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Global climate change has significantly impacted the production of various crops, particularly long-term fruit-bearing plants such as citrus. This study analyzed the fruit quality of 12 citrus orchards (Citrus Sinensis L.Osbeck cv. Bingtang) in a subtropical region in Yunnan, China from 2014 to 2022. The results indicated that high rainfall (>220 mm) and low cumulative temperature (<3150 °C) promoted increases in titratable acidity (>1.8 %) in young fruits. As the fruits further expanded (with a horizontal diameter increasing from 50 to 65 mm), excessive rainfall (300-400 mm), lower cumulative temperature (<2400 °C), and a reduced diurnal temperature range (<10 °C) hindered decreases in titratable acidity. Conversely, low rainfall (<220 mm), high cumulative temperature (>3150 °C), and a high diurnal temperature range (>14 °C) promoted the accumulation of soluble solids in young fruits (9 %) at 120 days after flowering (DAF). Furthermore, low rainfall (<100 mm) favored the accumulation of soluble solids (1.5 %) during fruit expansion (195-225DAF). To quantify the relationship between fruit acidity and climate variables at 120 DAF, we developed a regression model, which was further validated by actual measurements and accurately predicted fruit acidity in 2023. Our findings have the potential to assist citrus growers in optimizing cultivation techniques for the production of high-quality citrus under increasingly variable climatic conditions., Competing Interests: Declaration of competing interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The growth of fruit trees depends on the nitrogen (N) remobilization in mature tissues and N acquisition from the soil. However, in evergreen mature citrus (Citrus reticulata Blanco) leaves, proteins with N storage functions and hub molecules involved in driving N remobilization remain largely unknown. Here, we combined proteome and physiological analyses to characterize the spatiotemporal mechanisms of growth of new leaves and storage protein degradation in mature leaves of citrus trees exposed to low-N and high-N fertilization in the field. Results show that the growth of new leaves is driven by remobilization of stored reserves, rather than N uptake by the roots. In this context, proline and arginine in mature leaves acted as N sources supporting the growth of new leaves in spring. Time-series analyses with gel electrophoresis and proteome analysis indicated that the mature autumn shoot leaves are probably the sites of storage protein synthesis, while the aspartic endopeptidase protein is related to the degradation of storage proteins in mature citrus leaves. Furthermore, bioinformatic analysis based on protein-protein interactions indicated that glutamate synthetase and ATP-citrate synthetase are hub proteins in N remobilization from mature citrus leaves. These results provide strong physiological data for seasonal optimization of N fertilizer application in citrus orchards., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Nitrogen (N) remobilization is an important physiological process that supports the growth and development of trees. However, in evergreen broad-leaved tree species, such as citrus, the mechanisms of N remobilization are not completely understood. Therefore, we quantified the potential of N remobilization from senescing leaves of spring shoots to mature leaves of autumn shoots of citrus trees under different soil N availabilities and further explored the underlying N metabolism characteristics by physiological, proteome and gene expression analyses. Citrus exposed to low N had an approximately 38% N remobilization efficiency (NRE), whereas citrus exposed to high N had an NRE efficiency of only 4.8%. Integrated physiological, proteomic and gene expression analyses showed that photosynthesis, N and carbohydrate metabolism interact with N remobilization. The improvement of N metabolism and photosynthesis, the accumulation of proline and arginine, and delayed degradation of storage protein in senescing leaves are the result of sufficient N supply and low N remobilization. Proteome further showed that energy generation proteins and glutamate synthase were hub proteins affecting N remobilization. In addition, N requirement of mature leaves is likely met by soil supply at high N nutrition, thereby resulting in low N remobilization. These results provide insight into N remobilization mechanisms of citrus that are of significance for N fertilizer management in orchards., (© The Author(s) 2022. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The spring shoot leaves are important sites of nitrogen (N) metabolism in citrus trees. Understanding the physiological and metabolic response of the spring shoot leaves under varying N fertilization is fundamental to the fertilization management in citrus orchards. Thus, the processes affecting N composition, the activities of N metabolism related enzymes, and the expression of relevant genes were explored in spring shoot leaves under four N levels (0, 207, 275, 413 g N tree -1 y -1 , as N0, N207, N275, N413). The results showed that, compared with N0, N275 significantly increased total N by 24.81%, which was mainly attributed to enhancement of structural N by 30.92%, free amino acid N by 40.91% and nitrate N by 41.33%. The relative expression of nitrate reductase (NR) and glutamate dehydrogenase (GDH) under N275 increased by 19.32% and 73.48%, respectively, compared with that under N0 treatment. Compared with N0 treatment, the NR transcription level under N275 treatment increased by 381%. The relative transcription levels of NADP-GDH and GDH1 also increased with increasing N fertilization. However, compared with that under N275, the relative transcription of GDH2 under N413 treatment was inhibited. Therefore, the transcript abundance of NR, NADP-GDH,GDH1 and GDH2 affected the activities of NR and GDH and thereby contributed to the regulation of N composition in the leaves. In addition, the activities of glutamine synthetase and nitrite reductase were largely unaffected or even declined in the N207, N275 and N413 treatments compared with the N0. This study elucidated the mechanism of primary N metabolism and partitioning in citrus leaves and provided a theoretical basis for N management in citrus orchards., (Copyright © 2021 Elsevier GmbH. All rights reserved.)

