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High irradiance and increased air temperature during extreme weather conditions affect tree crops and impact the yield and quality of fruits. Moreover, flowering and fruit set of Citrus are likely impaired by UV radiation and/or reduced carbon assimilation, which increase reactive oxygen species production and damage the leaf photosynthetic apparatus. Particle coating films sprayed on leaves have been offered as a way to minimize crop losses due to the climate change scenario, even though the extent of leaf protection is not characterized. We evaluated the use of two protective films on the oxidative stress and leaf photosynthesis of sweet orange trees exposed to varying daylight levels. Trees were maintained under full sun light, sprayed or not (control) with kaolin or calcium carbonate, and under reduced irradiance using either aluminum shade cloth 50% or anti-UV transparent plastic. Kaolin or calcium carbonate reflected 20-30% of the incident light on the leaf surface compared to leaves not sprayed and under full sunlight. Leaves with coating exhibited improved CO

Background and Aims Citrus trees are severely affected by zinc (Zn) deficiency, which impairs plant growth and fruit yield. Zn fertilization is usually recommended in field orchards, and application strategies toward nutrient use efficiency are key for successful crop management. Methods Field studies were carried out with sweet orange trees for four growing seasons, testing Zn (nitrate, sulfate or EDTA) applied either via fertigation or foliar spray compared to a control without Zn. Results The Zn concentrations in the soil increased with nutrient supply by both application methods. Zn-EDTA via fertigation increased soil nutrient availability compared to the control. Likewise, Zn-nitrate via foliar application also increased soil Zn levels. Regarding the plant, Zn-EDTA via fertigation increased leaf nutrient levels only after the third year of fertilization, whereas Zn-nitrate via foliar spray increased leaf levels in the short term, up to 120 mg kg− 1. To a lesser extent, Zn-EDTA or sulfate also increased leaf Zn compared to the control. Accumulated fruit yield was ~ 20% higher in trees with Zn-EDTA via fertigation and ~ 20% higher in trees with Zn-nitrate or sulfate via foliar application compared to the control, with these latter exerting greater responses. Trees supplied with Zn exhibited lower H2O2 and higher CAT activity compared to the control, which correlated with a fruit yield increase. Conclusion In conclusion, Zn-EDTA via fertigation or nitrate or sulfate via foliar application improved the horticultural performance of trees, supporting the establishment of best nutrient management practices in fruit production.

Glyphosate excessive use is reported in Brazilian citrus orchards, whereas there is speculation about its consequences and the published studies are contradictory and inconclusive. This study aimed to describe the possible harmful effects by simulating glyphosate drift directly to the leaves of ∼4-yr-old citrus plants. As major results, glyphosate doses360 g ae ha

Optimizing the use of nitrogen (N) for food production is a major challenge in agricultural systems. The transformation of N into crop production results from intricate pathways, depending on plants, as well as the environment and fertilization regimes, which affect the N-use efficiency (NUE) of plants. In this context, lemon trees [Citrus limon (L.) Burm. f.] attain maximum harvest index at lower leaf N concentrations compared with sweet orange trees [Citrus sinensis (L.) Osbeck], and the processes governing these plant responses are not well known. The aim of this study was to understand how the higher NUE in lemons trees is constructed based on growth and biomass partitioning evaluation, as well as photochemical and biochemical characteristics of photosynthesis. To attain this goal, we evaluated growth, photosynthesis and biochemical characteristics in lemon and sweet orange trees under two different N levels over 14 months. We hypothesized that higher NUE in lemon trees is affected by plant capacity to grow with economy on nutrient resources. Furthermore, lemon trees could be more efficient in CO2 assimilation in non-limiting environmental conditions. We found that higher NUE in lemon trees was explained in part by the ability of trees to invest greater biomass in leaves instead of roots, even though this species exhibited lower relative recovery efficiency of N from the substrate than the sweet orange. We also found that lemon trees had a higher relative growth rate than sweet oranges, despite the fact that net CO2 assimilation and dark respiration were similar between the two species. As a consequence, we suggested that lemons could exhibit a lower biomass construction cost than oranges. Because lemon presented lower N concentration than sweet orange trees, the former exhibited better photosynthetic N-use efficiency (PNUE: 55–120 mmol CO2 g N−1 day−1) compared with the sweet orange (PNUE: 31–68 mmol CO2 g N−1 day−1). Lemon trees also exhibited a higher relative rate of electron transport per unit of chlorophyll (ETR/chlor: 350–850) compared with orange trees (ETR/chlor: 300–550) at both low and at high N supply. These characteristics were likely associated with transport facilitation of CO2 to the catalytic sites of plants. In fact, improved growth of lemon trees results from an array of events explained mostly by increase in leaf area and associated low construction cost despite N supply.

