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Calcium (Ca) plays a vital role as a macronutrient in the growth and development of plants. In order of decreasing solubility, Ca can be found in vegetal tissues as soluble Ca (Fraction I), bound Ca (mainly pectates, Fraction II), inorganic insoluble Ca (mainly phosphates and carbonates, Fraction III) and organic insoluble Ca or oxalate (Fraction IV). To explore the impact of Ca fertilizer application on plant growth and its allocation among different fractions, young citrus trees were fed over a complete vegetative cycle with a 44Ca labeled fertilizer (T1-Ca), while control plants (T2) received no Ca fertilizer. The results showed that plants receiving Ca exhibited significantly greater biomass. 44Ca derived from the fertilizer was localized mainly in sink organs (new flush leaves–twigs and fibrous roots). The primary fraction responsible for total Ca partitioning was Fraction II, followed by Fraction III or IV. Citrus plants, commonly found in calcareous soils, demonstrated improved growth with calcium treatments, indicating a positive link between calcium supplementation and enhanced development. The calcium supplied through the fertilizer (44Ca) was predominantly concentrated in sink organs (mainly in Ca-pectate fraction), including new flush leaves and twigs above ground, as well as fibrous roots below ground. [ABSTRACT FROM AUTHOR]

Abstract Background Improvement of Citrus, the most economically important fruit crop in the world, is extremely slow and inherently costly because of the long-term nature of tree breeding and an unusual combination of reproductive characteristics. Aside from disease resistance, major commercial traits in Citrus are improved fruit quality, higher yield and tolerance to environmental stresses, especially salinity. Results A normalized full length and 9 standard cDNA libraries were generated, representing particular treatments and tissues from selected varieties (Citrus clementina and C. sinensis) and rootstocks (C. reshni, and C. sinenis × Poncirus trifoliata) differing in fruit quality, resistance to abscission, and tolerance to salinity. The goal of this work was to provide a large expressed sequence tag (EST) collection enriched with transcripts related to these well appreciated agronomical traits. Towards this end, more than 54000 ESTs derived from these libraries were analyzed and annotated. Assembly of 52626 useful sequences generated 15664 putative transcription units distributed in 7120 contigs, and 8544 singletons. BLAST annotation produced significant hits for more than 80% of the hypothetical transcription units and suggested that 647 of these might be Citrus specific unigenes. The unigene set, composed of ~13000 putative different transcripts, including more than 5000 novel Citrus genes, was assigned with putative functions based on similarity, GO annotations and protein domains Conclusion Comparative genomics with Arabidopsis revealed the presence of putative conserved orthologs and single copy genes in Citrus and also the occurrence of both gene duplication events and increased number of genes for specific pathways. In addition, phylogenetic analysis performed on the ammonium transporter family and glycosyl transferase family 20 suggested the existence of Citrus paralogs. Analysis of the Citrus gene space showed that the most important metabolic pathways known to affect fruit quality were represented in the unigene set. Overall, the similarity analyses indicated that the sequences of the genes belonging to these varieties and rootstocks were essentially identical, suggesting that the differential behaviour of these species cannot be attributed to major sequence divergences. This Citrus EST assembly contributes both crucial information to discover genes of agronomical interest and tools for genetic and genomic analyses, such as the development of new markers and microarrays.

The main objective of this study was to compare the performance of two liquid organic fertilizers, an animal and a plant-based fertilizer, with mineral fertilization on citrus trees. The source of the fertilizer (mineral or organic) had significant effect in the nutritional status of the organic and conventionally managed mandarins. Nutrient uptake, vegetative growth, carbohydrate synthesis and soil characteristics were analyzed. Results showed that plants fertilized with animal based liquid fertilizers exhibited higher total biomass with a more profuse development of new developing organs (leaves and fibrous roots). Liquid organic fertilization resulted in an increased uptake of macro and micronutrients compared to mineral fertilized trees. Moreover, organic fertilization positively affected the carbohydrate content (fructose, glucose and sucrose) mainly in summer flush leaves. Liquid organic fertilization also resulted in an increase of soil organic matter content. Animal-based fertilizer, due to intrinsic composition, increased total tree biomass and carbohydrate leaves content, and led to lower soil nitrate concentration and higher P and Mg exchangeable in soil extract compared to vegetal-based fertilizer. Therefore, liquid organic fertilizers could be used as an alternative to traditional mineral fertilization in drip irrigated citrus trees. [ABSTRACT FROM AUTHOR]