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. [ABSTRACT FROM AUTHOR]

Purpose: This research is expected to shed light on soil contamination in citrus orchards and provide a reference for soil quality maintenance and improvement in orchards under the organic fertilizer substitution scenario. Materials and methods: Heavy metals (HMs) and veterinary antibiotics (VAs) in soils collected from citrus orchards in Meishan, Sichuan Province, China, were measured. Single factor index, potential ecological risk factor, and potential ecological risk index were employed to assess the contamination status and potential ecological risks of HMs in soils. Additionally, the risk of VAs was also evaluated based on their risk quotient values. Furthermore, Pearson's correlation analysis, hierarchical cluster analysis, and principal component analysis were employed to identify the potential sources of HMs and VAs in soils. Results and discussion: HMs and VAs exhibited great variance in the study area. Nickel (Ni) was the primary metallic contaminant, followed by cadmium (Cd), chromium (Cr), and arsenic (As). HMs accumulation in soils would not pose risks to the ecosystem. Atmospheric deposition and livestock manure could be the primary sources of HMs in soils. In terms of VAs, fluoroquinolones (FQs), sulfonamides (SAs), and tetracyclines (TCs) were the major VAs in soils, with the accumulative level averaged at 84.6 μg kg−1 (36.8–369 μg kg−1). Additionally, the residual VAs, except for enrofloxacin (ENR), would only pose limited to moderated risk to soil microbes or aqueous organisms. Conclusions: Clean and high-quality organic fertilizers, instead of livestock manure, were recommended to eliminate the potential environmental risks during organic fertilizer substitution. [ABSTRACT FROM AUTHOR]

New shoots (including newly formed leaves and twigs) and fruits of citrus tree are key organs for present yield formation and flower differentiation in the next season, but the relationship between yield fluctuation and accumulations of major nutrients in new shoots and fruits of citrus tree is still unclear. Thus, to quantify the biomass and mineral nutrient accumulation in new shoots and fruits of citrus trees under varied yield levels has essential significance for rational fertilization and pruning management for citrus orchards. The purpose of this study was, therefore, to investigate the accumulation of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg), and the distribution of biomass in new shoots and fruits of citrus trees under low, medium and high-yield levels in each of eight orchards located in Chongqing, China. The results showed that substantial variation of fruit yield was observed in all eight orchards with an average yield of 15.0 (low), 30.9 (medium) and 60.1 (high) kg/plant. The averaged biomass of new shoots ranged from 1.59 to 2.51 kg/plant, which was significantly and positively correlated with fruit yield. Nutrient accumulation in leaves was generally highest among new organs, while more than half of N (52.70–71.4%), P (66.5–80.4%) and K (68.9–85.9%) accumulated in fruit. Fruit yields closely correlated with total amounts of major nutrients in new shoots. Furthermore, the nutrient requirements per unit of newly developed shoots and fruits were gradually decreased with increased yield, but the removed nutrients per ton of fresh fruit were almost stable, indicating that more nutrients were distributed into fruit tissues. Taken together, these findings are valuable for optimizing nutrient and pruning management in citrus orchards in China and other similar countries. [ABSTRACT FROM AUTHOR]

