Citrus Seedlings Research Articles

Integrative DNA methylome and transcriptome analysis illuminate leaf phenotype alterations in tetraploid citrus

Whole-genome duplication (WGD) in plants triggers profound morphological and physiological changes, with DNA modification being a key epigenetic factor that helps neo-polyploids overcome challenges and gain adaptive advantages. Tetraploids were previously mined from diploid citrus seedlings, showing enhanced environmental adaptability and potential as rootstocks. These tetraploids exhibited increased leaf and cell wall thickness compared to their diploid counterparts. To explore the impact of WGD, transcriptomic and whole-genome bisulfite sequencing (WGBS) were conducted on two pairs of citrus tetraploids and their diploid controls, revealing significant molecular changes. Notably, tetraploid citrus displayed lower CG methylation levels in gene and transposable element (TE) bodies relative to diploids. Differentially methylated genes (DMGs) between tetraploids and diploids were primarily associated with immune stress, organ development, metabolic pathways, and secondary metabolism. In Trifoliate orange (Poncirus trifoliata L. Raf.) and Ziyang Xiangcheng (Citrus junos Sieb. ex Tanaka), only 150 and 58 differentially expressed genes (DEGs) were identified, respectively, with enrichment in critical cellular processes such as cell wall synthesis, plastid development, and pathways modulating chloroplasts and plasma membranes. A total of 70 genes showed both differential methylation and expression, including ACN1, Nac036, and ASMT1, which are involved in stomatal development, leaf morphology, and melatonin synthesis, respectively, offering insight into the regulatory mechanisms of phenotype alterations after polyploidization. These findings reveal the epigenetic modifications in polyploid citrus and highlight the role of polyploidization-induced methylation in driving phenotypic changes.

In vivo tetraploid induction of mono-embryonic citrus genotypes by colchicine treatment

Polyploid holds great significance in citrus breeding, to produce dwarf tetraploid rootstocks with wide adaptability and triploid scion varieties bearing seedless fruits. In citrus, tetraploids have been generated through somatic hybridization and autonomous chromosome doubling of nucellar embryos. However, most tetraploids were recovered from poly-embryonic (apomictic) citrus, while tetraploids of mono-embryonic (non-apomictic) citrus were seldomly reported. Herein, we developed an in vivo tetraploid induction method by colchicine treatment, which is especially high-efficient in mono-embryonic citrus. The epicotyl of citrus seedling was decapitated and incubated in dark for different time durations (0–15 d) to induce callus formation on the epicotyl wound. The callus on wound was then immersed in different concentrations (0.0125 %-0.100 %) of colchicine for 2 h, enabling regeneration of tetraploid shoots. We achieved high tetraploid induction rate (20.69 %) but low mixoploid induction rate (1.72 %), when 0.025 % colchicine was applied on wound cultured in dark for 9 d By using this method, 147 tetraploids were recovered from mono-embryonic mandarin, tangerine, sweet orange, pummelo and citron. Our method exhibited high tetraploid induction efficiency (14.28 %-20.69 %), low rate of mixoploid (1.72 %-5.30 %), short recovery period (45 d), and no need of tissue culture. The majority (75 %-100 %) of the recovered tetraploids maintained stable ploidy during subsequent growth, and the mono-embryonic nature of the tetraploids was identified by InDel marker analysis. The mono-embryonic tetraploids obtained herein enriched polyploid germplasm pool for citrus breeding.

Biochar-based phosphate fertilizer improve phosphorus bioavailability, microbial functioning, and citrus seedling growth

Phosphorus is essential for plant growth but often becomes immobilized in soils, reducing its availability to plants. This study explored the enhancement of phosphorus bioavailability through the co-pyrolysis of rice straw with magnesium oxide (MgO) at 800 °C, resulting in a novel biochar-based phosphate fertilizer, termed MRS8-P. The treatment involved enriching biochar with MgO to increase phosphorus loading and evaluating its effects on soil physicochemical properties and citrus seedling growth. Soil analysis revealed improved physicochemical properties, including increased available phosphorus (AP), enhanced soil pH, higher electrical conductivity (EC), and rapidly increased available potassium (RAK); however, a temporary immobilization of alkaline hydrolytic nitrogen (AHN) was noticed by biochar application. Notably, soil microbiota dynamics shifted with an increase in beneficial taxa, such as Proteobacteria and Actinobacteria, which enhanced phosphorus solubilization and organic matter decomposition, and a decrease in taxa, such as Methylomirabilota and Desulfobacterota. These microbial changes significantly contribute to soil fertility and nutrient availability. The application of MRS8-P significantly enhanced nitrogen, phosphorus, and potassium (NPK) uptake in citrus seedlings, as demonstrated by increased growth metrics, such as plant height, stem coarseness, and chlorophyll content measured by the SPAD index. These findings underscore the potential of MRS8-P in reducing reliance on traditional phosphate fertilizers, which are linked to environmental degradation through runoff and pollution. The study confirms that biochar-based phosphate fertilizers, such as MRS8-P, can promote more sustainable agricultural practices and improve crop health and soil management while minimizing environmental impacts.

