Search

In China, high copper (Cu) and low organic matter often occur in some citrus orchard soils. However, the underlying mechanisms by which humic acid (HA) stimulates growth and mitigates Cu toxicity of citrus seedlings are unclear. After being treated with 0, 0.1, or 0.5 mM sodium humate and 0.5 or 400 μM CuCl2 (Cu excess) for 24 weeks, sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] seedlings were used to examine the impacts of HA-Cu interactions on seedling growth, nutrient uptake, leaf pigments, and photosynthetic performance that was revealed by chlorophyll a fluorescence transient. Copper excess reduced root, stem, and leaf dry weight (DW) by 42.4%, 65.4%, and 61.6%, respectively at 0 mM HA, and by 17.3%, 25.4%, and 31.4%, respectively at 0.5 mM HA; and that the levels of Cu in leaves, stems, and roots declined with elevating HA supply. Copper excess caused some rotten and dead fibrous roots at 0 mM HA, but not at 0.5 mM HA. Adding HA lowered Cu uptake per root DW (UPR), the levels of Cu in leaves, stems, and roots, and the competition of Cu2+ with Mg2+ and Fe2+, and therefore mitigated root impairment caused by Cu excess. The HA-mediated alleviation of root damage caused by Cu excess increased the uptake per plant and UPR of nitrogen, potassium, magnesium, phosphorus, calcium, sulfur, boron, and manganese, and therefore alleviated Cu excess-induced decline in seedling growth, impairment to leaf photosynthetic electron transport chains, and decrease in leaf pigments. For 0.5 μM Cu-treated seedlings, adding HA promoted seedling growth by improving root nutrient uptake and leaf photosynthetic performance. Cu excess aggravated the impacts of HA supplementation on seedling growth, leaf photosynthetic performance, and root nutrient uptake. [ABSTRACT FROM AUTHOR]

Citrus species are prone to suffer from copper (Cu) toxicity because of improper application of Cu-based agrochemicals. Copper immobilization mediated by pectin methylesterase (PME) in the root cell wall (CW) is effective for Cu detoxification. However, the underlying mechanisms of the structural modification and stress responses of citrus root CW pectin to Cu toxicity have been less discussed. In the present study, seedlings of 'Shatian pummelo' (Citrus grandis L. Osbeck) and 'Xuegan' (Citrus sinensis L. Osbeck), which differ in Cu tolerance, were irrigated with nutrient solution containing 0.5 (as control), 100, 300 or 500 μM Cu for 18 weeks in sandy culture or 24 h in hydroponics. At the end of treatments in the 18-week sandy culture, Cu toxicity on CW pectin content, Cu distribution, degree of pectin methylesterification (DPM) and the PME enzyme activity were discussed. At the genome-wide level, PME gene family was identified from the two citrus species, and qRT-PCR array of citrus PMEs under control and 300 μM Cu stress for 18 weeks were performed to screen the Cu-responsive PME genes. Moreover, the candidate genes that responded to Cu toxicity were further examined within 24 h. The results showed that Cu toxicity increased the root CW pectin content. The root CW pectin under Cu toxicity was remodeled by upregulation of the expression of the Cu-responsive PME genes followed by increasing PME activity, which mainly promoted low methylesterased pectin level and the Cu content on root CW pectin. Compared with C. sinensis , C. grandis root CW had a lower DPM and higher Cu content on the Cu-stressed root CW pectin, contributing to its higher Cu tolerance. Our present study provided theoretical evidence for root CW pectin remodeling in response to Cu toxicity of citrus species. [ABSTRACT FROM AUTHOR]

