Swingle Citrumelo Research Articles

Candidatus Liberibacter asiaticus causes damage to citrus fibrous roots before visual decline of Huanglongbing-infected citrus trees

Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (Las), was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions. In recent seasons, concurrent with freeze and drought episodes, symptomatic HLBinfected trees were much more affected by the extremes of temperature and moisture than trees without HLB. Symptoms exhibited by the stressed trees were excessive leaf loss and premature fruit drop even when HLB-infected trees were managed with good nutritional and irrigation practices recommended to support health of HLB-affected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-infected trees on Swingle citrumelo rootstock (Citrus paradisi × Poncirus trifoliata), blocks of 2,307 three-yr-old Hamlin orange trees and 2,693 four-yr-old Valencia orange trees were surveyed visually and with a real time polymerase chain reaction (PCR) assay to determine Las infection status. The incidence of Las-infected trees (pre-symptomatic: Las+, visually negative, and symptomatic: Las+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. Las+ trees had 30 and 37% lower fibrous root mass density for pre-symptomatic and symptomatic trees, respectively, compared to Las- trees. In a second survey, 10- to 25-yr-old Valencia trees on ‘Swingle’ citrumelo or ‘Carrizo’ citrange (C. sinensis (L.) × P. trifoliata) rootstock were sampled within 3-6 months after identification of visual HLB status as symptomatic (Las+, visually positive) or non-symptomatic (Las-, visually negative) in orchards located in the central ridge, south-central and southwest flatwoods. Pairs of HLB symptomatic and non-symptomatic trees were evaluated for PCR status, fibrous root mass density and Phytophthora nicotianae progagules in the rhizosphere soil. Las+ trees had 27-40% lower fibrous root mass density and in one location higher P. nicotianae per root but Phytophthora populations per cm 3 soil were high on both Las+ and Las- trees. Fibrous root loss from HLB damage interacted with P. nicotianae depending on orchard location and time of year.

High resolution melting analysis for early identification of citrus hybrids: A reliable tool to overcome the limitations of morphological markers and assist rootstock breeding

Citrus rootstocks are highly polyembryonic. Seeds usually produce many nucellar seedlings that are identical to the mother plant and sometimes one zygotic plant. Nucellar seedlings hamper the identification of hybrids if the morphological markers that distinguish sexual and nucellar plants are missing, and competition among embryos might impair zygotic embryo viability. Accurate morphological and/or molecular screening methods are needed to identify and discard the nucellar plants, which is labor-intensive and increases the costs of rootstock breeding programs. This study describes an affordable and reliable method for an early identification of hybrids in progenies derived from highly polyembryonic female parents based on DNA isolation from pooled leaf samples (2 to 4 plants per sample) coupled with a high resolution melting (HRM) analysis at two SNP loci. The method was validated on more than one thousand progeny derived from the crosses alemow (Citrus macrophylla Wester)×Swingle citrumelo [Citrus paradisi Macf.×Poncirus trifoliata (L.) Raf.] (A×S), and Cleopatra mandarin (Citrus reshni Hort. ex Tan)×Swingle citrumelo (C×S). Bulking reduced the number of DNA isolations for population genotyping, while HRM unequivocally identified, in a single step, the bulks containing the unifoliate hybrids. A consistent difference was found between A×S and C×S populations in the frequency of uni- and multifoliate hybrids (69% unifoliate in A×S vs. 10% in C×S), suggesting that the combination of parents, rather than just the male parent carrying the trifoliate character, influenced the early multifoliate leaf expression. Moreover, several multifoliate nucellar seedlings were found among the C×S progenies, making the multifoliate leaf character inappropriate for the early identification of all hybrids.

