China accounts for about half of the world's apple area and production, and is a pillar industry that promotes farmers' income growth and rural revitalization. In recent years, with the dramatic change of the global climate, most apple (Malus domestica) producing areas in China have suffered salt damage. Soil salinization, especially secondary salinization, is common in apple producing areas in the North China Plain around the Bohai Sea. These phenomena indicate that the ability of apple industry to prevent and resist soil salinization still needs to be improved. In this article, we identified a novel salt sensitive gene MdGPCR in apple. We conducted salt stress experiments on apple callus and leaves using MdGPCR. The results showed that after salt stress, overexpression of MdGPCR in transgenic apple would accumulate a large amount of reactive oxygen species, leading to an imbalance of redox levels in their bodies. At the same time, MdGPCR interacts with MdSOS3 and promotes its degradation, further reducing its salt stress resistance.

There is a close relationship between potassium (K) and nitrogen (N). However, the roles of K under high N conditions remain unclear. Using a hydroponics approach, we monitored the morphological, physiological, and molecular changes in M9T337 apple (Malus domestica) rootstocks under different nitrate (10 and 30 mmol·L⁻¹) and K supply (0.5, 6, 10, and 20 mmol·L⁻¹ K⁺) conditions. Results revealed that high nitrate inhibited the root growth of M9T337 rootstocks, downregulated the expressions of K transporter genes (MdPT5, MdHKT1, and MdATK1), and reduced the net and K⁺ influx at the surface of roots, thereby resulting in an N/K imbalance in rootstocks. Further investigation showed that 10 mmol·L⁻¹ K increased the activity of N metabolic enzymes (NR, GS, NiR, and GOGAT), upregulated the expressions of genes related to nitrate uptake and transport (MdNRT1.1, MdNRT1.2, MdNRT1.5, and MdNRT2.4), promoted ¹⁵N transport from the roots to the shoots, optimized leaf N distribution, and improved photosynthetic N utilization efficiency under high nitrate conditions. These results suggest that the negative effects of high nitrate may be related to the N/K imbalance and that reducing N/K in plants by increasing K supply level can effectively alleviate the inhibition of N assimilation by high nitrate stress.

Soil microbial communities play an essential role in maintaining soil fertility and are considered as ecological indicators to evaluate soil health. In the present study, we examined the influence of almost 4 years of fertilization [no fertilizer (CK), nitrogen alone (N), nitrogen, phosphorus and potassium chemical fertilizer (NPK), organic manure (M), nitrogen plus organic manure (NM), and NPK plus organic manure (NPKM)] on soil fertility and the functional diversity of soil microbial communities in an apple orchard. Compared to CK, fertilization increased soil organic carbon, total nitrogen, and available nutrients, but reduced soil pH in N and NPK treatments. The highest microbial biomass carbon and nitrogen, most probable number of actinomycetes, bacteria, and fungi occurred in the NPKM treatment. The average well color development (AWCD) values followed the order of NPKM > M> NPK and NM > CK and N. The Shannon index in organic manure treatments were significantly higher than in control and in treatments without organic manure. The principal component analysis showed that manure treatment was significantly separated from other treatments. These results indicated that organic manure applied alone or in combination with chemical fertilizers would increase soil fertility and functional diversity of soil microbial communities. Moreover, applying balanced N, P, K fertilizer in combination with organic manure was found to be superior to the use of a single fertilizer in improving soil microbial community quality.

Soil and leaf nutrient analysis are widely used as effective methods of diagnosing nutrient deficiency in fruit trees, the results of which are used to properly manage fertilizer application. Therefore, a survey was conducted for assessment of the soil nutrient status and leaf nutrient concentration in 2 827 apple orchards in the Bohai Bay and Loess Plateau apple production regions of China. The soil organic matter, alkali hydrolyzable N, available P, and available K were 10.91 g·kg⁻¹, 73.21 mg·kg⁻¹, 70.22 mg·kg⁻¹, and 169.23 mg·kg⁻¹ in the Bohai Bay region, respectively, and 11.72 g·kg⁻¹, 56.46 mg·kg⁻¹, 14.91 mg·kg⁻¹, and 135.78 mg·kg⁻¹ in the Loess Plateau region, respectively. Soil organic matter was at a medium-to-low level in both regions, whereas the soil alkali hydrolyzable N was low. In the Bohai Bay region, soil available P was high, but soil available K was deficient. In contrast, both soil available P and K were insufficient in the Loess Plateau region. The Diagnosis and Recommendation Integrated System (DRIS) diagnostic results indicated that the most deficient elements were Ca and K in low-yielding orchards (<35 t·hm⁻²) of the Bohai Bay region followed by Fe, N, and Zn; however in the Loess Plateau region, the most deficient elements were P and K followed by N, Zn, and Cu. The findings imply that the application of Ca, K, Fe, N, and Zn fertilizer should be increased in the Bohai Bay region, whereas P, K, N, Zn, and Cu fertilizer should be enhanced in the Loess Plateau region. Meanwhile, use of organic manure is recommended to improve soil quality in the two apple producing regions.