Over-application of copper (Cu) is prevalent in citrus orchards of South China. Therefore, the evaluation of citrus Cu tolerance is of significance for better nutrient management and in the breeding of Cu-tolerant citrus species. In this study, seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] and 'Shatian pummelo' [Citrus grandis (L.) Osbeck] were treated with 0.5 µM Cu (Control) or 300 µM Cu (Cu toxicity) for 18 weeks in sandy culture medium. The results demonstrated that excess Cu decreased seedling biomass, plant height, and root length and significantly increased Cu accumulation of C. sinensis and C. grandis. The Cu distribution in citrus tissues was following the order: lateral roots > primary roots > stems > leaves. Compared to C. sinensis, C. grandis had a 22.4% higher Cu concentration in the lateral roots under Cu toxicity. According to the stomata limiting theory, the inhibition of the net photosynthetic rate of C. sinensis leaves by Cu toxicity was mainly attributed to non-stomata limitation. The chlorophyll a transient indicated much more severe damage to the structure and function of leaf photosynthetic system II (PS II) in C. sinensis than C. grandis by Cu toxicity. Conclusively, a higher Cu retention in the lateral roots, the maintenance of a stable PS II structure, and higher electron transferring rate from PS II to PS I implied the relatively higher Cu tolerance of C. grandis seedlings. These results provide essential information for further investigation on the Cu-tolerant mechanisms of citrus species and the future breeding of Cu-tolerant citrus species. [ABSTRACT FROM AUTHOR]

Low pH-induced alterations in gene expression profiles and organic acids (OA) and free amino acid (FAA) abundances were investigated in sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] leaves. We identified 503 downregulated and 349 upregulated genes in low pH-treated leaves. Further analysis indicated that low pH impaired light reaction and carbon fixation in photosynthetic organisms, thereby lowering photosynthesis in leaves. Low pH reduced carbon and carbohydrate metabolisms, OA biosynthesis and ATP production in leaves. Low pH downregulated the biosynthesis of nitrogen compounds, proteins, and FAAs in leaves, which might be conducive to maintaining energy homeostasis during ATP deprivation. Low pH-treated leaves displayed some adaptive responses to phosphate starvation, including phosphate recycling, lipid remodeling, and phosphate transport, thus enhancing leaf acid-tolerance. Low pH upregulated the expression of some reactive oxygen species (ROS) and aldehyde detoxifying enzyme (peroxidase and superoxidase) genes and the concentrations of some antioxidants (L-tryptophan, L-proline, nicotinic acid, pantothenic acid, and pyroglutamic acid), but it impaired the pentose phosphate pathway and VE and secondary metabolite biosynthesis and downregulated the expression of some ROS and aldehyde detoxifying enzyme (ascorbate peroxidase, aldo-keto reductase, and 2-alkenal reductase) genes and the concentrations of some antioxidants (pyridoxine and γ-aminobutyric acid), thus disturbing the balance between production and detoxification of ROS and aldehydes and causing oxidative damage to leaves. [ABSTRACT FROM AUTHOR]

Background: Many citrus orchards of south China suffer from soil acidification, which induces aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species are not well understood. Results: The seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] and 'Shatianyou' [Citrus grandis (L.) Osbeck], that differ in Al tolerance, were hydroponically treated with a nutrient solution (Control) or supplemented by 1.0 mM Al3+ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species followed the order: lateral roots > primary roots > leaves > stems. The concentration of Al extracted from the cell wall (CW) of lateral roots was found to be about 8 to 10 times higher than in the lateral roots under Al toxicity, suggesting that the CW was the primary Al-binding site at the subcellular level. Furthermore, the Al distribution in CW components of the lateral roots showed that pectin had the highest affinity for binding Al. The relative expression level of genes directly relevant to Al transport indicated a dominant role of Cs6g03670.1 and Cg1g021320.1 in the Al distribution of two citrus species. Compared to C. grandis, C. sinensis had a significantly higher Al concentration on the CW of lateral roots, whereas remarkably lower Al levels in the leaves and stems. Furthermore, Al translocation revealed by the absorption kinetics of the CW demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root CW than C. grandis. According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by a modification in the structure and components of the citrus lateral root CW. Conclusions: A higher Al-retention, mainly attributable to pectin of the root CW, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis. The aluminum distribution and translocation of two citrus species differing in aluminum tolerance were associated with the transcriptional regulation of genes related to Al transport and the structural modification of root CW. [ABSTRACT FROM AUTHOR]