Growth and inorganic composition of ‘Nova’ mandarin plants grafted on two commercial rootstocks in response to salinity and silicon

The effects of salinity and its combination with silicon (Si) were studied in ‘Nova’ mandarin plants grafted on Citrus aurantium L. or Swingle Citrumelo to determine: (1) which combination is more tolerant to salt stress and (2) the impact of Si in limiting the harmful effects of salinity. Six groups of plants were grown in a greenhouse for 120 days and irrigated with: (1) 50 % Hoagland’s solution (Control), (2) 50 % Hoagland’s solution plus 80 mM NaCl (NaCl), and (3) 50 % Hoagland’s solution plus 80 mM NaCl plus 0.5 mM Si (NaCl + Si). Grafted plants exhibited accumulation of Na and Cl in their tissues following exposure to salinity. The ability of S. Citrumelo to retain the toxic ions in the roots in corroboration with the observation that the dry weights (DWs) of S. Citrumelo tissues were not influenced by NaCl treatment indicates that this rootstock is more tolerant to salinity. Silicon supplementation into the saline medium promoted the accumulation of toxic ions, whereas, when compared to NaCl treatment, it increased the DW of S. Citrumelo roots. Mineral concentrations were significantly affected by rootstock, treatment, and their interaction with S. Citrumelo, which presented better nutrient status than Sour Orange; and Si which differed depending on citrus tissue. It appears that S. Citrumelo rootstock is the most tolerant for ‘Nova’ mandarin plants under salinity, whereas salt tolerance in grafted citrus plants is not improved by Si application, indicating that the beneficial role of Si depends on the cultivar or rootstock–scion combinations.

Nitrogen‐fertilizer forms affect the nitrogen‐use efficiency in fertigated citrus groves

AbstractThe 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 320 kg 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 240 kg 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.

Híbridos de trifoliata como porta-enxertos para laranjeira Pêra

O limão Cravo (Citrus limonia) tem sido utilizado como o principal porta-enxerto para a laranjeira Pêra (C. sinensis). Entretanto, sua susceptibilidade ao declínio e à morte súbita induziu o uso de porta-enxertos tolerantes a doenças, como as tangerinas Cleópatra (C. reshni) e Sunki (C. sunki) e o citrumelo Swingle (C. paradisi x Poncirus trifoliata), mais susceptíveis à seca que o limão Cravo. As tangerinas são, também, menos resistentes à gomose de Phytophthora, e o citrumeleiro Swingle revelou ser incompatível com a laranjeira Pêra. Na busca por outros porta-enxertos, este estudo objetivou avaliar a precocidade e a produção de frutos, a susceptibilidade à tristeza e ao declínio e a ocorrência de incompatibilidade de laranjeiras Pêra enxertadas em 12 híbridos de trifoliata, no limão Cravo EEL e na laranja Goutou, sem irrigação. Tristeza e declínio são endêmicos na região. Os citrandarins Sunki x English (1628), Changsha x English Small (1710) e duas seleções de Cleópatra x Rubidoux proporcionaram as maiores produções acumuladas, nas três primeiras safras e no total de seis safras avaliadas. Os citrandarins Cleópatra x Rubidoux (1660) e Sunki x Benecke (1697) induziram precocidade de produção, enquanto o citromonia Cravo x Swingle e o citrange C-13 foram os mais tardios. Nenhum dos porta-enxertos causou alternância de produção. As laranjeiras Pêra enxertadas no citrumelo Swingle, citrandarin Cleópatra x Swingle (1654) e em duas seleções de limão Cravo x citrange Carrizo apresentaram sintomas de incompatibilidade, formando linha de goma na região de enxertia. Nenhuma das plantas apresentou sintomas de tristeza ou declínio.

Substratos e recipientes para a produção de porta-enxertos de citros irrigados por subcapilaridade