Root plasma membrane H+-ATPase electrochemical equilibrium for optimum coffee plant growth can be modulated by specific ammonium:nitrate (NO3−:NH4+) ratio supply. This study aimed to evaluate the coffee seedlings responses to varying ammonium:nitrate (NO3−:NH4+) ratio and to depict how much NO3− and NH4+ plants can use in terms of growth, nitrogen metabolism, amino acids profile and regulation of root plasma membrane H+-ATPase. Coffee plants were grown in nutrient solution with the following NO3−:NH4+ ratios (%): 100:0; 87.5:12.5; 50:50; 0:100. Plants were grown in nutrient solution for 90 days and evaluated for growth, nitrate reductase activity as well as the modulation of H+-ATPase activity in the plasma membrane of the roots, amino acids profile, chlorophyll a fluorescence parameters and estimated cations and anions taken up by plants. The plants treated with the 87.5:12.5 and 50:50 NO3−:NH4+ ratio showed higher ability to absorb nutrients maintaining balanced uptake and as a consequence, 6% and 29%, the highest dry mass yield as compared to the 0:100 NO3−:NH4+ ratio. In addition, plants supplied with the 87.5:12.5 and 50:50 NO3−:NH4+ ratio had respectively, 58% and 94%, greater photosynthetic capability. Those data suggest that farmers and plant nurseries could implement the 50:50 NO3−:NH4+ ratio of nitrogen sources at coffee plantations and seedlings.

In the past 15 years, the global citrus industry has experienced significant losses in the fruit production, largely due to the huanglongbing (HLB). This bacterial disease impairs water and nutrient uptake by roots causing nutritional disorders and, reciprocally, metabolic imbalances associated to oxidative stress and carbohydrate distribution in trees. The sustainability of optimum yield and fruit quality of citrus are achieved by growing canopy and rootstock varieties with superior horticultural characteristics in well-established orchards, which relies on efficient irrigation and/or fertilization, as well crop protection. Then, attention to enhanced nutrient supply increased significantly in commercial groves. In order to better understand the pathological processes, this review discusses recent scientific advances and major findings in most citrus-producing regions of the world, critically analyzing nutrient management practices as a component of an intricate strategy to maintain tree health, fruit yield, and quality. Moreover, we consider the role of balanced and constant nutrition of citrus trees to sustain citrus production under endemic HLB or non-HLB conditions.

Despite efforts of research to establish best nutrient management practices in HLB-affected citrus orchards, there are still doubts about the contribution of such strategies to minimizing losses caused by the disease in the citrus industry. We evaluated the effects of micronutrient (Zn, Mn, and/or Cu) supply and ‘Candidatus Liberibacter asiaticus’ (CLas) infection on physiological and growth traits of sweet orange trees (Citrus sinensis (L.) Osbeck)) to understand if enhanced micronutrient supply would improve the growth of plants infected with CLas and reduce bacterial infection, as well as the acquisition of CLas by adults and nymphs of Diaphorina citri, correlated with nutritional status of trees. Plants were either grafted with buds obtained from micrografted plants (healthy, −) or buds infected with CLas (diseased plants, +). Infected plants were exposed to one of the five nutrient treatments, applied to leaf and root: (i) nilZnMnCu+: not fertilized with Cu, Mn, and Zn; (ii) Zn+: fertilized with zinc sulfate (ZnSO4.H2O); (iii) Mn+: fertilized with manganese sulfate (MnSO4.H2O); (iv) Cu+: fertilized with copper hydroxide [Cu(OH)2]; and (v) ZnMnCu+: fertilized with all three micronutrients. Likewise, healthy plants were exposed to one of the two treatments as above: (i) nilZnMnCu− or (v) ZnMnCu−. We found that CLas impairs plant biomass production regardless of nutrient treatments, especially root growth, and increases specific leaf dry weight as disease progresses because of starch accumulation. Moreover, individual supply with Zn, Mn, or Cu can mitigate such deleterious effects of HLB on starch metabolism. HLB also changes nutrient concentrations in both leaves and sap extract, regardless of nutrient treatments, although treatments do not reduce the CLas titer in plants as determined by RT-qPCR. On the contrary, micronutrients applied in combination (ZnMnCu+) can reduce the acquisition of CLas by adults and especially nymphs of D. citri, likely reducing the disease infection in citrus orchards.

This study addresses the interactive effects of deficit irrigation and huanglongbing (HLB) infection on the physiological, biochemical, and oxidative stress responses of sweet orange trees. We sought to answer: (i) What are the causes for the reduction in water uptake in HLB infected plants? (ii) Is the water status of plants negatively affected by HLB infection? (iii) What are the key physiological traits impaired in HLB-infected plants? and (iv) What conditions can mitigate both disease severity and physiological/biochemical impairments in HLB-infected plants? Two water management treatments were applied for 11 weeks to 1-year-old-trees that were either healthy (HLB–) or infected with HLB (+) and grown in 12-L pots. Half of the trees were fully irrigated (FI) to saturation, whereas half were deficit-irrigated (DI) using 40% of the water required to saturate the substrate. Our results demonstrated that: reduced water uptake capacity in HLB+ plants was associated with reduced root growth, leaf area, stomatal conductance, and transpiration. Leaf water potential was not negatively affected by HLB infection. HLB increased leaf respiration rates (ca. 41%) and starch synthesis, downregulated starch breakdown, blocked electron transport, improved oxidative stress, and reduced leaf photosynthesis (ca. 57%) and photorespiration (ca.57%). Deficit irrigation reduced both leaf respiration (ca. 45%) and accumulation of starch (ca.53%) by increasing maltose (ca. 20%), sucrose, glucose, and fructose contents in the leaves, decreasing bacterial population (ca. 9%) and triggering a series of protective measures against further impairments in the physiology and biochemistry of HLB-infected plants. Such results provide a more complete physiological and biochemical overview of HLB-infected plants and can guide future studies to screen genetic tolerance to HLB and improve management strategies under field orchard conditions.