Citrus sinensis (L.) Osbeck seedlings were fertigated with nutrient solution containing 2 [magnesium (Mg)-sufficiency] or 0 mM (Mg-deficiency) Mg(NO 3 ) 2 for 16 weeks. Thereafter, RNA-Seq was used to investigate Mg-deficiency-responsive genes in the veins of upper and lower leaves in order to understand the molecular mechanisms for Mg-deficiency-induced vein lignification, enlargement and cracking, which appeared only in the lower leaves. In this study, 3065 upregulated and 1220 downregulated, and 1390 upregulated and 375 downregulated genes were identified in Mg-deficiency veins of lower leaves (MDVLL) vs Mg-sufficiency veins of lower leaves (MSVLL) and Mg-deficiency veins of upper leaves (MDVUL) vs Mg-sufficiency veins of upper leaves (MSVUL), respectively. There were 1473 common differentially expressed genes (DEGs) between MDVLL vs MSVLL and MDVUL vs MSVUL, 1463 of which displayed the same expression trend. Magnesium-deficiency-induced lignification, enlargement and cracking in veins of lower leaves might be related to the following factors: (i) numerous transciption factors and genes involved in lignin biosynthesis pathways, regulation of cell cycle and cell wall metabolism were upregulated; and (ii) reactive oxygen species, phytohormone and cell wall integrity signalings were activated. Conjoint analysis of proteome and transcriptome indicated that there were 287 and 56 common elements between DEGs and differentially abundant proteins (DAPs) identified in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, and that among these common elements, the abundances of 198 and 55 DAPs matched well with the transcript levels of the corresponding DEGs in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, indicating the existence of concordances between protein and transcript levels. [ABSTRACT FROM AUTHOR]

Although the physiological and molecular responses of Citrus to Al-toxicity or low pH have been examined in some details, little information is available on Citrus responses to pH and aluminum (Al) interactions. Citrus sinensis seedlings were irrigated for 18 weeks with nutrient solution at a concentration of 0 or 1 mM AlCl3•6H2O and a pH of 2.5, 3.0, 3.5, or 4.0. Thereafter, biomass, root, stem, and leaf concentrations of Al and nutrients, leaf gas exchange, chlorophyll a fluorescence (OJIP) transients, and related parameters were investigated to understand the physiological mechanisms underlying the elevated pH-induced alleviation of Citrus toxicity. Increasing the nutrient solution pH from 2.5 to 4.0 alleviated the Al-toxic effects on biomass, photosynthesis, OJIP transients and related parameters, and element concentrations, uptake, and distributions. In addition, low pH effects on the above physiological parameters were intensified by Al-toxicity. Evidently, a synergism existed between low pH and Al-toxicity. Increasing pH decreased Al uptake per root dry weight and its concentration in roots, stems, and leaves and increased nitrogen, phosphorus, calcium, magnesium, sulfur, and boron uptake per plant and their concentrations in roots, stems, and leaves. This might be responsible for the elevated pH-induced alleviation of growth inhibition and the impairment of the whole photosynthetic electron transport chain, thus preventing the decrease of CO2 assimilation. [ABSTRACT FROM AUTHOR]

Little is known about the physiological and molecular responses of leaves to aluminum (Al)-toxicity. Seedlings of Al-intolerant Citrus grandis and Al-tolerant Citrus sinensis were supplied daily with nutrient solution containing 0 mM (control) and 1.0 mM (Al-toxicity) AlCl 3 ·6H 2 O for 18 weeks. We found that Al-treatment only decreased CO 2 assimilation in C. grandis leaves, and that the Al-induced alterations of gene expression profiles were less in C. sinensis leaves than those in C. grandis leaves, indicating that C. sinensis seedlings were more tolerant to Al-toxicity than C. grandis ones. Al concentration was similar between Al-treated C. sinensis and C. grandis roots, but it was higher in Al-treated C. grandis stems and leaves than that in Al-treated C. sinensis stems and leaves. Al-treated C. sinensis seedlings accumulated relatively more Al in roots and transported relatively little Al to shoots. This might be responsible for the higher Al-tolerance of C. sinensis . Further analysis showed that the following several aspects might account for the higher Al-tolerance of C. sinensis , including: (a) Al-treated C. sinensis leaves had higher capacity to maintain the homeostasis of energy and phosphate, the stability of lipid composition and the integrity of cell wall than did Al-treated C. grandis leaves; (b) Al-triggered production of reactive oxygen species (ROS) and the other cytotoxic compounds was less in Al-treated C. sinensis leaves than that in Al-treated C. grandis leaves, because Al-toxicity decreased CO 2 assimilation only in C. grandis leaves; accordingly, more upregulated genes involved in the detoxifications of ROS, aldehydes and methylglyoxal were identified in Al-treated C. grandis leaves; in addition, flavonoid concentration was increased only in Al-treated C. grandis leaves; (c) Al-treated C. sinensis leaves could keep a better balance between protein phosphorylation and dephosphorylation than did Al-treated C. grandis leaves; and (d) both the equilibrium of hormones and hormone-mediated signal transduction were greatly disrupted in Al-treated C. grandis leaves, but less altered in Al-treated C. sinensis leaves. Finally, we discussed the differences in Al-responsive genes between Citrus roots and leaves. [ABSTRACT FROM AUTHOR]