O conhecimento do porta-enxerto, do substrato e do recipiente são fundamentais para a definição do período necessário para produzir mudas cítricas de qualidade. O objetivo deste trabalho foi avaliar a emergência de plântulas e o desenvolvimento vegetativo de seis porta-enxertos cítricos, cultivados em dois substratos e mantidos em diferentes recipientes, em sementeira, irrigados por subcapilaridade. Foram testados dois substratos comerciais e a semeadura foi realizada em tubetes com 50cm³, tubetes com 120cm³ e bandejas de isopor contendo 72 células com volume de 120cm³. Os seis porta-enxertos cítricos avaliados foram: Trifoliata [Poncirus trifoliata [L.] Raf.]; Flying Dragon [Poncirus trifoliata (L.) Raf. var. monstrosa]; limoeiro Volkameriano (Citrus volkameriana Pasq.), citrangeiro Fepagro C37 [P. trifoliata x Citrus sinensis [L.] Osbeck.], tangerineira Sunki (C. sunki hort. ex Tan.) e citrumeleiro Swingle (P. trifoliata x C. paradisi). Avaliou-se emergência, altura e diâmetro das plantas ao longo do cultivo em casa de vegetação. Em laboratório, avaliou-se massa fresca e seca da parte aérea e raiz, número de folhas, área foliar por folha e área foliar por planta. Nas avaliações de emergência de plântulas, a tangerineira Sunki, o limoeiro Volkameriano e o citrumeleiro Swingle apresentaram maiores taxas em relação ao Trifoliata, ao citrangeiro Fepagro C 37 e ao Flying Dragon. Ao empregar-se o sistema de irrigação por subcapilaridade, o substrato comercial 1 é recomendado para todos recipientes. O substrato comercial 2 somente deve ser empregado em bandejas alveoladas de 120 cm³.

Drought tolerance in citrus trees is enhanced by rootstock-dependent changes in root growth and carbohydrate availability

Valencia orange scions grafted on Rangpur lime or Swingle citrumelo were grown under water deficit to evaluate how those rootstocks modulate the non-structural carbohydrate (NSC) availability and the drought tolerance of citrus trees. Additionally, the importance of young mature leaves as possible sources of carbon in citrus trees was studied. Herein, young mature leaves are those ones fully expanded and developed during water deficit. After 30 days under water limiting conditions, plant growth, leaf water status, photosynthetic rate and carbohydrate availability in old mature and young mature leaves, branches and roots were evaluated. Water deficit reduced the leaf water potential and caused diffusive limitation of photosynthesis in both rootstocks. Drought-induced decrease in total NSC content occurred only in plants grafted on Swingle. While plant growth on Swingle citrumelo was severely reduced by water deficit, plants grafted on Rangpur did not exhibited impairment of dry matter accumulation. The lower sensitivity of plant growth on Rangpur lime was associated with the enhanced root growth, the maintenance of the total carbohydrate pool and to a large shift in the carbohydrate partitioning, with the roots accumulating carbohydrates under water deficit. Regarding the young mature leaves, they exhibited higher photosynthetic rates than the old mature leaves after 30 days of treatment, regardless of the water conditions. As possible sources of carbohydrates, young mature leaves have equal importance as compared to branches under well-watered conditions.

Effect of the duration of inoculum exposure on development of citrus canker symptoms on seedlings of Swingle citrumelo

Citrus canker (Xanthomonas citri subsp. citri, Xcc) is one of the most serious diseases citrus in Florida, and elsewhere in the world. The disease causes yield loss and some fresh fruit trade restrictions may apply. Cultural management techniques such as windbreaks may work by not only reducing wind speed, but also reducing the period of exposure of susceptible foliage or fruit to those wind speeds that support infection from incoming inoculum. To investigate the effect of exposure period to inoculum of Xcc, seedlings of canker-susceptible Swingle citrumelo were exposed to sprayed inoculum for increasing periods at different wind speeds. The incidence and severity of citrus canker was assessed. In three experiments the incidence and severity of citrus canker most often increased with longer periods of exposure to inoculum, especially so at wind speeds of ≥16 m/s compared to wind speeds of ≤5 m/s (wind speed also increased disease incidence and severity). Regression analysis demonstrated relationships between period of exposure to inoculum and the percent infected leaves per plant, the number of lesions per plant, the number of lesions per infected leaf, and for the percent of infected leaves with lesions on the petioles at wind speeds of ≥16 m/s (R2 = 0.16–0.72). Due to the effect of inoculum exposure period and wind speed, attempts should be made to minimize exposure of canker-susceptible citrus when wind speed is highest and inoculum is available. Windbreaks should help minimize periods of exposure to splashed inoculum in high winds.