Large quantities of crop residue and animal manure from agricultural and livestock activities are annually produced worldwide. With proper management, these residues are potentially valuable sources of plant nutrients, mainly N. Recycling such subproducts in sustainably-based agricultural systems can minimise the use of mineral fertilisers, and hence reduce the potential risk of surface and groundwater pollution. Therefore, the purpose of this study was to obtain (small scale) two liquid labelled-organic fertilisers, an animal- and a vegetal-based organic (AO and VO, respectively) fertiliser, to be used as organic N sources in subsequent fertigation studies. Forage maize (Zea mays L.) grown under 15N-labelled fertiliser supply was used as raw material for VO fertiliser production, and also as 15N-labelled sheep feed to obtain 15N-labelled manure. The labelled faeces fraction was used as raw material for the AO fertiliser. The VO fertiliser was obtained after an acidic and an enzyme-driven hydrolysis. The AO fertiliser was obtained after acidic hydrolysis. The VO liquid fertiliser presented an N concentration of 330 mg·L-1, 85% of total N was organic, while ammonium and nitrate N accounted for 55% and 45% of the mineral nitrogen fraction, respectively. This fertiliser also exhibited high K, Ca and S concentrations and notable values for the remaining macro- and micronutrients. The AO liquid fertiliser had a similar total N concentration (496 mg·L-1, 82% of total N in an organic form) to that of VO, but its mineral N fraction significantly differed, which came in a predominantly (95%) ammonia form. It also had a high content of N, P, K and other macronutrients, and sufficient Fe, Zn, Mn, Cu and B levels, which suggests its suitability as a potential fertiliser. The percentage of 15N enrichment in both VO and AO liquid fertilisers exceeded 2% 15N atom excess, which enabled their use in subsequent assays run to assess nitrogen uptake efficiency. [ABSTRACT FROM AUTHOR]

Citrus fruits play an important part in the Spanish agricultural sector. These orchards could play an important role as a terrestrial C sink and contribute to C fixation. We proposed quantifying the net primary production in different-aged citrus trees through the measurement of annual increases in the C content of the biomass of trees of different ages completely excavated. The measured net primary production in mature citrus was 27 kg C tree-1 (1143 g C m-2 yr-1; 11.4 Mg C ha-1 yr-1) with a similar behavior to that of different forests. Moreover, in this experiment, allometric equations for the estimation of annual net primary production in citrus orchards based on the relationship between net primary production and biometric variables were established. These equations could be a useful tool to calculate the potential C fixation in citrus areas. Canopy volume was the best variable for predicting above- and belowground biomass. Leaf area index and trunk diameter could also be selected as reliable and easily measurable variables to estimate net primary production in the whole tree. [ABSTRACT FROM AUTHOR]