For the first time, we used targeted metabolome to investigate the effects of pH-aluminum (Al) interactions on energy-rich compounds and their metabolites (ECMs) and phytohormones in sweet orange (Citrus sinensis) roots. The concentration of total ECMs (TECMs) was reduced by Al-toxicity in 4.0-treated roots, but unaffected significantly in pH 3.0-treated roots. However, the concentrations of most ECMs and TECMs were not lower in pH 4.0 + 1.0 mM Al-treated roots (P4AR) than in pH 3.0 + 1.0 mM Al-treated roots (P3AR). Increased pH improved the adaptability of ECMs to Al-toxicity in roots. For example, increased pH improved the utilization efficiency of ECMs and the conversion of organic phosphorus (P) from P-containing ECMs into available phosphate in Al-treated roots. We identified upregulated cytokinins (CKs), downregulated jasmonic acid (JA), methyl jasmonate (MEJA) and jasmonates (JAs), and unaltered indole-3-acetic acid (IAA) and salicylic acid (SA) in P3AR vs pH 3.0 + 0 mM Al-treated roots (P3R); upregulated JA, JAs and IAA, downregulated total CKs, and unaltered MEJA and SA in P4AR vs pH 4.0 + 0 mM Al-treated roots (P4R); and upregulated CKs, downregulated JA, MEJA, JAs and SA, and unaltered IAA in P3AR vs P4AR. Generally viewed, raised pH-mediated increments of JA, MEJA, total JAs, SA and IAA concentrations and reduction of CKs concentration in Al-treated roots might help to maintain nutrient homeostasis, increase Al-toxicity-induced exudation of organic acid anions and the compartmentation of Al in vacuole, and reduce oxidative stress and Al uptake, thereby conferring root Al-tolerance. In short, elevated pH-mediated mitigation of root Al-stress involved the regulation of ECMs and phytohormones. [Display omitted] • High pH reduced P-containing energy-rich compounds (ECs) in Al-toxic Citrus roots. • High pH increased the utilization efficiency of ECs at Al-toxic roots. • High pH increased IAA, SA and JA levels and decreased CKs levels in Al-toxic roots. • ECs and phytohormones involved in high pH-mediated alleviation of root Al-toxicity. [ABSTRACT FROM AUTHOR]

Little is known about interactive effects of pH-aluminum (Al) on reactive oxygen species (ROS) and methylglyoxal (MG) metabolisms in plants. Citrus sinensis seedlings were fertilized with nutrient solution at an Al concentration of 1 or 0 mM and a pH of 4.0, 3.5, 3.0 or 2.5 for 18 weeks. Thereafter, gas exchange and chlorophylls in leaves, H 2 O 2 generation, electrolyte leakage, total soluble proteins, MG, malondialdehyde (MDA), antioxidants, sulfur-containing compounds, enzymes [viz., antioxidant enzymes, sulfur metabolism-related enzymes, ascorbate oxidase, phosphomannose isomerase, glyoxalase I and glyoxalase II] involved in ROS and MG detoxification in leaves and roots were measured. Effects of low pH and Al-toxicity on these parameters displayed obvious synergism. Without Al-toxicity, low pH increased H 2 O 2 production, electrolyte leakage, MDA and MG concentrations by 45.7%–90.3% (52.4%–73.6%), 24.3%–74.5% (26.7%–86.2%), 18.6%–44.8% (35.6%–53.7%) and 16.3%–47.1% (13.8%–51.7%) in leaves (roots) relative to pH 4, respectively; low pH-induced upregulation of enzymes involved in ROS and MG detoxification and sulfur-containing compounds in leaves and/or roots could not protect them against oxidative damage. At pH 2.5–3.0, Al-toxicity increased H 2 O 2 production, electrolyte leakage, MDA and MG concentrations by 34.2%–35.5% (23.9%–72.7%), 10.2%–29.5% (23.7%–56.8%), 15.6%–35.7% (27.5%–33.9%) and 21.5%–26.8% (21.0%–49.2%) in leaves (roots), respectively, and decreased total soluble protein concentration by 46.2%–47.4% (18.8%–20.8%) in leaves (roots); at pH 3.5–4.0, Al-toxicity did not affect significantly the five parameters in leaves and roots except for Al-induced increases in root MDA concentration at pH 3.5–4.0 and root electrolyte leakage at pH 3.5, and Al-induced decrease in root total soluble protein concentration at pH 4.0. Raised pH conferred the ability to maintain a balance between production and detoxification of ROS and MG in leaves and roots, thus protecting them against oxidative damage, and hence alleviating Al-induced increase in electrolyte leakage and decrease in total soluble protein level. Image 1 • Enhancing nutrient solution pH from 2.5 to 4.0 mitigated Citrus Al-toxicity. • With Al-stress, high pH reduced root and leaf ROS and MG production and level. • With Al-stress, high pH enhanced root and leaf ability to detoxify ROS and MG. • High pH prevented Al-treated roots and leaves from oxidative damage. Main finding: Increased pH alleviated Citrus Al-toxicity by enhancing the capacity to detoxify ROS and MG and decreasing their production and accumulation in leaves and roots. [ABSTRACT FROM AUTHOR]