The objectives of the present study were to investigate the influence of water supply combined with different rates and methods of nitrogen (N) and potassium (K) application on fruit yield and nutrient use efficiency (NutUE) of citrus by monitoring plant nutritional status and soil solution dynamic. The experiment was carried out with 4-yr-old Natal sweet orange trees on Rangpur lime over 4 consecutive growing seasons. Treatments were composed of 2 rates of N and K (NK): 50% and 100% of the recommended rates for maximum yield in rain-fed environments and 3 nutrient-water management systems: non-irrigation + broadcast granular fertilizer, irrigation + broadcast granular fertilizer and fertilizer application via irrigation (fertigation). Fruit yield was maximum under fertigation with application of 50% of the NK rate, and fertigation improved NutUE by 22% compared to non-irrigated trees. However, no difference in productivity and NutUE occurred across nutrient-water management systems under 100% of the NK rate. Differences in plant nutritional were not consistent across treatments and did not contribute to explain variations in fruit yield and NutUE. Soil solution analysis of fertigated trees receiving 100% of the NK rate revealed the lowest pH value and the highest concentration of N-NH4. It is concluded that fruit yield responses of citrus trees treated with established NK rates for rain-fed environments might be limited by soil solution acidification and N-NH4 toxicity when fertilizers are applied via fertigation, being necessary further adjustments on fertilization management of fertigated citrus groves.

Deficiencies of boron and zinc occur in coffee plantations despite the many fertilizer sources offered by the industry for the agriculture. Sparingly soluble fertilizers (SSF) have been used in several countries for many years. However, their effectiveness when applied to leaves is highly contested, and information about their use efficiency is still lacking. Experiments were set up to characterize the SSF of B and Zn according to particle size of fertilizers, and the spreading and retention of the particles in the leaf, comparing SSF (calcium borate, Zn oxide, and Zn borate) with soluble sources (boric acid and Zn sulfate) in four doses (mg.L-1): Experiment I (B 0, 130, 260, and 520), Experiment II (Zn 0, 200, 600, and 1,800), and experiment III (B 0, 43, 129, and 387, and Zn 0, 200, 600, and 1,800), to supply B and Zn for coffee plants. Microparticles of the SSF were found adhered to coffee leaves, and the Zn oxide was the one with the smallest particle size. SSFs were effective at increasing the Zn and B leaf concentrations in coffee. Dry matter of coffee increased 15% with applications of 270 mg.L-1 of B as calcium borate and 384 mg.L-1 of B as boric acid. Foliar application of Zn oxide at the dose of 1,800 mg.L-1 of Zn increased the leaf areas of the plants. High doses of Zn sulfate caused toxicity to coffee plants due to high saline concentrations. The microparticles found retained on the surface of the coffee leaves increased leaf concentration of B and Zn, which explained variations in the dry matter measurements and show the potential of the SSF for the development of fertilizers to improve the availability of micronutrients to coffee.

This study evaluated leaf supply of zinc (Zn) and boron (B) using either soluble or sparingly soluble fertilizers in young sweet orange trees. Three experiments were set up in a greenhouse to compare two sources and four doses (control, low, adequate and high) of fertilizers as follows: (i) Experiment I (B): boric acid and calcium borate; (ii) Experiment II (Zn): Zn sulfate and Zn oxide; and (iii) Experiment III (B + Zn): boric acid + Zn Sulfate and Zn Borate. The sparingly soluble sources were effective in increasing the Zn and B leaf concentration. Dry matter of the aerial part increased 18 % with B applications in adequate concentration independent of the B fertilizer sources. In contrast, trees did not grow well with applications of adequate concentration of Zn as Zn Sulfate or high Zn concentration as Zn borate. Superoxide dismutase activity in leaves increased with applications of low concentration of Zn as Zn oxide and decreased with high concentration of Zn from either source. Polyphenol oxidase activity increased with application of adequate concentration of B as boric acid and high concentration of B as calcium borate. Furthermore, the upper concentrations of Zn were toxic in orange trees when the source was Zn sulfate. Increases in plant growth without damage to leaf tissue and positive responses of key enzymes of orange trees in a range of nutrient concentration applications demonstrated the practical use of sparingly soluble fertilizers to supplying B and Zn foliarily to plants.

Techniques such as intercropping and minimum tillage improve soil quality, including soil microbial activity, which stimulates the efficient use of soil resources by plants. However, the effects of such practices in soil under citrus orchards have not been well characterized. In this study, we aimed to determine the effects of mowing and intercrop species on soil microbiological characteristics beneath a Tahiti acid lime orchard. The orchard was planted using minimum tillage and intercropped with two species of Urochloa species (U. ruziziensis&mdash, ruzi grass, U. decumbens&mdash, signal grass), with two types of mowers for Urochloa biomass (ecological, conventional) and herbicide applications. The study was conducted over 10 years. The ecological mower made the largest deposition of the intercrop biomass, thus providing the lowest disturbance of soil microbial activity and increasing, on average over all 10 years, the basal soil respiration (45%), microbial biomass carbon (25%), abundance of 16S rRNA (1.5%) and ITS (3.5%) genes, and arbuscular mycorrhizal fungi (30%), and providing a ca. 20% higher fruit yield. U. ruziziensis in combination with ecological mowing stimulated the abundance of the genes nifH (1.5%) and phoD (3.0%). The herbicide showed little influence. We conclude that the use of U. ruziziensis as an intercrop in citrus orchards subjected to ecological mowing can be recommended for improving and sustaining soil quality and citrus fruit production.