The environmental sustainability of the largest citrus plantation globally is facing a great challenge in China. Further, there is a lack of quantitative, regional hotspot studies. In this study, the life cycle assessment (LCA) was used to quantify the environmental cost of citrus production based on 155 farmers' surveys from typical citrus orchards in Danling County, southwest China, which produced 0.65% of the country's total citrus production. The results showed that the average values of environmental risk indicated by global warming potential (GWP), acidification potential (AP), and eutrophication potential (EP) were 11,665 kg CO2-eq ha−1, 184 kg SO2-eq ha−1, and 110 kg PO4-eq ha−1, respectively. The production and utilization of fertilizer ranked the first contribution to the environmental impacts among all the environmental impacts, which contributed 92.4–95.1%, 89.4–89.8%, and 97.8–97.9% to global warming potential, acidification potential, and eutrophication potential, respectively. Specific to the contribution of fertilizers to environmental costs, the production and utilization of nitrogen (N) fertilizer accounted for more than 95% of the total environmental costs. Thus, the spatial distribution of environmental costs in this county was well matched with that of N input. Compared with the average values of investigated 155 orchards, the high yield and high N use efficiency (HH) orchard group with younger and better educated owners achieved a higher citrus yield and N use efficiency with less fertilizer input and lower environmental costs. Five field experiments conducted by local government and Danling Science and Technology Backyard were used to further certify the reduction potential of environment costs. These field results showed that the local recommendation (LR) treatment increased citrus yield and N use efficiency by 1.9–49.5% and 38.0–116%, respectively, whereas decreased environmental costs by 21.2–35.2% when compared with the local farmer practice in the HH orchard group. These results demonstrated that an optimum nutrient management based on the local field recommendation in citrus-producing areas is crucial for achieving a win-win target of productivity and environmental sustainability in China and other, similar countries. [ABSTRACT FROM AUTHOR]

• Aboveground biomass in citrus trees is determined by tree age rather than N fertilizer application. • Accumulated temperature drives the growth and development of citrus trees in the field. • Stem diameter indicators can predict the growth trend of citrus tree. • Integrative view of N fertilizer management in citrus orchards. Tree growth is influenced by both, the availability of nutrients in the soil and temperature. However, how the interaction of nitrogen (N) fertilization and accumulated temperature (≥10 °C) determines the growth of evergreen citrus trees is still unknown. To characterize this interaction, we studied the biomass distribution of young citrus trees in a four-year field experiment (2018–2021, Chongqing city, China) and used the results to build an allometric growth model. Based on a completely randomized block design (Two-way ANOVA and Duncan´s test), three N levels were established, namely, N0 (no N fertilizer), N1 (80 % of the farmer's usual fertilization), and N2 (farmer's usual fertilization). Our results indicate that excessive N fertilizer application increases the inorganic N content in deep soil, whereas the aboveground biomass distribution was largely dependent on tree age, but independent of N fertilization. An allometric growth model of citrus aboveground biomass was established at different N levels and provided a realistic quantitative relationship between accumulated temperature and growth. It indicates that canopy volume (power function), stem diameter (linear function) and total aboveground biomass (exponential function) are highly positively correlated with accumulated temperature, rather than N fertilization. Since the canopy volume and total aboveground biomass are also significantly positively correlated with stem diameter, this parameter seems to be suitable for estimating the biomass of young citrus trees in the field. Further research is required to characterize the relationship between growth of citrus trees and other environmental factors. [ABSTRACT FROM AUTHOR]