A Transcriptomic Analysis of Sensitive and Tolerant Citrus Rootstocks under Natural Iron Deficiency Conditions

Iron chlorosis is one of the most serious abiotic stresses affecting citrus (Citrus sp.) culture in the Mediterranean Basin. A trial was performed with potted tolerant and sensitive rootstocks that were grown in volcanic and calcareous soils. Microarray analysis allowed for the identification of differentially expressed genes putatively involved in iron (Fe) deficiency. Most of the differentially expressed genes isolated from the root tips were of unknown function; the remaining genes were related to the oxidative stress response (e.g., glutathione peroxidase), hormone metabolism and signaling (e.g., small auxin up RNA family protein genes), biological regulation, protein turnover, and the tricarboxylic acid cycle (e.g., aconitase). Additionally, the majority of the Fe stress-related genes expressed in the sensitive Swingle citrumelo (Citrus paradisi × Poncirus trifoliata) and tolerant Carrizo citrange (Citrus sinensis × P. trifoliata) rootstocks identified using real-time reverse transcription–polymerase chain reaction (RT-PCR) were related to regulation, the oxidative stress response, and hormone metabolism and signaling, thereby confirming the array data. Furthermore, validation of the differentially expressed genes in seven tolerant and sensitive rootstocks grown in a field trial under chlorotic conditions was performed. In general, the gene expression profiles reflect the different responses of rootstocks, possibly as a result of the various genetic mechanisms involved in the response to Fe deficiency. Moreover, the expression of aconitase was analyzed in the roots and juice to evaluate the implication of the different aconitase isoforms (Aco), which are derived from specific cellular compartments, in the different tissues. The involvement of the mitochondrial isoform (Aco2) was directly correlated with the acidity of the juice, whereas the cytosolic one (Aco3), which corresponds to the aconitase isolated during the microarray analysis, was found specifically in the roots.

Presymptomatic Fibrous Root Decline in Citrus Trees Caused by Huanglongbing and Potential Interaction with Phytophthora spp.

Huanglongbing (HLB), associated with 'Candidatus Liberibacter asiaticus', was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions. In recent seasons, concurrent with freeze and drought episodes, symptomatic HLB-infected trees were much more affected by the extremes of temperature and moisture than trees without HLB. Symptoms exhibited by the stressed trees were excessive leaf loss and premature fruit drop even when HLB-infected trees were managed with good nutritional and irrigation practices recommended to support health of HLB-affected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-infected trees on 'Swingle' citrumelo rootstock (Citrus paradisi × Poncirus trifoliata), blocks of 2,307 3-year-old 'Hamlin' orange trees and 2,693 4-year-old 'Valencia' orange trees were surveyed visually and with a real-time polymerase chain reaction (PCR) assay to determine 'Ca. L. asiaticus' infection status. The incidence of 'Ca. L. asiaticus'-infected trees (presymptomatic: 'Ca. L. asiaticus'+, visually negative; and symptomatic: 'Ca. L. asiaticus'+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. 'Ca. L. asiaticus'+ trees had 30 and 37% lower fibrous root mass density for presymptomatic and symptomatic trees, respectively, compared with 'Ca. L. asiaticus'- trees. In a second survey, 10- to 25-year-old Valencia trees on Swingle citrumelo or 'Carrizo' citrange (C. sinensis (L.) × P. trifoliata) rootstock were sampled within 3 to 6 months after identification of visual HLB status as symptomatic ('Ca. L. asiaticus'+, visually positive) or nonsymptomatic ('Ca. L. asiaticus'-, visually negative) in orchards located in the central ridge, south-central, and southwest flatwoods. Pairs of HLB symptomatic and nonsymptomatic trees were evaluated for PCR status, fibrous root mass density, and Phytophthora nicotianae propagules in the rhizosphere soil. 'Ca. L. asiaticus'+ trees had 27 to 40% lower fibrous root mass density and, in one location, higher P. nicotianae per root but Phytophthora populations per cubic centimeter of soil were high on both 'Ca. L. asiaticus'+ and 'Ca. L. asiaticus'- trees. Fibrous root loss from HLB damage interacted with P. nicotianae depending on orchard location and time of year.