Citrus trees are subjected to variation of environmental conditions throughout the annual cycle, especially during the flowering and fruit set. Heat waves have affected the environmental conditions required for optimum crop growth and have reduced fruit yield and quality worldwide. This study aimed to evaluate an extra supply of magnesium (Mg) and/or nitrogen (N) to increase citrus tolerance to excessive increased irradiation and consequent air temperature. Young lemon trees were grown in a greenhouse with extra Mg, N and Mg + N supply for five months and then exposed to heat for 12 days. After imposing stress condition, photosynthetic and enzymatic antioxidant system parameters were assessed to evaluate the impact of the extra nutrient supply in alleviating stress induced by combination of elevated irradiance and air temperature. Extra nutrient supply increased plant tolerance to the environmental stress, allowing maintenance of high photosynthetic and transpiration rates and low ratio of apparent electron transport rate per photosynthetic carbon assimilated. Enhanced Mg supply also increased activity of the antioxidant enzyme system and decreased the oxidative stress of plants. Trees treated with extra N exhibited increased chlorophyll concentrations and ability to harvest light energy. Citrus trees under stressed condition, such as elevated air temperature and radiation, associated to heat waves, exhibited less damage in the photosynthetic mechanism and high activity of the antioxidant enzymatic system when supplied with extra nutrient. It was concluded that extra Mg and N supply are important tools to prevent damages and enhance lemon trees biochemical and physiological responses to alleviate the harmful effects of unfavorable environmental condition.

Citrus production is largely regulated by the availability of nutrients in the soil, which is affected by its physical, chemical, and biological characteristics. Soils under citrus cultivation vary in the major producing regions of the world from the subtropical to the tropical and Mediterranean-type areas. Understanding soil-plant relationships is important for the establishment of better management practices to increase production efficiency and fruit quality. To this end, tools are available to determine soil fertility and the nutritional status of citrus through soil and leaf analyses; the obtained information is associated with plant responses to nutrient supply and environmental conditions, allowing detailed recommendations for the optimal management of nutrients in citrus orchards. Thus, this chapter describes the main soil orders based on the world taxonomy system, the role of mineral nutrients in the production and quality of citrus, soil and plant monitoring, as well as the main management strategies of orchards based on the application of fertilizers in dry granular form, via fertigation, and through foliar application. This provides a basis to improve sustainability of global citrus production.

This study aimed to evaluate the fertiliser sources and application methods of copper (Cu) in citrus trees during the first years of fruit production. Two experiments were set up in an orchard with 3-year-old sweet orange trees, which were applied with three sources of Cu (nitrate, sulfate or EDTA) either via fertigation (Experiment 1) or via foliar sprays (Experiment 2). Regardless of the fertiliser source, Cu application via fertigation was not efficacious to increase the micronutrient concentration in leaves and, consequently, did not affect fruit yield. Conversely, foliar application of Cu, either as nitrate or sulfate, increased this nutrient level in leaves but when applied as copper nitrate, visual phytotoxicity symptoms were verified in leaves due to salt accumulation in the plant canopy, which reduced the fruit yield. Considering the plant growth and intensified phytosanitary management of the orchard with the application of copperbased products after the third year of the experiment, the effects of Cu treatments on fruit yield are expected to be negligible as the trees age.

Graphical abstract, Nitrate uptake by plants is mediated by specific transport proteins in roots (NRTs), which are also dependent on the activity of proton pumps that energize the reaction. Nitrogen (N) metabolism in plants is sensitive to copper (Cu) toxicity conditions. To understand how Cu affects the uptake and assimilation processes, this study assesses the inhibitory effects of elevated Cu levels on the expression of genes related to N absorption, transport and assimilation in roots of Arabidopsis. Plants were grown hydroponically for 45 days, being exposed to a range of Cu concentrations in the last 72 h or alternatively exposed to 5.0 μM Cu for the last 15 days. High Cu levels decreased the uptake and accumulation of N in plants. It down-regulated the expression of genes encoding nitrate reductase (NR1), low-affinity nitrate transporters (NRT1 family) and bZIP transcription factors (TGA1 and TGA4) that regulate the expression of nitrate transporters. Cu toxicity also specifically down-regulated the plasma membrane proton pump, AHA2, whilst having little effect on AHA1 and AHA5. In contrast, there was an up-regulation of high-affinity nitrate transporters from the NRT2 family when exposed to medium level of Cu excess, but this was insufficient for restoring N absorption by roots to control levels. These results demonstrate that plants display specific responses to Cu toxicity, modulating the expression of particular genes related to nitrate uptake, such as low-affinity nitrate transporters and proton pumps.