The feeding behavior of Diaphorina citri monitored by using an electrical penetration graph (DC-EPG) on citrus plants treated with Bacillus cereus and Bacillus velezensis

Diaphorina citri, an important pest and insect vector that can transmit the pathogenic bacteria Candidatus Liberibacter asiaticus, causing Huanglongbing disease, is one of many challenges in citrus agriculture. Integrated pest management by utilizing microorganisms is a wise and efficient alternative without damaging the environment. Utilization of Plant Growth Promoting Rhizobacteria (PGPR), such as Bacillus cereus and B. velezensis, is a potential strategy for the biological control of plant diseases or insect vectors. By inducing systemic resistance in plants, PGPR can enhance plant defense against diseases and insect pests while activating molecular and physiological changes in plants. This research aimed to determine the effect of B. cereus and B. velezensis on the plant growth and feeding behavior of D. citri. The height and volume of the plant canopy were observed periodically for 6 months, while the feeding behavior of D. citri was monitored using the Electrical Penetration Graph (DC-EPG). The results showed increased height and volume of the citrus plant canopy treated with B. cereus, indicating that B. cereus could act as a PGPR. The application of B. cereus and B. velezensis to citrus seedlings affected the feeding behavior of D. citri. D citri showed difficulty in penetrating the phloem tissue of citrus plants.

Physiological Indexes in Seed Germination and Seedling Growth of Rangpur Lime (Citrus limonia L. Osbeck) under Plant Growth Regulators

The propagation of citrus seedlings is accomplished through grafting, utilizing seeds for the production of rootstocks. The germination of certain seeds may be low and uneven, complicating the production of high-quality seedlings. The use of plant growth regulators (PGRs) is a viable alternative to improve the quality of seedling production, as these compounds can break dormancy, control the hydrolysis of reserves, induce cell division, and regulate permeability and protein functions. This study aimed to evaluate the germination of seeds and the growth of Rangpur lime (Citrus limonia L. Osbeck) seedlings under the influence of imbibition in solutions of gibberellic acid (GA3) and a combination of GA4+7 + 6Benzyladenine. The experiment was conducted under controlled laboratory and greenhouse conditions, using a completely randomized design in a 2 × 5 factorial scheme, with two types of plant regulators (GA3 and GA4+7 + 6BA) at five concentrations (0, 250, 500, 750, and 1000 mg L−1 a.i.). Quantitative and qualitative variables were evaluated, ranging from seed germination to seedling development and formation, including germination percentage and speed index, fresh and dry biomass of roots and shoots, enzymatic activity, and gas exchange. The results indicate that GA3 significantly accelerates the germination process of Rangpur lime cv. Santa Cruz seeds and promotes better seedling growth and development, resulting in vigorous seedlings. These findings demonstrate that the application of PGRs, particularly GA3, can substantially enhance the propagation efficiency of citrus rootstocks, offering a practical solution for improving the uniformity and quality of seedling production in commercial settings.

Induction of Somatic Embryogenesis of Pontianak Siamese Orange Cotyledon Cultures on Murashige Skoog Media with the Addition of 2,4-D and Kinetin