Abstract: The effect of bicarbonate ion (HCO3 −) on the mobilization of iron (Fe) reserves from cotyledons to roots during early growth of citrus seedlings and its influence on the components of the iron acquisition system were studied. Monoembryonic seeds of Citrus limon (L.) were germinated “in vitro” on two iron-deprived media, supplemented or not with 10mM HCO3 − (−Fe+Bic and −Fe, respectively). After 21d of culture, Fe concentration in seedling organs was measured, as well as gene expression and enzymatic activities. Finally, the effect of Fe resupply on the above responses was tested in the presence and absence of HCO3 − (+Fe+Bic or +Fe, respectively). −Fe+Bic seedlings exhibited lower Fe concentration in shoots and roots than −Fe ones but higher in cotyledons, associated to a significative inhibition of NRAMP3 expression. HCO3 − upregulated Strategy I related genes (FRO1, FRO2, HA1 and IRT1) and FC-R and H+-ATPase activities in roots of Fe-starved seedlings. PEPC1 expression and PEPCase activity were also increased. When −Fe+Bic pre-treated seedlings were transferred to Fe-containing media for 15d, Fe content in shoots and roots increased, although to a lower extent in the +Fe+Bic medium. Consequently, the above-described root responses became markedly repressed, however, this effect was less pronounced in +Fe+Bic seedlings. In conclusion, it appears that HCO3 − prevents Fe translocation from cotyledons to shoot and root, therefore reducing their Fe levels. This triggers Fe-stress responses in the root, enhancing the expression of genes related with Fe uptake and the corresponding enzymatic activities. [Copyright &y& Elsevier]

Loquat [Eriobotrya japonica(Thunb.) Lindl.] is a minor fruit-tree species which is grown commercially in the Mediterranean region. Given the current increase in loquat cultivation, there is a need to define basic crop-management procedures in order to obtain high yields and optimal fruit quality. The aim of this work was to develop a routine for loquat farmers to follow, in order to know the nutritional status of trees in order to establish a rational fertilization regime, and to correct nutrient deficiencies as soon as possible. This paper reports three experiments aimed at establishing: 1) leaf type, 2) time of leaf sampling and 3) the standard leaf nutrient levels as a function of maximum yield. Results indicate that the summer flush leaves could be the most representative of nutritional status. Thus, the most appropriate time to sample leaves for analysis is in summer (beginning of August to the end of September), taking mature 3–4 month old leaves. [ABSTRACT FROM PUBLISHER]

In citrus production, the combination of adequate irrigation and nitrogen (N) management is considered a key factor in improving N uptake efficiency, so as to minimize nitrate ( ) leaching below the root zone and maintain optimal crop yield. The response of eight-year-old citrus trees grown in lysimeters to two combinations of fertilizer and water management techniques – split N application in drip irrigation (DI-F) vs. a single broadcast application with flood irrigation (FI-B) – was compared by means of the use of a 15N-labeled fertilizer (ammonium nitrate). Soil sampling 54, 116, 175 and 238 days after the beginning of the experiment and destructive harvest of trees at the end of the growing cycle allowed the determination of the fate of the labeled fertilizer supplied. The 15N recovered in the different compartments of the plant-soil system and fertilizer nitrogen uptake efficiency (FNUE, 15N taken up by the plant per 15N supplied) was also quantified. Irrigation-fertilizer management practices had no effect on either total tree biomass or in fruit yield; however, a significantly greater development of coarse roots in the DI-F treatment was recorded. Fertilizer-N uptake was 25% higher in trees in the DI-F than in the FI-B treatment; fine roots and fruits of DI-F trees accumulated 2.1- and 1.4-fold higher fertilizer-N, respectively, compared to the corresponding organs of FI-B trees. At the end of the experiment, -15N concentrations in the 0–20 cm and 60–90 cm layers of DI-F trees were higher than those in FI-B trees, whereas no differences were recorded for -15N concentrations of both treatments. Potential nitrate leaching (PNL) 54 days after the beginning of the experiment represented 8% of fertilizer-15N delivered in FI-B, whereas in DI-F it was almost negligible. In DI-F trees, PNL become noticeable on day 175, highlighting the effect of this irrigation method in reducing and slowing down the movement through the soil profile. At the end of the experiment PNL represented 14% and 5% of fertilizer supplied for FI-B and DI-F, respectively. FNUE was significantly higher (12%) with the DI-F treatment when compared to FI-B treated trees. Comparison of the two water-fertilizer management practices studied in the present work allowed us to conclude that the DI-F treatment increased FNUE while diminishing nitrate leaching, when compared to the FI-B treatment. [ABSTRACT FROM AUTHOR]