Increasing the nitrogen (N) use efficiency of fruit trees to enhance fruit yield and decrease N rate and fertilization losses in the field is intensively discussed. Noteworthy, molybdenum (Mo) demand is likely to increase in high yielding citrus orchards. However, supply of this micronutrient through fertilization practices is not well-known. Thus, two experiments were carried out under greenhouse conditions to evaluate the nitrate reductase (NRase) activity and the Mo mobility in sweet orange plants (1-yr-old) after foliar application of Mo. For both experiments, the plants were supplied with two N levels via fertigation over 7-mo (totaling 2.8 and 17.5 g of N per plant), with Mo treatments applied in the final month. The first experiment consisted of leaf sprays to the whole plant canopy at 0 (control), 0.12, 0.60 and 1.20 g L −1 Mo. In the second experiment, the 0.60 g L −1 Mo spray was limited to one side of the canopy. The Mo supply enhanced the NRase activity either in leaves or roots and increased the nitrate uptake by roots. Consequently, the N content in the roots, twigs and leaves of plants increased. When the Mo was sprayed on one side of the canopy, the nutrient was translocated (30 − 40% from the absorbed) from the leaves to the roots, but at a lower percentage in plants grown with the highest N supply. Although the Mo concentration did not increase in leaves that did not directly receive the micronutrient spray, the NRase increased in both parts of the canopy, as well as in the roots, enhancing the N content in Citrus.

Boron (B) is a key element for citrus production, especially in tropical regions, where the nutrient availability is commonly low in the soil. In addition, information about doses, fertilizer sources, methods of application, and particularly, differential nutrient demand of scion/rootstock combinations are required for efficient fertilization of commercial groves. In a non-irrigated sweet orange orchard ( cv. Natal), grafted onto Rangpur lime, Swingle citrumelo or Sunki mandarin, we studied the application of two sources of B: boric acid (17% B, soluble in water) and ulexite (12% B, partially soluble in water) at four levels of supply (control without B, and soil application of 2, 4 and 6 kg ha −1 yr −1 of B). The experiment was carried out for three years (2004–2006). Boron availability in the soil and concentration in the leaves, as well as the fruit yield and quality of trees were evaluated. Soil B extracted with hot water and total leaf B positively correlated with doses of the nutrient applied to the trees. Levels of B in the soil and in the leaves did not vary with fertilizer sources. Fruit yield of trees grafted onto Rangpur lime and Swingle citrumelo was more responsive to B doses than those grafted onto Sunki mandarin. The maximum fruit yield of trees grafted onto Swingle was obtained with 3.2 kg ha −1 yr −1 of B, and leaf B level of 280 mg kg −1 that point out to a highest demand for B when this combination was compared with other rootstocks. Furthermore, fertilization with B did not affect the quality of fruits, but correlated with B and potassium (K) concentrations in the leaves. These results also support that the current recommendations for levels of B in leaves should be revisited.

Copper (Cu) deficiency and toxicity cause stresses in citrus orchards and limited information is available about which rootstocks and associated mechanisms would enhance plant resistance to such nutritional disorders. Therefore, this study evaluated the nutritional status and antioxidant system responses of citrus grafted onto selected rootstocks differing in horticultural performance [Swingle citrumelo (SW) or Rangpur lime (RL)], grown in nutrient solution with varying concentrations of Cu (0.015, 0.60 or 24.0 μM). The experiment was carried out in a greenhouse using young sweet orange trees. Once taken up, Cu mostly accumulated in roots (75% of total plant Cu content). Trees grafted onto RL were more responsive to enzyme activities related to oxidative stress and to nitrogen metabolism in leaves when grown in the presence of either the lowest or the highest Cu concentrations used. Those grown in 24.0 μM Cu displayed decreased overall nutrient uptake and accumulation, with the exception of iron, which was predominantly found in roots. Cu/Zn superoxide dismutase activity in leaves was dependent upon signalling regulated by rootstocks, being lower in SW than in RL. Therefore, the use of appropriate rootstock varieties contributes to alleviate the effects of Cu stress on the metabolism and nutritional status of citrus plants.

Nutritional stress caused by copper (Cu) deficiency or toxicity affects fruit production of citrus orchards worldwide, but this could be minimised by fine-tuned fertilisation in the orchards. Two experiments were performed aiming to evaluate the photosynthetic capacity and the antioxidant enzyme activities of Swingle citrumelo seedlings, grown in nutrient solution (NS) with two levels of nitrogen (N) in the first experiment (adequate-N and high-N) and two levels of calcium (Ca) in the second (low-Ca and adequate-Ca). Plants were then exposed to various Cu levels (low, medium and high) for 15 days. Plants under Cu-toxicity exhibited specific effects on reactive oxygen species formation and root-to-shoot plant signalling. Copper absorption was greater with increased Cu concentration in the NS, which reduced plant biomass accumulation, gas exchange measurements, the activity of nitrate reductase and affected Cu partitioning between roots and shoots. Despite these effects, oxidative stress induced by excess-Cu was reduced at the highest N dose when compared to control and, on the contrary, increased with low-Ca supply. Therefore, a rational supply of N or Ca minimises Cu-induced stress damages to roots and leaves of plants, by directly enhancing the antioxidant system and protecting the associated antioxidative enzyme activities, whilst maintaining photosynthesis.