Phosphorus availability and rootstock affect copper-induced damage to the root ultra-structure of Citrus

The control of several citrus diseases requires continuous applications of fungicides containing copper (Cu) which favor to the accumulation of this metal in the soil. Therefore, the evaluation of how nutrient availability and rootstock interact with Cu toxicity in the citrus trees is required to maintain sustainability of fruit production in Cu-contaminated soils. Valencia orange trees on Sunki mandarin (SM) or Swingle citrumelo (SC) rootstock were grown in nutrient solutions combining adequate Cu (1.0μmolL−1), excess Cu (50.0μmolL−1), deficient phosphorus (P) (0.01mmolL−1) and sufficient P (0.5mmolL−1). The excess Cu reduced root and shoot growth, chlorophyll and relative water content in the leaves of the trees compared to those under adequate Cu supply. Furthermore, excess Cu caused severe damage to the root ultra-structure, characterized by the degeneration of the middle lamella and the presence of a thin and sinuous cell wall, as well as, starch accumulation in the plastids, disruption of the mitochondrial membranes and cellular plasmolysis. The damage caused by excess Cu in the cell wall and middle lamella on the root cells of SC was less severe than SM. Sufficient P supply improved the structure of the cell wall and middle lamella of trees subjected to excess Cu in comparison to P-deficient ones. Thus, the occurrence of more preserved cell wall and middle lamella supports the idea that sufficient P availability in the rooting medium and the use of SC rootstock might contribute to increase the ability of young citrus trees to cope with Cu toxicity.

Root Protein Profiles of Two Citrus Rootstocks Grown under Iron Sufficiency/Deficiency Conditions

Two citrus rootstocks, one sensitive to iron deficiency (Swingle Citrumelo (SCO)) and the other tolerant (Carrizo Citrange, (CC)), were studied to characterize variation in their root protein profile induced by iron-deficient conditions. Plants of both rootstocks were grown in two different soils, one volcanic (v) and the other calcareous (c), containing 0% and 10% active Lime, respectively. To evaluate the effects of the calcareous soil on the protein accumulation of both rootstocks, the root protein profiles (SCc vs. SCv and CCc vs. CCv) were characterized by two-dimensional gel electrophoresis, thus obtaining, for the first time, a reference map of this previously uncharacterized proteome. A total of 219 spots, significantly changed in abundance, were analyzed by high-performance Liquid chromatography nano electrospray ionization tandem mass spectrometry. The identified proteins were classified according to their putative function and known biosynthetic pathways. Principal component analysis, comparing the four sets of data, indicated that each group clustered together with low variance and that CCv and CCc data sets were well differentiated, whereas SCv and SCc were similar.

The accumulation of endogenous proline induces changes in gene expression of several antioxidant enzymes in leaves of transgenic Swingle citrumelo

Plant exposure to abiotic stresses leads to an accumulation of reactive oxygen species with the concomitant increase in antioxidant defense mechanisms. Previous studies showed that exogenous application of proline mitigate the deleterious effects caused by oxidative stress due to its ability to increase the activity of antioxidant enzymes. However, there are no reports of the effects of high endogenous accumulation of proline in the transcriptional pattern of antioxidant enzymes genes under normal conditions of water supply or in response to water deficit. Here, we show that isoforms of four antioxidant enzymes genes (Ascorbate peroxidase-APX, Catalase-CAT, Superoxide dismutase-SOD and Glutathione reductase-GR) were differentially regulated in leaves of Swingle citrumelo transgenic plants with high endogenous proline accumulation submitted to water deficits and also under normal water supply condition. Proline per se caused a two-fold change in the transcription activity of APX1, APXcl, CAT2 and Cu/ZnSOD2, while during water deficit proline influenced mRNAs levels in APXs and Cu/ZnSODs isoforms, MnSODmit and GRcl. This study adds new information on the role of proline during drought conditions and, more important, without the potential confounding effects imposed by water deficiency. We showed that, in addition to its known effects on diverse plant physiological and biochemical processes, high endogenous proline can also acts as a regulatory/signalling molecule capable of altering the transcript levels of stress-related genes.

