low potassium treatments. The intercellular CO2 concentration was significantly lower in the low potassium treatment than that in the other treatments. The order of the root to crown ratio was low nitrogen＞low potassium＞normal nitrogenpotassium＞low nitrogen-potassium treatments. The potassium utilization efficiency and potassium utilization index of the low normal potassium treatment were significantly higher than those of the other treatments. The variability of the 23 traits of the melon seedlings at different levels of nitrogen and potassium ranged from 9.73% to 56.44%, and the coefficients of variation of all traits were greater than 10% except for the potassium content of the CK treatment. The coefficients of variation of the low nitrogen tolerance index, low potassium tolerance index and low nitrogen and potassium tolerance index for all the 23 traits were over 10%. The principal component analysis was conducted on the low nitrogen tolerance index, low potassium tolerance index and low nitrogen and potassium tolerance index of the 23 traits and the results showed that the cumulative contribution of the six principal components was as high as 84.613%, 85.898% and 85.214%, respectively. The results of the principal component analysis of the low nitrogen tolerance index, low potassium tolerance index and low nitrogen and potassium tolerance index were analyzed together, and the aboveground dry weight, nitrogen accumulation, aboveground fresh weight, K accumulation, stem thickness, potassium utilization efficiency, stomatal conductance, nitrogen content, intercellular CO2 concentration and instantaneous water utilization efficiency of leaves were finally established as screening indexes for the seedling nitrogen-potassium efficient varieties (lines) of the melon; The D values of the trait indicators determined by the principal component analysis were clustered to classify the 72 melon varieties (lines) into three categories of nitrogen (potassium) efficient, nitrogen (potassium) moderately efficient and nitrogen (potassium) sensitive varieties (lines). The results of D-value cluster analysis of nitrogen efficient, potassium efficient and nitrogen-potassium efficient varieties (lines) were comprehensively analyzed, and finally TS-16, TS-19, Yangchunbaixue, Sumi, Sumi No. 3 and Bingmeiren 128 were established as nitrogen-potassium efficient varieties (lines) of the melon, and Shijixingliangliguazi was a nitrogen and potassium sensitive variety (line). 【Conclusion】In this experiment, the results of the principal component analysis of the low nitrogen tolerance index, low potassium tolerance index and low nitrogen- potassium tolerance index were ana-lyzed, and the aboveground dry weight, nitrogen accumulation, aboveground fresh weight, potassium accumulation, stem thickness, potassium utilization efficiency, stomatal conductance, nitrogen content, intercellular CO2 concentration and instantaneous water utilization efficiency of leaves were finally established as the evaluation indexes of melon seedling nitrogen-potassium efficient varieties (lines), and finally TS-16, TS-19, Yangchunbaixue, Sumi, Sumi No. 3, and Bingmeiren 128 were selected as nitrogen and potassium efficient varieties (lines), and Shijixingliangliguazi was a nitrogen-potassium sensitive variety (line). This study synthesized and analyzed the screening results of the nitrogen efficient, potassium efficient, and nitrogen-potassium efficient. The accuracy and feasibility of the screening results of this experiment were determined to provide scientific basis for melon cultivation under stresses and breeding.