Antioxidant enzymatic responses in Citrus leaves under Cu‐induced stress depends on rootstock genotypes. However, there is a lack of information about how woody plants recover growth capacity after exposure to elevated Cu and whether growth is affected by the redistribution of the metal to new vegetative parts and consequently whether photosynthesis is affected. Therefore, the biomass of plants and Cu concentrations in new leaf flushes were determined in young citrus trees grafted onto contrasting rootstocks [Swingle citrumelo (SW) and Rangpur lime (RL)]. Photosynthetic rate, chlorophyll fluorescence and antioxidant enzymatic systems were evaluated in plants previously grown in nutrient solution with Cu varying from low to high levels and with no added Cu. Both rootstocks exhibited reduced plant growth under Cu toxicity. However, trees grafted onto RL exhibited better growth recovery after Cu excess, which was dependent on the modulation of antioxidant enzyme activities in roots and leaves that maintained the integrity of the photosynthetic apparatus. In contrast, plants grafted onto SW exhibited a lower photosynthetic rate at the lowest available Cu concentration. Although the highest accumulation of Cu occurred in citrus roots, the redistribution of the nutrient to new vegetative parts was proportional to the Cu concentration in the roots.

Nutrient management recommendations for fruit crops lack the understanding of the efficiency of soil fertilisation with manganese (Mn) and zinc (Zn), which could substitute, in part, the traditional foliar applications. Fruit yield of trees in response to Zn and Mn supply via soil may be limited either by sorption reactions with soil colloids or low solubility of fertilisers. We investigated the effects of fertiliser sources and rates of Mn and Zn applied to soils with different sorption capacities on nutrient uptake, biochemical responses and biomass of Citrus. Two experiments were carried out with 2-year-old sweet orange trees that received applications of Mn or Zn. The first experiment evaluated the application of Mn fertilisers (MnCO3 and MnSO4) at three levels of the nutrient (0, 0.7 and 3.5 g plant−1 of Mn) in two types of soil (18.1% and 64.4% of clay, referred to as sandy loam and clay soils, respectively). The second experiment, likewise, evaluated Zn fertilisers (ZnO and ZnSO4) and nutrient levels (0, 1.0 and 5.0 g plant−1 of Zn). Application of Mn and Zn increased nutrient availability in the soils as well as leaf nutrient concentrations in the trees. The lowest rates, 0.7 g plant−1 of Mn and 1.0 g plant−1 of Zn, both as sulphate, were sufficient to supply these micronutrients to sufficient levels in leaves, flowers and fruits. Metal toxicity to plants occurred with higher doses of both nutrients and to a large extent in the sandy soil. In this case, protein bands lower than 25 kDa were observed as well a decrease on leaf chlorophyll content. In the clay soil, despite increased micronutrient concentrations in the plant, responses were less pronounced because of higher adsorption of metals in the soil. Superoxide dismutase (SOD, EC 1.15.1.1) isoenzyme activity was determined by non-denaturing polyacrylamide gel electrophoresis (PAGE). The Cu/Zn-SOD isoenzymes increased with increased Zn rates, but in contrast, when Mn was applied at the highest rate, the activity of Cu/Zn-SODs decreased. The SOD activity pattern observed indicated increased production of superoxide and consequently an oxidative stress condition at the highest rates of Zn and Mn applied. The results demonstrated that the soil application of Mn and Zn can supply nutrient demands of orange trees, however the low solubility of fertilisers and the high sorption capacity of soils limit fertilisation efficiency. On the contrary, application of sulphate source in sandy soils may cause excess uptake of Mn and Zn and oxidative stress, which impairs the photosynthetic apparatus and consequently tree growth.

Nutritional disorders caused by copper (Cu) have affected citrus orchards. Since Cu is foliar sprayed as a pesticide to control citrus diseases, this metal accumulates in the soil. Thereby, we evaluated the effects of Cu leaf absorption after spray of different metal sources, as well as roots absorption on growth, nutritional status, and oxidative stress of young sweet orange trees. Two experiments were carried out under greenhouse conditions. The first experiment was set up with varying Cu levels to the soil (nil Cu, 0.5, 2.0, 4.0 and 8.0 g of Cu per plant as CuSO4.5H2O), whereas the second experiment with Cu application via foliar sprays (0.5 and 2.0 g of Cu per plant) and comparing two metal sources (CuSO4.5H2O or Cu(OH)2). Copper was mainly accumulated in roots with soil supply, but an increase of oxidative stress levels was observed in leaves. On the other hand, Cu concentrations were higher in leaves that received foliar sprays, mainly as Cu(OH)2. However, when sulfate was foliar sprayed, plants exhibited more symptoms of injuries in the canopy with decreased chlorophyll contents and increased hydrogen peroxide and lipid peroxidation levels. Copper toxicity was characterized by sap leakage from the trunk and twigs, which is the first report of this specific Cu excess symptom in woody trees. Despite plants with 8.0 g of Cu soil-applied exhibiting the sap leakage, growth of new plant parts was more vigorous with lower oxidative stress levels and injuries compared to those with 4.0 g of Cu soil-applied (without sap leakage). With the highest level of Cu applied via foliar as sulfate, Cu was eliminated by plant roots, increasing the rhizospheric soil metal levels. Despite citrus likely exhibiting different mechanisms to reduce the damages caused by metal toxicity, such as responsive enzymatic antioxidant system, metal accumulation in the roots, and metal exclusion by roots, excess Cu resulted in damages on plant growth and metabolism when the metal was taken up either by roots or leaves.

