Nitrogen (N) is one of the most required mineral elements for plant growth, and potassium (K) plays a vital role in nitrogen metabolism, both elements being widely applied as fertilizers in agricultural production. However, the exact relationship between K and nitrogen use efficiency (NUE) remains unclear. Apple dwarf rootstock seedlings (M9T337) were used to study the impacts of different K levels on plant growth, nitrogen metabolism, and carbon (C) assimilation in water culture experiments for 2 years. The results showed that both deficiency and excess K inhibited the growth and root development of M9T337 seedlings. When the K supply concentration was 0 mM and 12 mM, the biomass of each organ, root-shoot ratio, root activity and NO 3 − ion flow rate decreased significantly, net photosynthetic rate (P n) and photochemical efficiency (F v /F m) being lower. Meanwhile, seedlings treated with 6 mM K + had higher N and C metabolizing enzyme activities and higher nitrate transporter gene expression levels (NRT1.1; NRT2.1). 13 C and 15 N labeling results showed that deficiency and excess K could not only reduce 15 N absorption and 13 C assimilation accumulation of M9T337 seedlings, but also reduced the 15 N distribution ratio in leaves and 13 C distribution ratio in roots. These results suggest that appropriate K supply (6 mM) was optimal as it enhanced photoassimilate transport from leaves to roots and increased NUE by influencing photosynthesis, C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport.

We examined the effects of three fertilization treatments (CK, NPK, and NPKM) on soil pH, soil organic carbon (SOC), total nitrogen (TN), C/N ratio, and available nutrients [alkali extractable-N (AN), available phosphorus (AP), exchangeable K (EK)] in the plow layer (0-40 cm) in an apple orchard from 1988 to 2016 in North China, as well as the mechanisms of nitrogenous fertilizer action on soil acidification through a three-year experiment.

Excessive application of nitrogen (N) fertilizer is common in Chinese apple production. High N reduced the contents of soluble sugar and total flavonoids by 16.05 and 19.01%, respectively, resulting in poor fruit quality. Moreover, high N increased the total N and decreased the total C and C/N ratio of apple fruits. On the basis of the transcriptomic, proteomic, and metabolomic analyses, the global network was revealed. High N inhibited the accumulation of carbohydrates (sucrose, glucose, and trehalose) and flavonoids (rhamnetin-3-O-rutinoside, rutin, and trihydroxyisoflavone-7-O-galactoside) in fruits, and more C skeletons were used to synthesize amino acids and their derivatives (especially low C/N ratio, e.g., arginine) to be transferred to N metabolism. This study revealed new insights into the decline in soluble sugar and flavonoids caused by high N, and hub genes (MD07G1172700, MD05G1222800, MD16G1227200, MD01G1174400, and MD02G1207200) and hub proteins (PFK, gapN, and HK) were obtained.

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In order to improve the problem of poor coloring caused by high fruit nitrogen in apple production, we studied the effects of different concentrations of abscisic acid (ABA: 0, 50, 100, and 150 mg/L) and fluridone (ABA biosynthesis inhibitor) on the fruit of 'Red Fuji' apple (Malus Domestica Borkh.) in the late stage of apple development (135 days after blooming) in 2017 and 2018. The effects of these treatments on the distribution of 13 C and 15 N and anthocyanin synthesis in fruit were studied. The results showed that the expression levels of ABA synthesis and receptor genes in the peel and flesh were upregulated by exogenous ABA treatment. An appropriate concentration of ABA significantly increased the expression of anthocyanin synthesis genes and transcription factors and increased the content of anthocyanin in the peel. The results of 13 C and 15 N double isotope labeling showed that exogenous ABA coordinated the carbon-nitrogen nutrient of apple fruit in the late stage of the development, reduced the accumulation of fruit nitrogen, increased the accumulation of fruit carbon and sugar, provided a substrate for anthocyanin synthesis, or promoted anthocyanin synthesis through the sugar signal regulation mechanism. Comprehensive analysis showed that the application of 100 mg/L ABA effectively improved the problem of poor coloring caused by high fruit nitrogen in the late stage of apple development and is beneficial to the accumulation of carbon in fruit and the formation of color.

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In order to define the effects of fruit crop load on the distribution and utilization of carbon and nitrogen in dwarf apple trees, we conducted three crop load levels (High-crop load, 6 fruits per trunk cross-sectional area (cm2, TCA)), Medium-crop load (4 fruits cm−2 TCA), Low-crop load (2 fruits cm−2 TCA)) in 2014 and 2015. The results indicated that the 15N derived from fertilizer (Ndff) values of fruits decreased with the reduction of crop load, but the Ndff values of annual branches, leaves and roots increased. The plant 15N-urea utilization rates on Medium and Low-crop load were 1.12–1.35 times higher than the High-crop load. With the reduction of crop load, the distribution rate of 13C and 15N in fruits was gradually reduced, but in contrast, the distribution of 13C and 15N gradually increased in annual branches, leaves and roots. Compared with High-crop load, the Medium and Low-crop load significantly improved fruit quality p < 0.05. Hence, controlling fruit load effectively regulated the distribution of carbon and nitrogen in plants, improved the nitrogen utilization rate and fruit quality. The appropriate crop load level for mature M.26 interstocks apple orchards was deemed to be 4.0 fruits cm−2 TCA.