PRODUCTIVIDAD DEL LIMÓN ‘PERSA’ INJERTADO EN CUATRO PORTAINJERTOS EN UNA HUERTA COMERCIAL DE VERACRUZ, MÉXICO

There were 36,166.36 ha planted with ‘Tahiti’ lime (Citrus latifolia Tan.) in Veracruz, Mexico in 2010, which produced 437,460.80 tons of fruit valued at 88.07 million dollars. Because rootstocks influence vegetative growth, citrus fruit yield and quality, depending on the species of rootstock, soil type and climate where they are cultivated, we compared the agronomic performance of 11-year-old ‘Tahiti’ lime trees grafted onto Carrizo citrange, Swingle citrumelo and Volkamerian and Rough lemons, planted with 4 x 6 m spacing, in a thin, strongly acidic (pH 4.5) clay soil. Tree dimensions, yield, production efficiency and fruit quality of eight harvests, from June 2008 to July 2009, were recorded. The most important results were: Carrizo, Swingle and Volkamerian induced lower height and canopy volume than Rough lemon, but the canopy formation of ‘Tahiti’ lime was wider than high, with all of them, b) fruit yield per tree was higher with Rough, but its production efficiency equaled that obtained with Carrizo and Swingle, c) Carrizo, Rough and Swingle induced a higher volume of export- grade (United States) ‘Tahiti’ lime fruit, with no differences among them in terms of the commercial sizes of the fruit produced, and d) the agronomic performance of Carrizo and Swingle, in terms of vigor, tree size, production efficiency, fruit quality and tolerance to termites, offers greater advantages for growing ‘Tahiti’ lime in savanna soils.

Effect of Brassinolide ,Cytokinin and Auxins on In Vitro propagation of Citrus Rootstock (Swingle Citrumelo)

Astudy on in vitro micropropagation of a citrus Rootstock (Swingle Citrumelo)Was Conducted at the tissue culture lab.located in postgraduate building /Collage of Agric./Uni.of Baghdad from September 2010 till August 2011. single nodal segment or terminal shoots were explanted on MS medium supplemented with som plant growth regulators( Brassinolide ,Cytokinin and Auxins ). The aims of the study were increasing number of length shoots ; rooting and plantlet acclimatization. Results showed that 3% of NaOCl was the best for explant disinfestation only 10% of explants were contaminated although 4.5 or6.0 %of NaOCl gave 100% disiafestation ، the concentration were poisonous. Single nodal segments responded better than terminal shoots. in the establishment stage. MS medium supplemented with 2.0 mg / L BA + 0.4mg / L IAA was superior on shoot multiplication ( 11.10 shoots / explant) . While MS medium supplemented with 2.0 mg / L NAA + 0.048 mg / L BL. Was more effective on root number 9.44 roots / explant. Most plantlets 95% were acclimatized when They were cultured on soil consisted of 1 : 1 sand : peatmoss.

Sodium Distribution in Salt-stressed Citrus Rootstock Seedlings

Although citrus trees are considered relatively salt-sensitive, there are consistent differences in Na+ and Cl– tolerance among different citrus rootstocks. We grew uniform seedlings of rough lemon (RL) and the more Na+-tolerant Swingle citrumelo (SC) with and without 50 mm NaCl for 42 days. Salinity reduced leaf chlorophyll and plant transpiration rate (Ep) more in RL than SC. Confocal laser scanning analyses using the Na+-specific cell-permeant fluorescent probe CoroNa-Red revealed a higher capacity for Na+ sequestration in root tissue vacuoles of SC than in RL roots and that cell walls within the stele acted as Na+ traps. In leaves, however, RL had significantly higher Na+-dependent fluorescence than SC. Thus, the sequestration of Na+ in root tissue vacuoles and its immobilization by cell walls were key contributing mechanisms enabling SC leaves to maintain lower levels of Na+ than RL leaves. Examination of intracellular distribution of CoroNa-Green fluorescence in SC root protoplasts verified a vacuolar localization for Na+ in addition to the presence of a 2- to 6-μm unidentified endosomal compartment containing significantly higher Na+ concentrations.