The fertigated area of the Brazilian citrus industry has grown rapidly during recent years, and an efficient management of nitrogen (N) application at these sites is required for sustainable citrus production. Therefore, a field trial with Valencia orange trees [Citrus sinensis (L.) Osbeck] on Swingle citrumelo rootstock (Citrus paradise Macfad. x Poncirus trifoliata L. Raf.) was conducted for 8 years to evaluate the effects of N rates (80, 160, 240 and 320kg ha–1 y–1) applied by fertigation, either as ammonium nitrate (AN) or calcium nitrate (CN), on soil solution dynamics, fruit yield, nutritional status, and N-use efficiency (NUE) of trees. The maximum fruit yield was reached with 240kg N ha–1 for AN, whereas a linear response and greater fruit yield was observed for N supplied as CN. The NUE was reduced for both N forms with increasing N rates. However, the NUE for CN was 14 to 38% greater than the NUE for AN. The lower fruit yield and NUE for AN compared to CN-treated trees was associated with the increased acidification of the soil solution with increased AN rates (pH ≤ 4.0). This limited nitrification resulted in a high ammonium (NH 4 ) concentration in the soil solution and a reduction in the net absorption of cations by the trees, particularly calcium (Ca). Due to the improved ion balance as well as the higher pH of the soil solution (pH ≥ 6.3) and diminished NH 4 availability, gains in both fruit yield and NUE in fertigated citrus groves in tropical soils can be obtained with the use of CN as a source of N.

Because low-phosphorus (P) availability limits citrus growth, rootstocks with a relatively high capacity for P uptake are desirable. An experiment was conducted with trees on Cleopatra mandarin (CM) and Rangpur lime (RL). Treatments consisted of P rates (20, 40, and 80 mg kg−1 of soil) applied in soil layers of 0–0.30 m and/or 0.31–0.60 m, besides an unfertilized control. The P fertilization increased root and shoot growth, and P nutrition was improved as indicated by greater leaf P concentration, P uptake, and P root uptake efficiency (PUE). The P applied in both soil layers improved shoot growth, P uptake, and PUE. Trees on RL took up 23–126% more P and had root systems with greater growth and PUE compared to those on CM. Thus, P uptake by citrus trees in low-P soils can be improved by augmenting the depth of fertilizer application and the use of more adapted rootstocks.

In Citrus, water, nutrient transport and thereby fruit production, are influenced among other factors, by the interaction between rootstock and boron (B) nutrition. This study aimed to investigate how B affects the anatomical structure of roots and leaves as well as leaf gas exchange in sweet orange trees grafted on two contrasting rootstocks in response to B supply. Plants grafted on Swingle citrumelo or Sunki mandarin were grown in a nutrient solution of varying B concentration (deficient, adequate, and excessive). Those grafted on Swingle were more tolerant to both B deficiency and toxicity than those on Sunki, as revealed by higher shoot and root growth. In addition, plants grafted on Sunki exhibited more severe anatomical and physiological damages under B deficiency, showing thickening of xylem cell walls and impairments in whole-plant, leaf-specific hydraulic conductance and leaf CO2 assimilation. Our data revealed that trees grafted on Swingle sustain better growth under low B availablitlity in the root medium and still respond positively to increased B levels by combining higher B absorption and root growth as well as better organization of xylem vessels. Taken together, those traits improved water and B transport to the plant canopy. Under B toxicity, Swingle rootstock would also favor plant growth by reducing anatomical and ultrastructural damage to leaf tissue and improving water transport compared with plants grafted on Sunki. From a practical point of view, our results highlight that B management in citrus orchards shall take into account rootstock varieties, of which the Swingle rootstock was characterized by its performance on regulating anatomical and ultrastructural damages, improving water transport and limiting negative impacts of B stress conditions on plant growth.

Gaseous losses of ammonia (NH3) have been observed in citrus orchards when urea is surface-applied to the soils, and this loss might significantly limit the effectiveness of the nitrogen (N) fertilizer. However, a portion of the volatilized NH3 might be absorbed by the plants through the leaves. To quantify the contribution of the leaf absorption of 15NH3, a study with sweet oranges was conducted in two field areas where trees were grown at standard (480 trees ha−1) and high densities (617 trees ha−1). Plastic trays were filled with soil, covered with mown grass to simulate field management conditions, fertilized with 15N labeled urea (12 atom % excess) and placed under each of three trees in the orchards. This experimental procedure prevented the uptake of N from the labeled urea by the roots. Two weeks after 15N fertilization, the trays were removed from the field, and the soil was homogenized and sampled for chemical analyses. The citrus trees under which the trays were placed were destructively harvested, and the total N concentrations and 15N/14N ratios were determined. After urea application, the NH3 losses peaked within three days and subsequently decreased to negligible amounts after 10 days. The total NH3 losses accounted for 55–82 % of the applied N. Although the NH3 absorption by the citrus leaves was proportional to the tree density in the field, only 3–7 % of the 15NH3 volatilized from the soil was recovered by the citrus trees, and the NH3 absorption was also influenced by the proximity of citrus trees to the site of urea application and the leaf areas of the trees. The citrus trees can absorb the NH3 volatilized from urea, even though, the amount recovered by the trees is small and does not represent a significant proportion of total gaseous N losses, what demonstrates the importance of enhanced N use efficiency practices in field to reduce losses of NH3 when urea is applied to soil surfaces.