The fate of N-labeled potassium nitrate (8.5% N excess) was determined in 3-year-old Valencia orange trees grown in 1-m containers filled with different textured soils (sandy and loamy). The trees were fertilized either in spring (24 March) or summer (24 July). Spring fertilized trees gave higher fruit yields in sandy than in loamy soils, which exceeded summer fertilized trees in both cases. Summer fertilized trees had greater leaf biomass than spring fertilized trees. Fibrous root weight was 1.9-fold higher in sandy than in loamy soil. At the end of the cycle, tree N recovery from spring application was 45.7% for sandy and 37.7% for loamy soil; from summer fertilization, N recovery was 58.9% and 51.5% for sandy and loamy soils, respectively. The N recovered in the inorganic soil fraction (0-90 cm) was higher for loamy (1.3%) than for sandy soil (0.4%). Fertilizer N immobilized in the organic matter was lower in sandy (2.5%) than in loamy soil (6.0%). Potential nitrate leaching from fertilizer (NO-N in the 90-110-cm soil layer plus NO-N in drainage water) was 34.8% higher in sandy than in loamy soil. The low N levels in sandy soil resulted from both higher NO-N leaching losses and higher N uptake of plants grown in the former. The great root mass and higher soil temperatures could account for raised plant N uptake in sandy soil and in summer, respectively. [ABSTRACT FROM AUTHOR]

The effects of two rootstocks on tree growth, fruit set and development, yield, CO2 assimilation, 13C-photoassimilates transport, and carbohydrate distribution were investigated. Cleopatra mandarin (CM) (Citrus reshni hort. ex Tanaka) and Forner-Alcaide 13 (FA-13) [Citrus reshni hort. ex Tanaka × Poncirus trifoliata (L.) Raf.] rootstocks were used for this study. The first experiment was performed with 2-yr-old plants grown under greenhouse conditions. The results showed that leaves from navel orange [Citrus sinensis (L.) Osbeck] grafted onto FA-13 rootstock (navel onto FA-13) had higher net CO2 assimilation (Aco2) rates than those from navel orange grafted onto CM rootstock (navel onto CM). Also, FA-13 rootstock induced an increase in leaf transpiration rate with respect to CM rootstock resulting in similar water use efficiency (WUE) values. The higher of photosynthetic activity in navel onto FA-13 plants resulted in higher photoassimilate transport (13C- labeled) from leaves to roots compared to navel onto CM plants, causing higher sucrose and starch accumulation in FA-13 roots. The second experiment was performed under field conditions using 7-yr-old trees. Navel orange grafted onto FA-13, which had the highest Aco2 values, displayed higher photoassimilate transport (13C-labeled) from leaves to developing fruits than those on CM. navel onto FA-13 leaf sucrose content decreased compared to navel onto CM and leaf concentrations of this sugar were inversely correlated with Aco2 values. On the other hand, sucrose and starch concentrations in root bark appeared to be related with fruit sink strength. In conclusion, FA-13 rootstock enhances photosynthetic rate in navel orange leaves compared to CM and differences in this parameter affected growth and increased yield. [ABSTRACT FROM AUTHOR]

This study assesses the nutritional behavior of the new citrus rootstocks Forner-Alcaide no.5 (FA-5) and Forner-Alcaide no.13 (FA-13) under saline conditions compared to that of their parents, Cleopatra mandarin (CM) and Poncirus trifoliata (PT). Eighteen month-old plants grafted with Valencia orange scions were used in the experiment. The plants were grown in a greenhouse and irrigated over an eight-week period with nutrient solutions to which different amounts of sodium chloride (NaCl) had been added, namely 0, 20, 40 and 60 mM. Relative growth and the uptake of major mineral elements [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)] were then determined. It was noted that the reduction in relative growth caused by salt treatment was greater in plants grafted on PT than those on FA-13, FA-5 and CM. Increasing the salt level in the growth medium reduced the absorption of the above mineral elements in all scion-rootstock combinations. However, this decrease was generally more marked in plants grafted on PT than in those on CM and FA-5. Plants on FA-13 showed an intermediate behavior. Positive correlations were found between growth and mineral element uptake by salt treated plants. Nutrient uptake was also plotted against chloride (Cl) and sodium (Na) concentrations in leaves and roots at increasing salt levels. This showed that N absorption was closely correlated (inversely) with Cl content in leaves, whereas K, Ca and Mg uptakes were correlated (inversely) with Na concentration in roots. This suggests that the accumulation of saline ions impacts growth and nutrient uptake by citrus plants. [ABSTRACT FROM AUTHOR]