Background: Limited data are available on the responses of reactive oxygen species (ROS) and methylglyoxal (MG) metabolisms to low pH in roots and leaves. In China, quite a few of Citrus are cultivated in acidic soils (pH < 5.0). 'Xuegan' (Citrus sinensis) and 'Sour pummelo' (Citrus grandis) (C. sinensis were more tolerant to low pH than C. grandis) seedlings were irrigated daily with nutrient solution at a pH of 2.5, 3 or 5 for nine months. Thereafter, we examined low pH effects on growth, and superoxide anion production rate (SAP), malondialdehyde (MDA), MG, antioxidants, and enzymes related to ROS and MG detoxification in roots and leaves in order to (a) test the hypothesis that low pH affected ROS and MG metabolisms more in roots than those of leaves, and (b) understand the roles of ROS and MG metabolisms in Citrus low pH-tolerance and -toxicity. Results: Compared with control, most of the physiological parameters related to ROS and MG metabolisms were greatly altered at pH 2.5, but almost unaffected at pH 3. In addition to decreased root growth, many fibrous roots became rotten and died at pH 2.5. pH 2.5-induced changes in SAP, the levels of MDA, MG and antioxidants, and the activities of most enzymes related to ROS and MG metabolisms were greater in roots than those of leaves. Impairment of root ascorbate metabolism was the most serious, especially in C. grandis roots. pH 2.5-induced increases in MDA and MG levels in roots and leaves, decreases in the ratios of ascorbate/(ascorbate+dehydroascorbate) in roots and leaves and of reduced glutathione/(reduced+oxidized glutathione) in roots were greater in C. grandis than those in C. sinensis. Conclusions: Low pH affected MG and ROS metabolisms more in roots than those in leaves. The most seriously impaired ascorbate metabolism in roots was suggested to play a role in low pH-induced root death and growth inhibition. Low pH-treated C. sinensis roots and leaves had higher capacity to maintain a balance between ROS and MG production and their removal via detoxification systems than low pH-treated C. grandis ones, thus contribute to the higher acid-tolerance of C. sinensis. [ABSTRACT FROM AUTHOR]

