Nitrogen (N) and potassium (K) are key elements for crop growth, yet studies on the impact of N–K interactions on plant N and K status and yield are lacking.  This study aimed to develop effective indicators for diagnosing N and K nutrition and predicting the yield of wheat under N–K interactions based on the theoretical framework of a critical nutrient dilution curve.  A 4-year N–K interaction experiment involving three wheat cultivars was employed for building and validating nutrient indices (NIs) based on the critical N dilution curve (CNDC) and the critical K dilution curve (CKDC).  In addition, relevant data from the literature were collected for supplementary validation.  The results revealed that changes in parameter A1 of the critical K dilution curves (CKDCs) can reflect the impact of nitrogen application on K absorption and utilization.  However, the difference in K nutrition index (KNI) values calculated by CKDC under different N levels was not significant.  Based on the aboveground biomass (AGB), a universal CKDC was established and defined as K_c=3.63AGB^–0.37 under N–K interactions.  The results showed that the direct effects of N or K deficiency on crops could be quantified by the N–K interaction index (NKI) calculated by integrating CNDC and CKDC, and the changes in crop growth in response to proportional N and K concentrations could be determined by NKI as well.  In addition, topdressing N fertilizer at the jointing stage significantly improved the N–K interaction effect on the N nutrition index (NNI) and NKI at the booting stage (P<0.05), but it had no significant N–K interaction effect on the KNI.  All indicators at the heading stage demonstrated the best predictive capability for relative yield (RY) compared to other stages.  Compared with NNI and KNI, the prediction accuracy of yield with NKI improved by 11.63 and 17.44%, respectively.  The NKI has better performance in diagnosing N and K nutrition and predicting yield under N–K interactions than either NNI or KNI.  This result enhances our understanding of the effects of N–K interactions on wheat growth and has important applications for improving the accuracy of N and K nutrition diagnosis and yield prediction.

Global climate warming is characterized by diurnal and seasonal asymmetry, with greater increases at nighttime and in winter and spring.  Growing evidence has recognized that night-warming in winter and spring significantly impacts winter wheat production.  Pre-crop straw returning is the principal method for straw utilization, but the interactions between straw returning and night-warming on wheat yield and N use efficiency (NUE) remain unclear.  Here, a consecutive three-year field experiment with two straw treatments (S0, straw removal; S1, straw returning) and two warming treatments (W0, no warming control; W1, night-warming) found that both S1 and W1 improved wheat grain yield and NUE, with W1 exhibiting more pronounced improvements.  Notably, the interaction between S1 and W1 (S1W1) further enhanced yield and NUE by 13.0 and 16.5%, respectively, compared to S0W0 through increasing grain number and 1,000-grain weight (three-year average).  Additionally, root growth and topsoil inorganic N content decreased in S1 before jointing, thereby reducing plant dry matter and N accumulation.  However, W1 exhibited an opposite trend, thereby mitigating these negative effects.  Simultaneously, under S1W1, increased N translocation to grain and post-anthesis dry matter accumulation, driven by greater N distribution to leaves and higher N metabolism enzyme activity, enhanced both yield and NUE.  This improvement was supported by better root morphology and biomass, particularly in the 0–40 cm soil layer, boosting plant N absorption.  Additionally, elevated soil N-acquiring enzyme activity after jointing increased the net N mineralization rate and microbial biomass N, enhancing soil N-supply capacity.  As a result, post-jointing inorganic N content rose in the 0–20 cm layer while decreasing at 20–60 cm, thus reducing the apparent N surplus.  Collectively, straw returning, night-warming, and their interactions enhanced root distribution and N-supply capacity after jointing in the topsoil layer, thereby increasing plant N uptake and its translocation to grains, along with post-anthesis dry matter accumulation, ultimately improving grain yield and NUE.

Late sowing is a critical factor that hinders achieving high-yield, good-quality wheat under rice–wheat rotation.  Understanding the physiological basis and regulatory pathways that lead to high yield and sound quality late-sown wheat is crucial for developing effective cultivation strategies.  A 2-year field experiment was conducted to investigate the effects of sowing date, nitrogen (N) application rate, and planting density on wheat yield, grain quality, population characteristics, and the underlying physiological factors.  The results revealed significant interactions among the sowing date, planting density, and N application in regulating both yield and quality.  Late sowing reduced grain yield primarily by reducing the number of spikes and kernels.  However, the latter was improved by increasing N application and the planting density, thus mitigating the yield losses caused by late sowing.  Moreover, the grain protein content (GPC) and wet gluten content (WGC) increased with delayed sowing dates and higher N rates but decreased with increased planting densities.  For wheat yields over 9,000 or 7,500 kg ha^–1, the latest sowing date should not be later than Nov. 4 or 15, respectively.  In addition, specific criteria should be met, including a maximum of 1.5 and 1.0 million stems and tillers ha^–1, a maximum leaf area index of 6.7 and 5.5, and a dry matter accumulation (DMA) at anthesis of 14,000 and 12,000 kg ha^–1, respectively.  For high-yield, good-quality late-sown wheat, the optimal combination is a 25% increase in the N rate (300 kg N ha^–1) and a planting density of 2.25 million (N300D225) or 3.75 million (N300D375) plants ha^–1 for 10- or 20-day delays in sowing, respectively.  These combinations result in a higher leaf net photosynthetic rate, higher activities of leaf nitrate reductase, glutamine synthetase, grain glutamic pyruvic transaminase, and a lower sugar-N ratio during post-anthesis.