Background and aims Phosphorus (P) is a mobile nutrient in the plant so growth depends on its internal remobilization and a plant’s ability to respond to its availability in the growing media. This study was conducted to evaluate the influence of P status and rootstocks on the patterns of P uptake and remobilization in orange trees. Methods Sweet orange trees on Cleopatra mandarin (CM) or Rangpur lime (RL) rootstocks were grown for nine months in nutrient solution (NS) that was either P-deficient (DNS) or was P-sufficient (SNS). After this period, half of the trees were reciprocally transferred between DNS and SNS (from D to S and S to D), while the others remained in their initial P availability. Results Trees on RL had more shoot and root growth, accumulated more P and had greater efficiency of P absorption and transport to the shoot (PAE) than those on CM. The major source of P for growth was previously stored P even with an adequate current P supply to the roots. This suggested the dominance of P remobilization over P uptake and the requirement that trees had sufficient stored P to meet P demand of new growth. Trees on CM had greater concentrations of remobilized P in new shoots than trees on RL. Conclusion Trees grafted on rootstocks less able to take up P (CM) were more dependent on the internal reserves of P for new growth than rootstocks with higher PAE (RL).

In low fertility tropical soils, boron (B) deficiency impairs fruit production. However, little information is available on the efficiency of nutrient application and use by trees. Therefore, this work verified the effects of soil and foliar applications of boron in a commercial citrus orchard. An experiment was conducted with fertigated 4-year-old ‘Valencia’ sweet orange trees on ‘Swingle’ citrumelo rootstock. Boron (isotopically-enriched 10B) was supplied to trees once or twice in the growing season, either dripped in the soil or sprayed on the leaves. Trees were sampled at different periods and separated into different parts for total B contents and 10B/11B isotope ratios analyses. Soil B applied via fertigation was more efficient than foliar application for the organs grown after the B fertilization. Recovery of labeled B by fruits was 21% for fertigation and 7% for foliar application. Residual effects of nutrient application in the grove were observed in the year after labeled fertilizer application,...

A produção de mudas cítricas em ambiente protegido, associada ao uso de substratos, requer estratégias adequadas de manejo no viveiro. Problemas de falta e/ou excesso de nutrientes freqüentemente causam prejuízos ao setor. Em citros ocorrem diferentes respostas à disponibilidade de boro (B) para a planta e são comuns danos causados pela toxidez desse nutriente. Este trabalho teve como objetivo avaliar os efeitos da adição de B no substrato sobre a absorção e o crescimento de duas variedades de porta-enxertos de citros em fase de sementeira. O experimento foi desenvolvido em esquema fatorial, com limão 'Cravo' e citrumelo 'Swingle' e quatro doses de B misturadas ao substrato na forma de H3BO3 (0, 3, 6, e 12 g m-3 de B), em três repetições. Avaliaram-se os teores de B no substrato no início do experimento, aos 55 e 110 dias de sementeira, e o crescimento dos seedlings e a absorção de B pelas plantas nos mesmos intervalos. Os teores de B no substrato variaram entre 0,1 e 4,7 mg L-1. Aos 55 e 110 dias, houve decréscimo nos teores do nutriente nas plantas provavelmente devido às perdas do nutriente do substrato por lixiviação. Observaram-se sintomas visuais de fitotoxicidade de B nos porta-enxertos cultivados no substrato tratado com 3 g m-3 de B, quando o teor na planta foi >100 mg kg-1. O excesso de B causou redução no crescimento dos porta-enxertos. O maior crescimento e acúmulo de B na massa seca de plantas de citrumelo 'Swingle', comparado ao 'Cravo, indicam maior tolerância do primeiro à fitotoxicidade de B. The production of citrus nursery trees under protected environment using growth media requires healthy management practices. Problems associated with deficiency and/or nutrient excess have frequently caused economic losses in the nursery. Citrus grow differently depending on boron (B) availability in the growth media, even tough B toxicity commonly cause damage to plants. This research aimed to evaluate the effects of B supply in the growth media on nutrient uptake and seedling growth of two rootstock citrus varieties. A factorial design was set up with two factors: two rootstocks (Rangpur lime and Swingle citrumelo) and four B levels added to the substrate (0, 3, 6, e 12 g m-3 B) as boric acid, with 3 replications. Boron availability in the substrate was evaluated at beginning the experiment, 55 and 110 days after rootstock seeding, as well as seedling growth and B uptake by seedlings in the same time intervals. Boron concentration in the treated substrate varied from 0.1 to 4.7 mg L-1, whereas B concentration in the whole plant tissue decreased in both evaluation periods probably because of nutrient leaching losses from substrate. Visual symptoms of B toxicity were observed for seedlings grown in substrate with B added at 3 g m-3, which were associated with B concentration in the plant tissue >100 mg kg-1. Excess B caused reduced growth of seedlings. The greater growth and B content in the dry mass of 'Swingle' compared to 'Cravo' rootstock plants denoted greater tolerance of the former to B toxicity.