Citrus clementine cv. ‘Nules’ is a seedless mandarin with excellent quality for consumption as fresh fruit; however, its poor fruit set leads to an excess of small-sized fruits. Currently, the only available practice to increase fruit set is the use of the growth regulator gibberellic acid (GA3) as a foliar spray. A trial was conducted to compare an environmentally friendly new bioregulator (MX), containing methyl-xanthine, to GA3 in terms of increasing fruit set and yield of clementine cv. Nules mandarin trees. Fruit yield was significantly increased in treated trees from 17.4 to 25.7% for GA3 and MX treatments, respectively, compared to untreated control trees, without any significant differences between the growth regulators. At the tested doses, MX performed similarly to GA3 in terms of fruit quality and nutrient content, while colour index was not affected significantly by this new bioregulator. A single foliar application of MX was enough to increase fruit set. Spray application of MX on clementine mandarins at the end of the flowering period, when all petals have fallen showed a similar response to GA3 treatments. [ABSTRACT FROM PUBLISHER]

Presents an experiment evaluating the fate and recovery of nitrogenous fertilizers (N) applied with different seasonal frequency and irrigation systems in citrus trees. Aim of the efforts to advance best management practices in plant-soil-water systems for citrus crops; Experimental conditions; Analytical determinations; Statistical analysis; Dry weight of the different organs and of whole citrus trees; N derived from fertilizer; Allocation of fertilizer-N within the trees.

A field experiment was carried out in a drip-irrigated orchard of Clementine (Citrus clementina Ort. ex. Tan) grafted on Troyer citrange (C. sinensis × Poncirus trifoliata) rootstock located in the Valencian Citrus area (Spain). The trees received a single iron (Fe) EDDHA (ethylene diamine diorthohydroxyphenyl acetate) rate (3 g Fe tree−1) supplied in different application frequencies from April to September (8-, 4-, 2-, or 1-week intervals). Leaf chlorophyll (Chl) concentrations were estimated every month by using an SPAD-502 meter. The foliar contents of Fe were also evaluated with time. Mineral composition of leaves, total Chl concentration, yield, and fruit quality were also evaluated at the end of the assay. SPAD readings, Chl, N, K, Mg, Fe, and Mn concentration in leaves increased as a result of Fe application. The concentration of Zn, however, significantly decreased in comparison to the control trees. Iron treatment increased yield and some of the fruit quality parameters, like total juice, sugar, and acid contents. Iron application frequency had not a consistent effect on the concentrations of macro and micronutrients in leaves, yield, and fruit quality. The highest values of SPAD readings and the leaf Chl content were obtained when Fe was applied at 4-week intervals along the year. These results suggest that soil Fe-EDDHA application with a moderate frequency could be recommended to the Citrus farmers in the area for a more rational Fe application along the growth cycle in Citrus orchards. [ABSTRACT FROM AUTHOR]