Citrus sinensis seedlings were irrigated with nutrient solution at a concentration of 0 (Mg-deficiency) or 2 (Mg-sufficiency) mM Mg (NO3)2 for 16 weeks. Mg-deficiency-induced interveinal chlorosis, vein enlargement and corkiness, and alterations of gas exchange, pigments, chlorophyll a fluorescence (OJIP) transients and related parameters were observed in middle and lower leaves, especially in the latter, but not in upper leaves. Mg-deficiency might impair the whole photosynthetic electron transport, including structural damage to thylakoids, ungrouping of photosystem II (PSII), inactivation of oxygen-evolving complex (OEC) and reaction centers (RCs), increased reduction of primary quinone electron acceptor (QA) and plastoquinone pool at PSII acceptor side and oxidation of PSI end-electron acceptors, thus lowering energy transfer and absorption efficiency and the transfer of electrons to the dark reactions, hence, the rate of CO2 assimilation in Mg-deficiency middle and lower leaves. Although potassium, Mg, manganese and zinc concentration in blades displayed a significant and positive relationship with the corresponding element concentration in veins, respectively, great differences existed in Mg-deficiency-induced alterations of nutrient concentrations between leaf blades and veins. For example, Mg-deficiency increased boron level in the blades of upper leaves, decreased boron level in the blades of lower leaves, but did not affect boron level in the blades of middle leaves and veins of upper, middle and lower leaves. To conclude, Mg-deficiency-induced interveinal chlorosis, vein enlargement, and corkiness, and alterations to photosynthesis and related parameters increased with increasing leaf age. Mg-deficiency-induced enlargement and corkiness of veins were not caused by Mg-deficiency-induced boron-starvation. [ABSTRACT FROM AUTHOR]

• Aluminum (Al) affected the contents of N-containing compounds in Citrus. • Al affected the activities of N metabolism–related enzymes in Citrus. • Al differentially affected N metabolism between Citrus sinensis and C. grandis. • Increased leaf GOGAT might be an index of Al–tolerance in Citrus plants. Citrus are mostly planted in tropical/subtropical acidic soil areas, and are susceptible to aluminum (Al) toxicity, which affects tree growth and fruit quality. However, the effects of Al stress on the nitrogen (N) metabolism in different Al-tolerant Citrus varieties, and whether N metabolism is related to Al-tolerance in different Citrus varieties, were remained unveiled. In the current study, Al-tolerant "Xuegan" (Citrus sinensis) and Al-sensitive "Sour pummelo" (C. grandis), were irrigated with different nutrient solutions with or without Al. Al decreased the dry weights (DW) of leaves, stems and roots, as well as the N content, the N accumulation, total soluble proteins (TSPs) and total free amino acids (TFAAs) of different tissues of C. grandis , but it increased the ratio of root DW to shoot DW. In contrast, Al did not change the parameters mentioned above, except for decreasing the content and accumulation of N in C. sinensis roots. Al did not change the content of NO 3 – in different position leaves and roots, but decreased the content of NH 4 + in C. grandis. Al decreased the content of NO 3 – in different position leaves and roots, but except for the lower leaf, Al did not change that of NH 4 + in C. sinensis. Although Al affected the activities and gene expression levels of N metabolism–related enzymes in the leaves and roots of these two Citrus species, the effects of Al on these parameters were more pronounced in C. grandis than in C. sinensis. Results from correlation analysis (CoA) and principal components analysis (PCA) indicated that the increased root DW/shoot DW and NO 3 --N content in the roots and middle/upper leaves might be the related indicators of Al toxicity, whereas the enhanced activity of leaf GOGAT might be related to the Al–tolerance in Citrus plants. In conclusion, this study preliminarily revealed that the impact of Al on N metabolism and the response of N metabolism–related indicators to Al stress in Citrus , providing scientific reference for high-yield and high-quality cultivation of Citrus grown in acidic soil areas. [Display omitted] [ABSTRACT FROM AUTHOR]

• N-deficient citrus leaves and roots kept high abilities to detoxify ROS and MG. • N-deficiency (ND) affected ROS and MG metabolism more in roots than in leaves. • ND enhanced n distribution in roots and R/S, and delayed root senescence. • ND reduced MDA levels and electrolyte leakage in citrus leaves and roots. Nitrogen-deficiency (ND) usually occurs in some citrus orchard soils in China. The roles of reactive oxygen species (ROS) and methylglyoxal (MG) generation and their detoxification systems in ND tolerance of horticultural woody plants still need to be revealed. For the first time, we examined the effects of ND on ROS and MG generation and their detoxification systems in leaves and roots of Citrus sinensis seedlings. The objectives are to test the hypotheses that N-deficient leaves and roots can keep high abilities to scavenge ROS and MG, thereby protecting them from oxidative damage, and that ND-induced alterations of ROS and MG formation and their detoxification systems in leaves and roots are different. ND augmented superoxide anion production rate and MG concentrations, but it decreased malondialdehyde (MDA) concentrations and electrolyte leakage in leaves and roots. ND increased the activities of most enzymes involved in ROS (ascorbate-glutathione cycle-related enzymes, antioxidant enzymes, and sulfur metabolism-related enzymes) and MG (glyoxalases) detoxification expressed on a protein basis with a few exceptions, and the concentrations of ascorbate, phytochelatins, and total non-protein thiols in leaves and roots. These results suggested that nitrogen-deficient leaves and roots could keep high abilities to detoxify ROS and MG, and protect them from oxidative damage. Generally viewed, ND affected the production and removal of ROS and MG more in roots than in leaves. [ABSTRACT FROM AUTHOR]