Conventional citrus propagation has weaknesses, one of which is susceptible to CVPD(Citrus Vein Phloem Degeneration) and other diseases which are the main cause of the decline in population and production of pontianak siam orange plants in West Kalimantan. One alternative to in vitro propagation of citrus seedlings is the induction of somatic embryogenesis. The purpose of the study was to obtain the best combination of 2,4-D and kinetin concentrations for callus induction and somatic embryo induction from Pontianak siam orange cotyledon explants, observing the developmental phase (stage) of embryos formed. The implementation of the study used a factorial completely randomized design (CRD) consisting of 2 factors. The first factor is the concentration of 2,4-D consisting of 4 concentration levels, namely 0; 0.5; 1; 1.5; 2 mg/L. The second factor is kinetin concentration consisting of 4 concentration levels, namely 0; 0.5; 1; 1.5; 2 mg/L.The observation parameters include the time of callus emergence (hst), the percentage of explants forming callus (%), the texture and color of the callus, observing the stage of the embryo found. The results showed that 72% of treatments were able to induce callus from pontianak siam orange cotyledons with an average callus appearance at 7 hst. The callus formed is compact and crumbly with varied callus colors, namely white, yellowish, greenish and brownish. The best treatment that produces embryogenic callus is the combination of D0.5K0.5; D1K1; D0.5K1.5; D1.5K1.5 and D2K2 with the characteristics of crumbly callus and yellowish and brownish colors. The results of microscopic observations found that the somatic embryo phase formed was the pro-embryo phase.

Design and Test of Automatic Feeding Device for Substrate Filling

An automatic feeding device for substrate filling was designed to address the challenges of difficult feeding and low efficiency in the citrus seedling pot filling and transferring machine. The device comprises a framework, tracks, overturning frame, drive system, etc. In order to ensure optimal performance, the frame’s turning angle was set at a minimum of 110° and the angle between the frame’s horizontal plane and slope was determined to be 120°. Following optimization, the number and intensity of sudden changes in velocity, angular velocity, and thrust were reduced, thereby prolonging the device’s service life. The prototype test demonstrated that the device has an average feeding time of 9.86 s, is capable of raising 0.14 m3 of substrate in a single cycle, and has a handcart turning angle of 111°. Furthermore, no residual substrate remained in the handcart, and the handcart fixing mechanism operated correctly. The torque measurement results of the motor output shaft were found to be consistent with the simulation results in ADAMS, with a maximum force difference of only 298 N. The simulation was found to be accurate, with an error rate of only 3.67%. This model can be utilized as a dependable reference for the optimization of the design of the automatic feeding device.

A Monitoring, Evaluation, and Prediction System for Slight Water Stress in Citrus Seedlings Based on an Improved Multilayer Perceptron Model

To address the lack of effective monitoring, evaluation, and prediction methods for water stress in citrus seedlings, we conducted 10 sets of water stress gradient experiments. Based on the experimental dataset, we constructed, trained, and improved an MLP classification model for citrus seedling water stress. In addition, we developed a monitoring, evaluation, and prediction system based on this model. The experiments demonstrated that 7 days of slight water stress can induce changes in overall root wilting and growth stagnation, and the chlorophyll content in the leaves can decrease by up to 11.78%. Furthermore, the optimal VWC for seedlings was [45%, 50%], the boundary of drought was [20%, 25%], and the boundary of waterlogging was [50%, 55%]. We validated the effectiveness of the system in assessing the growth status of seedlings over the past 7 days and predicting it after 7 days through testing sets and experiments on slight water stress. We found that the system achieved non-destructive remote monitoring, evaluation, and prediction of slight water stress in citrus seedlings, thus enhancing seedling quality. These research findings provide valuable insights into water stress management in citrus seedlings and other crops.

Root Reduction Caused Directly or Indirectly by High Application of Nitrogen Fertilizer Was the Main Cause of the Decline in Biomass and Nitrogen Accumulation in Citrus Seedlings.

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.

Seedlings of Poncirus trifoliata exhibit tissue-specific detoxification in response to NH4 + toxicity.

Ammonium nitrogen (NH4 +-N) is essential for fruit tree growth, but the impact of excess NH4 +-N from fertilizer on evergreen citrus trees is unclear. In a climate chamber, 8-month-old citrus plants were exposed to five different hydroponic NH4 +-N concentrations (0, 5, 10, 15 and 20 mm) for 1 month to study effects of NH4 +-N on growth characteristics, N uptake, metabolism, antioxidant enzymes and osmotic regulatory substances. Application of 10 mm NH4 +-N adversely affected root plasma membrane integrity, root physiological functions, and plant biomass. MDA, CAT, POD, APX and SOD content were significantly correlated with leaf N metabolic enzyme activity (GOGAT, GDH, GS and NR). GDH was the primary enzyme involved in NH4 +-N assimilation in leaves, while the primary pathway involved in roots was GS-GOGAT. Under comparatively high NH4 + addition, roots were the main organs involved in NH4 + utilization in citrus seedlings. Our results demonstrated that variations in NH4 + concentration and enzyme activity in various organs are associated with more effective N metabolism in roots than in leaves to prevent NH4 + toxicity in evergreen woody citrus plants. These results provide insight into the N forms used by citrus plants that are important for N fertilizer management.

