Citrus is the largest fruit crop around the world, while high nitrogen (N) application in citrus orchards is widespread in many countries, which results not only in yield, quality and environmental issues but also slows down the establishment of citrus canopies in newly cultivated orchards. Thus, the objective of this study was to investigate the physiological inhibitory mechanism of excessive N application on the growth of citrus seedlings. A pot experiment with the citrus variety Orah (Orah/Citrus junos) at four N fertilization rates (0, 50, 100, and 400 mg N/kg dry soil, denoted as N0, N50, N100, and N400, respectively) was performed to evaluate the changes of root morphology, biomass, N accumulation, enzyme activities, and so on. The results showed that the N400 application significantly reduced the total biomass (from 14.24 to 6.95 g/Plant), N accumulation (from 0.65 to 0.33 g/Plant) and N use efficiency (92.69%) in citrus seedlings when compared to the N100 treatment. The partial least squares pathway model further showed that the decline of biomass and N accumulation by high N application were largely attributed to the reduction of root growth through direct and indirect effects (the goodness of fit under the model was 0.733.) rather than just soil N transformation and activity of root N uptake. These results are useful to optimize N management through a synergistic N absorption and utilization by citrus seedlings.

Top grafting is an efficient and practical technique for the renewal and rejuvenation of citrus trees in old orchards. However, root death after top grafting restricts plant growth and canopy reconstruction. Thus, applications of rooting promotion substances before citrus top grafting may increase the amount and activity of roots, thereby enhancing top-grafted plant performance. To test this assumption, four rooting promotion substances, i.e., rooting promotion powder, biochar, organic fertilizer, and potassium fulvic acid, were applied before top grafting, and the effects on biometric and physiological parameters were analyzed after top grafting. The results showed that the application of all rooting promotion substances before top grafting has a positive effect on growth and mineral nutrient acquisition, as well as on foliar C and N assimilates and the activity of anti-oxidative enzymes of top-grafted plants. Rooting promotion powder and biochar had the best effect on top-grafted tree performance in the short term. In conclusion, pre-grafting root promotion reduced root damage, enhanced nutrient acquisition, and improved the physiological performance of top-grafted plants. Therefore, this approach can play a crucial role in accelerating canopy reconstruction in old citrus orchards and in improving citrus plant development.

China produces 61.0% of the world’s watermelons, yet a life cycle assessment (LCA) of its production system is lacking. Here, we combined farmer surveys and field experiments to assess resource depletion and environmental impacts across North China (NC), Northwest China (NW), and Southwest China (SW), and to quantify the mitigation potential through optimized nitrogen (N) management. NC achieved the highest yield but also the highest resource use and emissions per hectare. In contrast, SW performed best per ton of fruit produced due to lower input intensity. Nitrogen fertilizer dominated greenhouse gas emissions and eutrophication potential, with over 85% of its impact arising during the field application stage. Grouping farms by yield and N partial factor productivity revealed a mitigation potential of 46.5–55.4%, enabling both high yield and high efficiency. Field experiments confirmed that reducing N input by 14.3–40.0%, as recommended regionally, stabilizes yield while significantly lowering environmental burdens. Our findings validate that region-specific N optimization is a key strategy for achieving sustainable watermelon production in China.

The global consumption of resources and environmental problems are becoming increasingly serious. China produces 61.0% of the world's watermelons, yet no comprehensive assessment of the resource consumption and environmental effects of China's watermelon production system has been reported. This study applies life cycle assessment (LCA), combining farmer surveys with field experiments, to systematically evaluate and verify the reduction potential of land occupation (LO), water depletion (WD), energy depletion (ED), global warming potential (GHG), acidification potential (AP), and eutrophication potential (EP), as well as the resource budget and environmental impact index (REI), in north China (NC), northwest China (NW), and southwest China (SW). The results reveal significant differences in resource consumption and environmental effects. The NC region was found to achieve the highest yield, 146% and 174% greater than the two other regions, but also to exhibit the highest resource use and emissions per unit area, with the lowest REI. By contrast, the SW region was found to achieve the most favorable performance in resource consumption and environmental emissions per unit yield. During the material stage (MS), more than 86.6% of energy and water resources are consumed, while in the farm stage (FS), 47.3% to 98.7% of total pollutants are emitted. Nitrogen (N) fertilizer is identified as the main contributor to GHG, WD, and EP, with field application accounting for more than 85% of the total impact. Although REI values across the three regions remain low, yield management and improved N fertilizer efficiency (PFPN) present a reduction potential of 46.5% to 55.4%, enabling both high yield and high efficiency. Field experiments further verify that reducing N fertilizer use by 14.3% to 40.0% can lower environmental impacts, which is of great significance for promoting green watermelon production.

Top-grafting is widely used for cultivar renewal in old citrus orchards, yet some hetero-top-grafted combinations show poor tree vigor and restricted growth. However, the physiological mechanisms underlying this growth limitation remain unclear. Here, we investigated carbon allocation, carbohydrate metabolism, phytohormone accumulation, anatomical structure, and multi-omics responses in citrus plants subjected to non-top-grafted, self-top-grafted, and hetero-top-grafted. Hetero-top-grafted plants showed significantly lower aboveground, root, and whole-plant biomass than both non-top-grafted and self-top-grafted plants, with reductions of 41.0%, 38.3%, and 39.9%, respectively, relative to non-top-grafted plants. Stable ¹³C tracing revealed that hetero-top-grafted caused strong retention of newly fixed carbon in the top-grafted union bark, accompanied by reduced carbon allocation to the roots, indicating impaired downward carbon allocation. Consistently, hetero-top-grafted plants accumulated starch and soluble sugars in the top-grafted union bark, but showed reduced carbohydrate availability in roots. Anatomical observations further revealed swelling, bark cracking, and disrupted phloem structure at the hetero-top-grafted union. Hormone analysis showed increased auxin and abscisic acid contents but decreased cytokinin and gibberellin contents in the union top-grafted union bark, indicating disturbed hormone status. Integrated metabolomic and transcriptomic analyses further revealed coordinated changes in starch and sucrose metabolism, hormone signaling, and phenylpropanoid/flavonoid biosynthesis. These findings support a model in which hetero-top-grafted impairs downward carbon allocation and induces physiological and molecular reprogramming at the graft union, thereby contributing to growth inhibition in citrus.