• Elevated pH ameliorated Al-induced reduction of Citrus sinensis seedling growth. • Raised pH enhanced leaf ability to maintain energy and phosphate homeostasis. • Raised pH enhanced the ability to maintain cell wall and lipid metabolism stability. • Raised pH increased SM accumulation, lessened Al-induced downregulation of SMs. • Raised pH ameliorated Al-induced oxidative damage and increase of callose. The contribution of aluminum (Al)-pH-interaction-responsive metabolites and genes to Al-tolerance in Citrus leaves is still poorly understood. Seedlings of 'Xuegan' (Citrus sinensis) were supplied with nutrient solution at a pH of 4.0 or 3.0 and an Al concentration of 1 or 0 mM for 18 weeks. Thereafter, we investigated the impacts of Al-pH interactions on plant growth; Al concentrations in leaves and roots; and metabolome, transcriptome, and related physiological indices in leaves. Elevated pH ameliorated Al-toxicity-induced reduction of seedling (leaf) growth. Here, we screened 867 upregulated and 1041 downregulated genes, and 43 [eight primary metabolites (PMs) + 35 secondary metabolites (SMs)] and 28 downregulated (23 PMs + five SMs) in pH 3.0 + 1 mM Al-treated leaves (P3AL) vs. pH 3.0 + 0 mM Al-treated leaves (P3L); and 410 upregulated and 149 downregulated genes, and 75 upregulated (20 PMs + 55 SMs) and 32 downregulated (25 PMs + seven SMs) metabolites in pH 4.0 + 1 mM Al-treated leaves (P4AL) vs. pH 4.0 + 0 mM Al-treated leaves (P4L), implying that elevated pH ameliorated Al-toxic impacts on gene expression, but it enhanced the adaptability of metabolites to Al-stress. Further analysis suggested that raised pH-mediated-amelioration of leaf Al-stress involved the several aspects, including: (a) decreased accumulation of callose and low methylesterified pectin, increased accumulation of high methylesterified pectin, and enhanced cell wall stability; (b) less alterations of lipid metabolism and downregulation of PMs; (c) increased biosynthesis and accumulation of SMs; and (d) enhanced ability to maintain energy, phosphate and cell redox homeostasis, and less oxidative injury due to higher ability to maintain the balance between reactive oxygen species and aldehyde generation and detoxification. Some metabolic pathways such as glycolysis / gluconeogenesis and phenylpropanoid pathway; metabolites such as callose and phenolic compounds; and genes such as xyloglucan endotransglucosylase/hydrolases, purple acid phosphatases, phytochelatin synthase 3, and cysteine synthase , might involve in raised pH-induced amelioration of leaf Al-stress. Also, some similarities and differences existed between leaves and roots in Al-pH-interaction-responsive metabolites and genes. [ABSTRACT FROM AUTHOR]