The effect of rootstocks on morphological, physiological, and gene expression characters of citrus seedlings grown under drought condition

The effect of rootstocks on morphological, physiological, and gene expression characters of citrus seedlings grown under drought condition

Allelopathic effects of rhizobacteria on Fusarium wilt and on the growth of citrus seedlings in Adjara, Georgia

Allelopathic effects of rhizobacteria on Fusarium wilt and on the growth of citrus seedlings in Adjara, Georgia

Growth and Functional Traits of Citrus (Citrus Sp.) Provenance as Effected by Watering Regime in the Nursery

Five citrus proevancnes were subjected to watering levels in the nursery. The effects of watering regime were evaluated on the growth, physiological and biochemical attributes while the measured variables were deployed to rank tolerance to soil moisture deficits by citrus species. The watering regimes were 100 % field capacity (1.2 litres at four days interval), moderately watered: 75% field capacity (0.9 litres at four days interval) and 50% field capacity (0.6 litres at four days interval) which imposed at 16 weeks after transplanting. Data were collected on height and leaf development, root and shoot biomass, chlorophyll content, stomata architecture, proline, membrane lipid peroxidation and antioxidant enzymes (Superoxide dismutase (SOD), Catalase (CAT), Gluthathione (GSH). Citrus provenances differed in growth traits, stomata architecture, proline, membrane lipid peroxidation and and enzymatic activities. Watering regime induced significant changes in leaf chlorophyll and phenolic contents and enzymatic activities of catalase (CAT), superoxide dismutase (SOD), membrane lipid peroxidation (malonaldehyde (MDA) and Guaiacol peroxidase (GPx) in citrus. The 75 and 50% FC watering significantly enhanced MDA and proline and activities of enzymatic antioxidant in citrus seedlings compare with 100% FC watering. Activities of CAT and GPx were enhanced by 75% FC while CAT, SOD and GPx were enhanced by 50 % FC watering. Under water limitations, 75% FC watering can support citrus seedling growth and 50% FC for lemon under soil moisture limitation.

Bacillus thuringiensis translocation in citrus scion/rootstock combinations and binding of cry toxins with the Asian citrus psyllid gut receptors

Bacillus thuringiensis (Bt) can be effective in controlling insect pests either in spray products or by their toxins expressed in transgenic crops. The discovery of Bt as endophytic in citrus opened new perspectives for the control of Diaphorina citri Kuwayama, the vector of the bacteria Candidatus Liberibacter spp. associated with Huanglongbing. In this study, the endophytic translocation of Bt in citrus seedlings (small and large plants) and in different scion-rootstock combinations were investigated. We also assessed the lethal concentration of D. citri nymphs caused by systemic Bt in citrus and, moreover, the binding of Cry toxins with the Asian citrus psyllid (ACP) gut receptors. Endophytic translocation of Bt in citrus seedlings and nursery trees was confirmed by colony isolation, PCR detection, and Btk-GFP microscopy analyses, which demonstrated that, after Bt inoculation into the substrate, the bacteria enter through lateral root emergence sites and are localized in stem xylem vessel elements. Mean lethal concentrations for D. citri third instar nymphs were 4.92 × 104 and 2.19 × 104 spores mL−1 for S1450 and S1302 strains, respectively. Nymphal mortality induced by the bacteria was higher in citrus seedlings than in citrus nursery trees. All Cry-toxins tested were able to bind to D. citri brush border membrane vesicles, suggesting the potential pathogenicity of Bt strains against psyllid. This study clearly demonstrates the penetration and Bt translocation in the xylem of small and large citrus plants and the potential of the bacteria to cause ACP nymphs mortality that fed on shoots from Bt-treated citrus.