SEASONAL NITROGEN BUDGETS OF MATURE CITRUS TREES ON A SANDY ENTISOL

Approximately 30% of Florida citrus is grown on well-drained Entisols with low nutrient-holding capacity, which are prone to high nitrogen (N) leaching losses. However, increasing application frequency of N-fertilizer via multiple fertigations does not increase crop yield, whereas in agronomic crops, such an approach typically enhances N uptake efficiency. We assessed seasonal tree N tissue concentration dynamics as affected by N rate for mature fourteen-year-old ‘Hamlin’ orange (Citrus sinensis L. Osbeck) trees on either Carrizo citrange (C. sinsensis L. Osbeck X Poncirus trifoliata L. Raf.) or Swingle citrumelo (C. paradisi Macf. X P. trifoliata L. Raf.) rootstocks. Nitrogen was applied as ammonium nitrate in six split fertigation applications with N target values of 179 and 269 kg ha−1yr−1. Leaf, twig, and branch bark tissue N concentrations decreased through the spring to minima in May and June. This time period corresponds to a period of high N demand associated with both vegetative and reproductive growth. Tissue N concentrations increased from late spring minimums to fall and winter maximum concentrations. Reduction in branch bark and wood tissue N concentrations may have been due to a redistribution of N to leaf, twig, and fruit tissues in response to low N supply. The majority of the spring N should be supplied prior to May.

Effects of arbuscular mycorrhizal fungi on resistance to Phytophthora parasitica of citrus seedlings and on growth of Thai honey tangerine scions on citrus rootstocks

Thai honey tangerine (Sainamphueng tangerine) is generally grown by grafting on rootstocks of another variety of tangerine or citrus species which may differ in their reaction to beneficial and pathogenic soil organisms. The objectives of this study were to evaluate responses to arbuscular mycorrhizal (AM) fungi and Phytophthora parasitica of different citrus genotypes and the effect of AM fungi on the growth of scions of Thai honey tangerine grafted on different citrus rootstocks including Cleopatra tangerine, lime, pomelo, Swingle citrumelo and Troyer citrange. Significant differences were found among citrus species in the percentage of root colonization by the AM fungi and in the severity of root rot disease when inoculated with P. parasitica alone. Thai honey tangerine was most susceptible and Cleopatra tangerine was most resistant to P. parasitica . Inoculation with AM fungi could reduce disease severity of all the citrus plants from P. parasitica . AM fungi enhanced the growth of seedling to the greatest extent in lime. Variation in the response to AM fungi was found among the scions of Thai honey tangerine on different citrus rootstocks. The scion of Thai honey tangerine grew best on the lime rootstock inoculated with AM fungi. Key words: Phytophthora parasitica , citrus, rootstock, arbuscular mycorrhizal (AM) fungi, root rot.

Citrus Viroid IV Detected in Citrus sinensis and C. reticulata in South Africa.