During ripening, citrus fruit-peel undergoes ‘colour break’, a process characterized by the conversion of chloroplast to chromoplast. The process involves the progressive loss of chlorophylls and the gain of carotenoids, changing peel colour from green to orange. In the present work, the in vivo and in vitro effects of supplemented nutrients (sucrose and nitrogen) and phytohormones (gibberellins [GA] and ethylene) on colour change in fruit epicarp of Satsuma mandarin (Citrus unshiu (Mak.) Marc., cv. Okitsu), were studied. The rate of colour break was correlated positively with sucrose content and negatively with nitrogen content. The removal of leaves blocked natural sucrose build-up and nitrogen reduction in the peel. Defoliation also inhibited chlorophyll disappearance and carotenoid accumulation, thereby preventing colour break. In vivo sucrose supplementation promoted sucrose accumulation and advanced colour break. In both in vivo and in vitro experiments, colour change promoted by sucrose was unaffected by ethylene but delayed by GA3. In non-supplemented plants, ethylene accelerated colour break while GA3 had no detectable effects. Ethylene inhibitors effectively counteracted the sucrose effects on colour change. Collectively, these results suggest that the chloroplast to chromoplast conversion in citrus fruit epicarps is stimulated by sucrose accumulation. The sugar regulation appears to operate via ethylene, whereas GA may act as a repressor of the sucrose-ethylene stimulation. [ABSTRACT FROM AUTHOR]

This work quantifies Fe uptake in young citrus trees, its partitioning among plant compartments, and the contribution of the Fe absorbed from fertilizer to the development of new tissues. A soil pot experiment was conducted using 4-year-old clementine trees (Citrus clementina Hort ex Tan), and a dose of 240 mg Fe was applied by labeled fertilizer (92% atom 57Fe excess). Plants were uprooted at five different phenologic states: end of flowering (May 15), end of fruit setting and fruit drop (July 1), two fruit growing moments (August 1 and October 15), and at complete fruit maturity (December 10). The Fe accumulated in the root system exceeded 90% of the total Fe content in the plant. All organs progressively enriched with 57Fe (8.5–15.5% and 7.4–9.9% for young and old organs, respectively). Reproductive ones reached the highest increase (111% between May and October). 57Fe enrichment from woody organs reflects an increasing gradient to sink organs. The root system accumulated 80% of the Fe absorbed from the fertilizer, but the young organs accumulated relatively more Fe uptake during flowering and fruit setting (15.6% and 13.8%, respectively) than old organs (around 9.8%). Although iron derived from fertilizer (Fedff) preferably supplied young organs (16.7–31.0%) against old ones (2.5–14.9%), it only represented between 13.8% and 21.4% of its content. The use efficiency of the applied Fe (FeUE) barely exceeded 15%. The lowest FeUE were found in young and old organs of the aerial part (1.1–1.8% and 0.7–1.2%, respectively). Since the pattern of the seasonal absorption of Fe is similar to the monthly distribution curve of the supplied Fe, it is recommended that the application of Fe chelates in calcareous soils should be performed in a similar way to that proposed in this curve. [ABSTRACT FROM AUTHOR]

Abstract: The present work studies the effect of salinity on PIP aquaporins gene expression in citrus roots and its relationship with root hydraulic conductance (Kr), transpiration rate (E) and chloride transport to leaves. To this end, ten-month-old seedlings of Cleopatra mandarin (CM), Carrizo citrange (CC) and Poncirus trifoliata (PT) were tested. No effect was detected of salt treatments on PIP1 and PIP2 aquaporin mRNA transcript abundances from citrus roots, although PIP1 expression in CM roots was lower than in CC and PT. The lowest Kr and E values were detected in CM, whereas PT had the highest. CC seedlings presented intermediate values for these parameters. Addition of HgCl2 to either control or salt solution (200mM NaCl) led to a decrease in Kr and E, thus implying aquaporin involvement. By contrast, salinity strongly reduced Kr and E in all plants, with this effect being unrelated to aquaporin activity. In salinized seedlings, E values appear to be related with Cl− concentration in leaves. Thus, CM seedlings treated with 80mM NaCl presented a lower Cl− uptake by leaves than PT, whereas this trend was intermediate in CC. Moreover, Hg2+ treatments significantly reduced leaf Cl− concentration in salt stressed plants, probably through the reduction of E. We can conclude that differences among genotypes in PIP1 expression affect Cl− exclusion from leaves, probably due to effects on water movement. Nevertheless, long-term salt treatments did not affect PIP expression in citrus plants, but reduced root hydraulic conductance and transpiration. [Copyright &y& Elsevier]