Towards the completion of Koch's postulates for the citrus huanglongbing bacterium, Candidatus Liberibacter asiaticus

Abstract Candidatus Liberibacter asiaticus (Las) is one of the causal agents of huanglongbing (HLB), the most devastating disease of citrus worldwide. Due to the intracellular lifestyle and significant genome reduction, culturing Las in vitro has proven to be extremely challenging. In this study, we optimized growth conditions and developed a semi-selective medium based on the results of nutritional and antibiotic screening assays. Using these optimized conditions, we were able to grow Las in the LG liquid medium with ca.100- to 1000-fold increase, which peaked after 4 to 6 weeks and were estimated to contain 106 to 107 cells/ml. The cultured Las bacteria remained in a dynamic state of growth for over 20 months and displayed limited growth in subcultures. The survival and growth of Las was confirmed by fluorescence in situ hybridization with Las-specific probes and expression of its metabolic genes. Growth of Las in the optimized medium relied on the presence of a helper bacterium, Stenotrophomonas maltophilia FLMAT-1 that is multi-drug resistant and dominant in the Las co-culture system. To recapitulate the disease, the co-cultured Las was inoculated back to citrus seedlings via psyllid feeding. Although the Las-positive rate of the fed psyllids and inoculated plants were relatively low, this is the first demonstration of partial fulfillment of Koch’s postulates with significant growth of Las in vitro and a successful inoculation of cultured Las back to psyllids and citrus plants that resulted in HLB symptoms. These results provide new insights into Las growth in vitro and a system for improvement towards axenic culture and anti-Las compound screening.

Spatial Distribution and Damage Severity of Citrus Leafminer (Phyllocnistis spp. Stainton) in Major Citrus Producing Regions of Tanzania

Field survey was conducted to determine the occurrence and damage severity of the citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracilariidae) in major citrus producing regions of Tanzania between December 2011 and September 2012. In total, 14,725 citrus trees and seedlings were assessed to determine the presence or absence of P. citrella as well as its associated damage severity. A total of 10,000 citrus trees (>5 years) were sampled from Morogoro Rural and Muheza Districts, while 4,725 nursery seedlings (<5 years) from Kinondoni District. The damage severity was assessed as described by Horsfall-Barratt scale. Results revealed that leafminers were recorded in nearly all surveyed areas. Kruskal Wallis analysis suggested minimal spatial distribution of the pest incidence among the surveyed locations. Kinondoni district recorded the highest (34.11%) leafminer incidence while Muheza district had the lowest (3.74). However, the damage severity indicated an increasing trend from December 2011 through June 2012 in nursery seedlings at Kinondoni. Weather variables (especially temperature) played an important role to the pest’s development and perpetuation. Further studies need to be conducted to have a wider knowledge of the pest in all the citrus growing regions in the country.

Salicylic acid and fertilizers affect the performance of Panonychus citri (McGregor) (Acari; Tetranychidae) on citrus seedlings

ABSTRACT The citrus red mite, Panonychus citri (McGregor), is one of the most important pests of citrus trees. The use of non-selective and wide-spectrum insecticides leads to a reduction in the natural enemy presence against this pest and turns P. citri into a serious pest in citrus orchards. In the present study, the induction of resistance was carried out in two-year-old citrus seedlings using a foliar application of amino acid, humic acid, potassium, vermiwash, and salicylic acid (SA) against P. citri. The data indicated a lower survival rate, longer immature duration, shorter adult longevity, and lower fecundity due to an increase in the resistance of citrus. The developmental time of immature stages, survival, and fecundity was lowest in combined treatments, intermediate in individual treatments, and highest in control and vermiwash treatments. Population growth parameters of P. citri including net reproductive rate, intrinsic rate of increase, finite rate of increase, and generation time were lower when the mites fed on combined treatments, especially on potassium+SA. These results suggest that the application of a combination of fertilizer and SA on citrus trees can lead to a significant reduction in caused damage by citrus red mites. http://www.zoobank.org/urn:lsid:zoobank.org:pub:unique-alpha-numeric-id

Effect of fertilization of Seaweed extracts and CuSO4 on some vegetative growth indicators of Citrus limon L. grafted seedlings on rootstock aurantifolia

Citrus aurantifolia rootstock seedlings were treated with Seaweed extracts and copper sulfate. Seaweed extracts were added as a ground feed during the growing season at four levels (0, 10, 20, and 30 ml. seedling-1) and copper sulfate sprayed at four concentrations (15,10,5, 0 ml. L-1) alone or in combination with the control (watering with water only) during the growing seasons 2020-2021. The results showed that the treatment of seedlings grafted on rootstock aurantifolia with Seaweed extracts and spraying with copper sulfate affected (height, stem diameter, area Leaf, root length, and chlorophyll content of leaves (the results also showed that the addition of Seaweed extracts at a concentration of 30 ml overlapping with copper sulfate spraying at a concentration of 15 ml. L-1 is recommended to improve the vegetative, root and nutritional growth of lemon citrus seedlings.