Citrus is an evergreen broad-leaved fruit tree including numerous species and cultivars with an extensive global planting area. However, the relationship between physiological parameters and seasonal fruit ripening of different citrus cultivars has not been reported. With the aim to obtain information on this relationship, we selected two late-maturing (Newhall Navel and Ponkan) and two middle-maturing citrus cultivars (Fertile orange and Tarocco) and compared the seasonal patterns of foliar CO 2 and H 2 O gas exchange, leaf water content and macro-nutrient concentrations in leaves and roots. Our results showed that seasonal changes of foliar CO 2 and H 2 O gas exchange and water content greatly differed between cultivars at defined developmental stages, but especially during fruit ripening. For example, in December, the net rates of photosynthetic CO 2 fixation (P n) of the late-maturing cultivars were significantly higher than in middle-maturing cultivars. Seasonal fluctuations of total nitrogen (N), total phosphorus (P), and inorganic phosphorus (P i), as well as C/N, C/P, and N/P ratios in leaves and roots differed between middle and late-maturing cultivars. These differences were most obvious at fruit ripening compared to other developmental stages and were significantly higher in leaves than in roots. We conclude that seasonal differences in foliar P n and macro-nutrient contents between leaves of citrus cultivars are largely determined by seasonal fruit ripening. In addition, seasonal changes in root macro-nutrient contents were also affected by seasonal fruit ripening of citrus cultivars. However, this effect was much lower compared to the leaves, but still indicates particular shoot-root interactions during fruit ripening. • Foliar photosynthesis and WUE greatly differed between citrus cultivars. • Such differences also at defined developmental stages during fruit ripening. • Seasonal N, P contents and their stoichiometry differed between middle and late-maturing cultivars. • A long-distance transport and signaling interaction process in citrus shoot with root. [ABSTRACT FROM AUTHOR]

Reducing greenhouse gas (GHG) emissions and increasing carbon sequestration (CS) in agricultural systems are critical to realize carbon neutrality, and slowdown global climate change. However, a comprehensive analysis of GHG emissions, CS and the ways to achieve carbon neutrality of citrus production systems in China has not been reported. In this study, a NUFER-Citrus-LCA (citrus nutrient flows in food chains, environment, and resource use model combined with life cycle assessment) combined model was used to quantify the spatial and temporal characteristics of GHG emissions and CS from citrus production in China from 2004 to 2018 to determine the potential and the means to achieve carbon neutrality. The results showed that the GHG emissions varied remarkably among major citrus-producing provinces, with a decreasing trend in the last decade except for Guangdong province. However, the estimated emission for 2014–2018 (12,261 kg CO 2 -eq ha−1 yr−1) was 2–3 times greater than that of other countries, with Guangdong province recording the highest GHG emissions (24,403 kg CO 2 -eq ha−1 yr−1). Nitrogen fertilizer accounted for 71.5% of the total GHG emissions. The total carbon sequestration (TCS) during 2014–2018 was 24.1–74.5% of the total greenhouse gas (TGHG) emissions, depending on the province, indicating a net positive carbon emission in China's citrus production system. The scenario analysis showed considerable potential for transforming the Chinese citrus production system from a carbon source to a carbon sink. This will, however, require a crop management system that combines optimized nitrogen fertilizer management, 50% replacement of chemical fertilizer with organic fertilizer and use of cover crops, resulting in a total net carbon sequestration of 15.2 Mt CO 2 -eq yr−1 nationally. Implementation of this crop management model is imperative for transitioning the Chinese citrus production toward carbon neutrality. [Display omitted] • The GHG emissions from citrus orchard system in China reached 22.3 Mt CO 2 -eq in 2018. • Nitrogen (N) fertilizer contributed 71.5% of GHG emissions. • Total carbon (C) sequestration was 47.8% of GHG emissions. • Current citrus production system in China is far from C-neutral. • Optimizing N input, use of organic fertilizer and cover crop make citrus orchards C-neutral. [ABSTRACT FROM AUTHOR]