Citrus Viroid IV or Bark cracking viroid (CVd-IV) has been reported from various countries, mostly in the Near East, but was unknown in southern Africa. It can cause severe bark cracking of trifoliate orange (Poncirus trifoliata), but symptoms on the indicator, Arizona 861-S-1 Etrog (Citrus medica), are mild and transient, usually leaf epinasty (2-4). CVd-IV was detected for the first time in South Africa during the 2009 biennial indexing of mother trees maintained for the South African Citrus Improvement Scheme (CIS). Symptoms were observed on Etrog indicators budded with material from mother trees of DuRoi Valencia sweet orange (C. sinensis). Initially, leaf bend of a single leaf was noted on one plant, and later, petiole browning developed on all test plants. Reverse transcription (RT)-PCR testing confirmed CVd-IV infection by amplification of a 286-bp PCR product representing the full-length CVd genome using the primers CVd-IV-F3 5'-GGTGGATACAACTCTTGGG-3' (1) and CVd-IV-SL3 5'-GGGTAGTTTCTATCTCAG-3' (N. Duran-Vila personal communication). Samples of 78 field and 30 nursery trees of DuRoi Valencia supplied over a 7-year period from the CIS were budded on Etrog and tested 3 months after inoculation. Ninety positive samples were obtained from trees up to 7 years old. These trees were on either Swingle citrumelo (P. trifoliata × C. paradisi) or rough lemon (C. jambhiri) rootstocks and no symptoms of bark-cracking or other abnormalities were noted on these trees. Following the initial interception, a comprehensive screening of the CIS gene source, consisting of 450 cultivars, was initiated and CVd-IV was detected in five other accessions: SB navel (C. sinensis) collected in 1984 in Kwa-Zulu Natal Province of South Africa, Gillemberg navel (C. sinensis) collected in 1986 in Limpopo Province, Tarocco midseason (C. sinensis) imported from Sicily in 1987, Fortuna mandarin (C. reticulata) imported from Spain in 1987, and Westin midseason (C. sinensis) imported from Brazil in 1994. None of these accessions are planted on a commercial scale and field observations are therefore not possible. PCR amplicons of all positive samples were bidirectionally sequenced and sequences submitted to GenBank. Sequence comparisons showed those obtained from DuRoi Valencia (JN903763), Tarocco (JN903764), and SB Navel (JN903765), and GenBank reference sequences AB054633 (Japan) and HM042747 (China) to be identical. Likewise, the South African sequences from Westin (JN903766) and Gillemberg (JN903767) were identical to reference sequences X14638 and NC_003539 (Israel) and GQ260216 (Iran). The Fortuna sequence (JN903762) differed by two base pairs from the latter group. On the basis of the entry dates of the accessions, it is probable that CVd-IV has been in South Africa since at least 1984, but was not detected during indexing because of the erratic symptom expression of CVd-IV on Etrog. Existing CVd-IV-contaminated field material will be monitored for possible disease expression, but thus far the trifoliate hybrid and rough lemon rootstocks do not show the same susceptibility as reported for P. trifoliata (4). The detection of CVd-IV in field and gene-source material emphasizes the importance of intensive and specific indexing to ensure distribution of pathogen-free citrus in South Africa.

Influence of rootstock variety on Huanglongbing disease development in field-grown sweet orange (Citrus sinensis [L.] Osbeck) trees

Huanglongbing (HLB), a bacterial disease of citrus, is causing substantial economic losses to the citrus industry worldwide. Sweet oranges are highly susceptible to the disease, and account for nearly 90% of all varieties grown in Florida. Rootstock is an important component of commercial citrus production, and tolerance to HLB has been reported for some rootstock varieties. Our objective was to investigate if rootstock selection has an effect on HLB disease development under natural conditions in the field. Four field trials with sweet orange scion on 15 different rootstocks were evaluated for incidence of Candidatus Liberibacter asiaticus (Las), the suspected causal agent of HLB, as well as incidence of foliar disease symptoms, canopy damage, and stem growth during the first years after Las began to spread into the trials. Trials ranged from two to nine years in age and included hybrids of trifoliate orange along with other rootstocks standard for citrus production in Florida. Fruit yield and soluble solids content were analyzed for the oldest trial. Our study showed that rootstock did not affect disease incidence and that trees on all rootstocks were considerably damaged by HLB. However, tolerance to HLB was higher in trees grafted on some rootstock selections. In the youngest trial, stem diameters on Volkamer lemon increased 53% while trees on US-852, Benton citrange and Swingle citrumelo grew the least at 21–26% from 2008 to 2010. Depending on trial and time of observation, foliar HLB symptoms on US-897 were less than on many other rootstocks, with ranks of 3.3–4.0 compared with 4.5–4.9 observed for US-812, US-852, Carrizo citrange and Kinkoji. Canopy damage ranks were 1.4–2.6 for trees on US-802 compared with 3.9–4.1 for trees on US-812, U-852, Sour orange, and Swingle in the oldest trial. In the youngest trials, trees on Volkamer showed least canopy damage while trees on US-852 and Benton were more affected. Highest fruit yields of 30–64kg/tree were obtained from trees on US-802 and Carrizo. In 2010, highest juice soluble solids content of 2.8–2.9kg was observed for trees on US-897 and US-812.