Limited data are available on the molecular and physiological mechanisms for increased pH-mediated amelioration of copper (Cu) toxicity in plants. Citrus sinensis seedlings were fertilized with a nutrient solution at the Cu concentration of 300 (Cu toxicity) or 0.5 μM (control) and a pH of 3.0, 4.0, or 4.8 for 17 weeks. Subsequently, we examined the interactive effects of low pH and Cu toxicity on transcriptomics, metabolomics, and some physiological parameters in leaves. Our results demonstrated that increased pH reduced Cu toxicity-induced leaf Cu accumulation and oxidative damage by reducing reactive oxygen species (ROS) production and maintaining the homeostasis of sulfur (S)-containing compounds (reduced glutathione), ascorbate, and cell redox potential, thus mitigating Cu toxic effects on leaf chlorophyll biosynthesis, photosynthesis, and metabolisms of carbohydrates, lipids, amino acids, and secondary metabolites. The increased pH mitigated Cu toxicity-induced impairment of cell wall metabolism by reducing cell wall Cu concentration, thus improving leaf growth. Under low pH (pH 3.0), C. sinensis leaves also displayed some adaptive responses to Cu toxicity to meet the increased demand for the dissipation of excess light energy and the detoxification of Cu and ROS, including: (a) increased distribution of Cu in cell wall; (b) elevated photorespiration and thermal dissipation; (c) increased accumulation of nonstructural carbohydrates [fructose, glucose, starch, total nonstructural carbohydrates (the summation of fructose + glucose + sucrose + starch), maltotetraose and 1,1-kestotetraose] and upregulation of metabolism (glycolysis/gluconeogenesis, pyruvate metabolism and pentose phosphate pathway) related to energy production; (d) downregulation of phospholipid [LysoPC 18:3(2 n isomer)] and phosphate-containing compounds (2′-deoxycytidine-5′-monophosphate and adenosine 5′-monophosphate) and upregulation of L -tryptophan metabolism and related amino acids (L -tryptophan and 5-hydroxy- L -tryptophan); and (e) increased accumulation of some secondary metabolites [total phenolics, lignin, alkaloids (3-indoleacrylic acid, N-acetyl-5-hydroxytryptamine and methyl nicotinate), plumerane (indole and 3-indolepropionic acid) and coumarins (isoscopoletin, scopoletin, skimming and scopolin)]. However, these adaptive responses could not protect low pH-treated leaves from Cu toxicity, as indicated by elevated malondialdehyde accumulation and electrolyte leakage and decreased photosynthesis and chlorophyll level in leaves as well as reduced leaf growth due to impaired cell metabolism. Cu toxicity intensified the adverse effects of low pH on C. sinensis leaves. • High pH reduced Cu level and Cu toxicity-induced oxidative damage in leaves. • High pH alleviated Cu toxic effects on photosynthesis and related metabolisms. • High pH reduced Cu toxic effects on amino acids, lipids, and cell wall metabolism. • Under low pH, adaptive changes occurred in leaves affected by Cu toxicity through detoxification of Cu and ROS. • Cu toxicity intensified the adverse effects of low pH on Citrus sinensis leaves. [ABSTRACT FROM AUTHOR]

• Magnesium deficiency induces leaf chlorosis, leaf curling and impaired growth of C.sinensis seedlings. • Magnesium deficiency upregulates the relative abundances of most monosaccharides and amino acids in C.sinensis leaves. • Magnesium deficiency decreases the relative abundances of most pigment compounds and organic acids in C.sinensis leaves. In south China, Magnesium (Mg) deficiency is commonly found in Citrus orchard due to soil acidification. However, less information is available discussing the global metabolic profile of Mg-deficient leaves in Citrus species. In the study, seedlings of 'Xuegan' [ Citrus sinensis (L.) Osbeck] were treated with Mg-plus (2 mM, as Control) or Mg-deficiency (0 mM) for 16 weeks by sandy culture, respectively. The untargeted metabolomics based on UHPLC-Q-TOF/MS were investigated to reveal the metabolic responses to Mg-deficiency in leaves of C. sinensis. The results indicated severe growth inhibition and obvious leaf chlorosis in Mg-deficient seedlings of C. sinensis compared to control. The principal component analysis (PCA) and orthogonal partial least squares-discriminate analysis (OPLS-DA) on the LC/GC data demonstrated a clear discrimination between the profiles of metabolites in control leaves and Mg-deficient leaves of C.sinensis. Besides, the identification and classification of those differential expressed metabolites (DEMs) revealed Mg-deficiency upregulated relative abundances of 78.42 % lipids and lipid-like molecules. In contrast, Mg-deficiency downregulated 89.42 % phenylpropanoids and polyketides in leaves of C.sinensis. Remarkably, relative abundances of most monosaccharides and amino acids increased while relative abundances of most leaf pigment compounds and organic acids decreased in response to Mg-deficiency in C. sinensis leaves. Those results support both primary and secondary metabolic modifications in C. sinensis leaves exposed to Mg-deficiency. Overall, the findings of present study contribute to a better understanding on the metabolic changes of Mg-deficient Citrus species, which will provide references for nutrient management and quality control in Citrus production. [ABSTRACT FROM AUTHOR]