• Chlorosis of mature leaves caused by fruiting is related to N allocation plasticity. • Fruiting stimulated the transport of proline from mature leaves to phloem sap. • N supply affects leaf N allocation via N metabolism and consequently also C allocation. • N supply leads to a decrease in the content of TCA cycle intermediates. Mature leaves are the central organs involved in carbon (C) and nitrogen (N) metabolism to support the growth and development of citrus trees. These metabolisms may be greatly altered by fruiting and N supply, which can be verified by a typical field phenomenon in which leaf chlorosis appeared on leaves of fruiting branches but not on leaves of adjacent nonfruiting branches on citrus trees. To elucidate the underlying strategy of fruiting on C and N partitioning in citrus leaves, citrus trees were cultivated with low- and high-N supplies, and then leaves and phloem sap from fruiting and nonfruiting branches were analyzed. The results showed that fruiting drives N allocation but does not improve N assimilation while maintaining the nonstructural carbohydrate content of the leaves. Combined physiological analysis and nontargeted metabolomics showed that proline is a key metabolite of mature citrus leaves and phloem sap. Due to fruiting, the proline content in mature leaves and phloem sap decreased by 71∼73% and 17∼30%, respectively. Fruiting, followed by N supply, resulted in proline remobilization from mature leaves into phloem sap and enhanced the consumption of TCA cycle intermediates for N assimilation and energy generation. Catabolism of organic N, rather than N assimilation or C metabolism, is primarily accelerated in leaves of fruiting branches, becoming a major source of organic N (mainly proline) upon phloem transport to developing fruits. Furthermore, N supply strengthened N storage in leaves of both fruiting and nonfruiting branches and then facilitated proline turnover at higher levels, indicating the importance of timing for N management in orchards. [ABSTRACT FROM AUTHOR]

• Farm survey revealed widespread magnesium (Mg) deficiency in Chinese citrus orchards. • Acid soil and little knowledge of Mg nutrition by growers caused Mg deficiency. • Optimized Mg management improved citrus yield, quality and reduced nutrient input. • Optimized Mg management reduced eutrophication and other environmental costs. Citrus is the biggest fruit crop in the world. Magnesium (Mg) deficiency is becoming a major limiting factor for sustainable citrus production globally, especially in China. Acidified soil, low Mg availability and poor nutrient management by small growers impact citrus fruit yield and quality, and its ecological environments. Grower surveys were therefore conducted to evaluate the extent and the underlying causes of Mg deficiency in the citrus orchards in Danling County, a typical citrus production county in Southwest China. A large, detailed citrus grower survey, along with soil and plant analyses, revealed that 73% of orchard soil and 93% of citrus leaf samples were Mg-deficient in the study region. Further, more than 50% of the local agrochemical technology providers did not identify citrus Mg deficiency symptoms. As a result, the orchard soil had negligible input of Mg (average 11 kg MgO ha−1), but received a large excess of nitrogen (average 712 kg N ha−1), phosphorus (average 364 kg P 2 O 5 ha−1), and potassium fertilizers (average 565 kg K 2 O ha−1) annually. This unbalanced fertilization aggravated soil Mg depletion in 89% of citrus orchards. To rectify Mg deficiency, field experiments with crop nutrient demand and supply-based optimal nutrient management, including Mg application and reduced rates of NPK fertilizer (OPT+Mg), were conducted in two orchards over two cropping seasons. Compared with farmer practices, OPT+Mg treatment resulted in higher fruit yield (average 3.7 t ha−1), nutrient use efficiency, fruit quality and gross profit, with reduced environmental costs. In summary, this case study helped develop a social and technological strategy to mitigate Mg deficiency for profitable and sustainable citrus production in the major citrus producing region in China, and possibly in other counties with similar soil and environmental conditions. [Display omitted] [ABSTRACT FROM AUTHOR]