The Effect of Spraying with Jasmonic Acid and Sugar Alcohols on some Chemical and Hormonal Properties of Lemon and Benzher Seedlings

In 2022, researchers from the University of Samarra’s Department of Horticulture and the College of Agriculture built a wooden canopy to perform their experiment. To study the effect of spraying with jasmonic acid and sugar alcohols on some chemical and hormonal properties of lemon and benzher seedlings, the seedlings were chosen as homogeneous as possible in size at the age of one and a half years. The experiment included three factors: the first being two Speciess of citrus seedlings, lemon and benzher. The second factor is spraying with three levels of jasmonic acid 0, 10, 20 mmol L-1, and the third factor is spraying seedlings with three concentrations of sugar alcohols 0, 3, 6 ml L-1. The experiment was designed according to a randomized complete block design (RCBD), the results showed the superiority of lime over benzher in the leaves content of nitrogen, chlorophyll, and jasmonic leaves content, and the values were 2.332%, 12.12 mg gm-1, 64.78 mg kg-1, respectively. As for benzher, it excelled in the rate of increase in the percentage of phosphorus, potassium, Carbohydrates, as it recorded 0.476%, 1.240%, and 3.284%, respectively.Also spraying with jasmonic resulted in a significant increase, especially the treatment Ja2, which was superior to the two treatments Ja1 and Ja0 in the leaves content of nitrogen, chlorophyll, leaf content of jasmonic, percentage of phosphorus, potassium, carbohydrates, and the results were 2.364%, 12.34 mg gm-1, 69.13 mg kg -1, 0496%, 1.263%, 3.300%, respectively. Spraying with sugar alcohols had a significant effect at the concentration of 6 ml L-1 S2 in the leaf content of nitrogen, chlorophyll, phosphorus, potassium, and carbohydrates, and the values were 2.339%, 12.20 mg gm-1., 0.491, 1.243%, 3.277%, respectively, while the average of these characteristics was lowest in the control group that received treatment S0. All of the qualities analyzed displayed substantial variations in the binary and triple interactions of the research components.

Regulation of magnesium and calcium homeostasis in citrus seedlings under varying magnesium supply

Magnesium (Mg) and calcium (Ca) are two essential macronutrients in plants; however, the characteristics of Mg and Ca concentrations in organ, subcellular and chemical forms and their relationships in citrus plants, especially under varying Mg supply, are not well understood. In this study, Citrus sinensis seedlings (cv. Xuegan) were cultivated in conditions of Mg deficiency (0 mmol Mg2+ L−1) and Mg sufficiency (2 mmol Mg2+ L−1) to investigate the responses of Mg and Ca homeostasis in different organs and fractions. Compared with Mg sufficiency, Mg deficiency significantly decreased root and shoot growth, with the shoot biomass reduction of branch organs was greater than that of parent organs. In addition to increasing the Ca concentration in the parent stem and lateral root organs, Mg deficiency significantly decreased the concentrations and accumulations of Mg and Ca in citrus seedlings, further altering their distribution in different organs. More than 50% of Ca and Mg were sequestrated in the cell wall and soluble fractions, respectively, with Mg concentration decreasing by 15.4% in roots and 46.9% in leaves under Mg deficiency, while Ca concentration decreased by 27.6% in roots and increased by 23.6% in parent leaves. Approximately 90% of Mg exists in inorganic, water-soluble, and pectate and protein-bound forms, and nearly 90% of Ca exists in water-soluble, pectate and protein-bound, phosphate and oxalate acid forms. Except for the decreased inorganic Mg in roots and water-soluble Mg and Ca in leaves, Mg deficiency increased the proportions of Mg and Ca in all chemical forms. However, Mg deficiency generally increased the Ca/Mg ratio in various organs, subcellular and chemical forms, with negative relationships between Mg concentration and Ca/Mg ratio, and the variations of Mg and Ca were highly separated between Mg supply and organs. In conclusion, our results provide insights into the effects of Mg supply on Mg and Ca homeostasis in citrus plants.