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 NO₃^– 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). ¹³C and ¹⁵N labeling results showed that deficiency and excess K could not only reduce ¹⁵N absorption and ¹³C assimilation accumulation of M9T337 seedlings, but also reduced the ¹⁵N distribution ratio in leaves and ¹³C 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. The application of NPK and NPKM fertilizers improved SOC, TN, and available nutrients compared with the CK. The average C/N ratio decreased with the number of years of fertilization, and reached lowest in NPK treatment (6.80). By 2016, the soil pH in CK, NPK, and NPKM treatments was 6.17, 5.29, and 5.59, which were 0.16, 1.04 and 0.74, respectively, units lower than initial values. Compared with the previous 14 years (1988-2002), the latter 14 years (2002-2016) showed greater change rates of SOC, TN, C/N ratio, AN, AP, EK, and pH in all treatments. Further analysis showed that the highest net H⁺ production rate was found in the N900 treatment (136.8 kmol ha^-1 y^-1), followed by the N600 treatment (88.6 kmol ha^-1 y^-1), and the lowest appeared in the N300 treatment (50.6 kmol ha^-1 y^-1). Thus, long-term fertilization can remarkably increase SOC, TN, and available nutrients, especially in NPKM treatment; however, it also clearly decreased soil C/N ratio and pH due to the continuous excessive application of N fertilizers.

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 ¹³C and ¹⁵N 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 ¹³C and ¹⁵N 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.

Apple (Malus domestica Borkh. cv. “Fuji”), an important cash crop, is widely consumed around the world. Accurately predicting preharvest apple fruit yields is critical for planting policy making and agricultural management. This study attempted to explore an effective approach for predicting apple fruit yields based on time-series remote sensing data. In this study, time-series vegetation indices (VIs) were derived from Planet images and analyzed to further construct an accumulated VI ($\sum \mathrm{VIs}$)-based random forest (${\mathrm{RF}}_{\sum \mathrm{VI}}$) model and a Carnegie–Ames–Stanford approach (CASA) model for predicting apple fruit yields. The results showed that (1) $\sum \mathrm{NDVI}$ was the optimal predictor to construct an RF model for apple fruit yield, and the R², RMSE, and RPD values of the ${\mathrm{RF}}_{\sum \mathrm{NDVI}}$ model reached 0.71, 16.40 kg/tree, and 1.83, respectively. (2) The maximum light use efficiency was determined to be 0.499 g C/MJ, and the CASA_SR model (R² = 0.57, RMSE = 19.61 kg/tree, and RPD = 1.53) performed better than the CASA_NDVI model and the CASA_Average model (R², RMSE, and RPD = 0.56, 24.47 kg/tree, 1.22 and 0.57, 20.82 kg/tree, 1.44, respectively). (3) This study compared the yield prediction accuracies obtained by the models using the same dataset, and the ${\mathrm{RF}}_{\sum \mathrm{NDVI}}$ model (RPD = 1.83) showed a better performance in predicting apple fruit yields than the CASA_SR model (RPD = 1.53). The results obtained from this study indicated the potential of the ${\mathrm{RF}}_{\sum \mathrm{NDVI}}$ model based on time-series Planet images to accurately predict apple fruit yields. The models could provide spatial and quantitative information of apple fruit yield, which would be valuable for agronomists to predict regional apple production to inform and develop national planting policies, agricultural management, and export strategies.

Introduction: Both nitrogen (N) and magnesium (Mg) play important roles in biochemical and physiological processes in plants. However, the application of excessive N and insufficient Mg may be the factor leading to low nitrogen utilization rate (NUE) and fruit quality degradation in apple production.Methods: In this study, we analyzed the effects of different application rates of Mg (0, 50, 100, 150, 200 kg/ha) on the photosynthetic nitrogen use efficiency (PNUE), the accumulation and distribution of carbon (C), N metabolism, anthocyanin biosynthesis and fruit quality of the ‘Red Fuji’ apple in 2018 and 2019.Results: The results showed that the application of Mg significantly increased the ¹⁵NUE and increased the allocation rate of ¹⁵N in the leaves whereas the ¹⁵N allocation rate in the perennial organs and fruits was decreased. With the increase in Mg supply, the activities of N metabolism enzymes (NiR, GS, and GOGAT) were significantly promoted and the content of intermediate products in N metabolism (NO2−, NH4+, and free amino acid) was significantly decreased. Furthermore, an appropriate rate of Mg significantly promoted the net photosynthetic rate (P_n) and photosynthetic nitrogen use efficiency (PNUE), enhanced the enzyme activities of C metabolism (SS, SPS, S6PDH), and increased the contents of sorbitol and sucrose in leaves. In addition, Mg upregulated the gene expression of sugar transporters (MdSOT1, MdSOT3, MdSUT1, and MdSUT4) in fruit stalk and fruit fresh; ¹³C isotope tracer technology also showed that Mg significantly increased the ¹³C allocation in the fruits. Mg also significantly increased the expression of anthocyanin biosynthesis genes (MdCHS and MdF3H) and transcription factors (MdMYB1 and MdbZIP44) and the content of anthocyanin in apple peel.Conclusion: The comprehensive analysis showed that the appropriate application of Mg (150 kg/ha) promoted PNUE, C–N metabolism, and anthocyanin biosynthesis in apple trees.