Highlights: [•] The carbon assimilation of representative citrus plantations in Eastern Spain was estimated. [•] Our plantation was responsible for a net carbon fixation rate of higher than 10MgCha−1 yr−1. [•] Leaf assimilatory processes mainly determined the annual pattern of C sequestration. [•] Under typical culture conditions, soil respiration rates accounted for low C losses. [•] The results of this study are relevant to climate change mitigation. [Copyright &y& Elsevier]

Abstract: The expression of iron (Fe) acquisition-related genes in roots was studied in roots of two different citrus seedlings, namely, Carrizo citrange (CC, Fe chlorosis-sensitive) and Cleopatra mandarin (CM, Fe chlorosis-tolerant), growing either with (control) or without (−Fe) Fe in the nutrient solution. Fe-deficiency increased expression of the gene HA1 coding for proton-ATPase (H+-ATPase) enzyme in both genotypes, although no differences were observed between treatments among rootstocks. Furthermore, while the gene expression levels of FRO2 – which encodes the Ferric Chelate Reductase (FC-R) enzyme–, increased under −Fe condition in both genotypes, CM always recorded the highest activity. CC showed the greatest induction of genes IRT1 and IRT2 encoding two iron transporters, however only IRT1 was significantly induced by Fe starvation. Analysis of the enzymatic activities (H+-ATPase and FC-R) regulated by the aforementioned genes confirmed these results. Thus, in agreement with the acidification pattern registered, H+-ATPase activities were higher in −Fe plants than in controls, although no significant differences were detected between each treatment among rootstocks. Fe starvation also induced FC-R activity; however, this was greater in CM than in CC roots. Interestingly, root 57Fe uptake rates from 57Fe-EDDHA solutions were increased by Fe-deficiency, especially in the CM genotype, and CM accumulated a much larger Fe pool in the root apoplast than CC. Taken together, the main trait determining Fe-chlorosis tolerance among these genotypes is the ability to boost Fe3+ reduction in response to Fe-deficiency through enhanced FRO2 gene expression. Moreover, Fe chlorosis resistance in these plants could be related to the amount of Fe stored in the root apoplast. [Copyright &y& Elsevier]

Abstract: Soil flooding has been widely reported to affect large areas of the world. In this work, we investigated the effect of waterlogging on citrus carbon and nitrogen pools and partitioning. Influence on their uptake and translocation was also studied through 15N and 13C labeling to provide insight into the physiological mechanisms underlying the responses. The data indicated that flooding severely reduced photosynthetic activity and affected growth and biomass partitioning. Total nitrogen content and concentration in the plant also progressively decreased throughout the course of the experiment. After 36 days of treatment, nitrogen content of flooded plants had decreased more than 2.3-fold compared to control seedlings, and reductions in nitrogen concentration ranged from 21 to 55% (in roots and leaves, respectively). Specific absorption rate and transport were also affected, leading to important changes in the distribution of this element inside the plant. Additionally, experiments involving labeled nitrogen revealed that 15N uptake rate and accumulation were drastically decreased at the end of the experiment (93% and 54%, respectively). 13CO2 assimilation into the plant was strongly reduced by flooding, with δ13C reductions ranging from 22 to 37% in leaves and roots, respectively. After 36 days, the relative distribution of absorbed 13C was also altered. Thus, 13C recovery in flooded leaves increased compared to controls, whereas roots exhibited the opposite pattern. Interestingly, when carbohydrate partitioning was examined, the data revealed that sucrose concentration was augmented significantly in roots (37–56%), whereas starch was reduced. In leaves, a marked increase in sucrose was detected from the first sampling onwards (36–66%), and the same patter was observed for starch. Taken together, these results indicate that flooding altered carbon and nitrogen pools and partitioning in citrus. On one hand, reduced nitrogen concentration appears to be a consequence of impaired uptake and transport. On the other hand, the observed changes in carbohydrate distribution suggest that translocation from leaves to roots was reduced, leading to significant starch accumulation in leaves and further decreases in roots. [Copyright &y& Elsevier]