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Safwan Shiyab

important ( Ali et al., 2000 ). A species of multiple use, the sour orange ( Citrus aurantium L.) is also known as bitter or seville orange. It is a universal rootstock for citrus and is used widely in the Mediterranean region ( Navarro et al., 1975 ) It is

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Mark Rieger and Antonio Motisi

Estimates of root hydraulic conductivity (Lp) were obtained on intact peach (Prunus persica × P. davidiana `Nemaguard') and sour orange (Citrus aurantium L.) rootstock over a broad range of transpiration rates. Within a species, Lp was lower when estimated using the Ohm's law analog than the reciprocal of the slope of the linear regression between transpiration (E) and stem xylem water potential (Ψ). Nonzero y-intercepts in linear regressions of Ψ vs. E resulted in the lack of agreement between Lp estimates. Removal of the root system caused xylem Ψ to rapidly approach zero in both species when E ≈ 0, suggesting that factors responsible for nonzero y intercepts resided within roots. Lp was 2.2 and 3.5 times lower for sour orange than peach when calculated by the Ohm's law and regression methods, respectively.

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Mongi Zekri and Lawrence R. Parsons

We determined whether the ability of sour orange seedlings to withstand saline irrigation water could be improved by the addition of calcium to the water. Sour orange (Citrus aurantium L.) seedlings were treated for 4 months with a nutrient solution containing either no NaCl, 40 mm NaCI, or 40 mm NaCl plus various concentrations of CaSO4, CaCl2, or KCl. After 4 months, the NaCl alone reduced root and shoot dry weights by ≈ 30% with no leaf necrosis. Addition of 1, 5, or 7.5 mm CaSO4 to solutions containing 40 mm NaCl significantly inhibited the NaCl-induced reductions in shoot dry weight. Addition of 7.5 mm CaCl2 or 7 mm KCl to the NaCl solution reduced leaf Na, but increased Cl to the toxicity level; hence, growth was not improved. The beneficial effect of CaSO4 was mainly attributed to a reduction in the accumulation of Na and Cl below the toxicity level in the leaves (0.4% and 0.5%, respectively) without a major increase in total dissolved salts. This study demonstrated that the beneficial effect of adding Ca depended on the anion associated with the Ca salt. Calcium sulfate, but not CaCl2, was able to overcome the damaging effect of NaCl to sour orange seedlings.

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Francesco Loreto, Harold H. Burdsall Jr., and Alfio Tirro'

The effect of inoculating seedlings of Mediterranean cultivated trees grown under greenhouse conditions with North American isolates of Armillaria mellea (Vahl: Fr) Kumm. and A. ostoyae (Romagn.) Herink on net photosynthesis (A), stomatal conductance (gs), and water potential was examined. The effect of water stress was determined also on the same plant species independently and in combination with Armillaria infection. Red oak (Quercus rubra L.) was used as a control to indicate Armillaria virulence on North American trees. Carob (Ceratonia siliqua L.) was resistant to infection. Infection was successful in sour orange (Citrus aurantium L.), but A, gs, and water potential were unchanged over the 60-day experiment. In olive (Olea europea L.) and oak, A and gs were reduced following inoculation with A. mellea. A and gs of all species but carob were reduced under water stress. Olive and oak responses to water stress and Armillaria infection were quantitatively similar; however, the two stresses combined did not reduce A and gs further. Red oak was strongly susceptible to A. ostoyae infection, but Mediterranean trees were not infected by the same Armillaria isolate. Our results show that Armillaria infection may reduce A and gs in susceptible species.

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Bhimanagouda S. Patil

Field studies were conducted for 2 years, at three sites in the Lower Rio Grande Valley of Texas, to evaluate the effects of location, rootstock, and irrigation on sheepnosing (elongation of the apex) of `Rio Red' grapefruit (Citrus paradisi Macf.). Based on the ratio of equatorial to polar diameter, grapefruit budded on sour orange (Citrus aurantium L.) rootstock grown at Weslaco had a significantly higher percentage of sheepnosed fruits (63%) than did fruit grown at Mission (57%), while the grove at Bayview produced a negligible percentage of sheepnosed fruit (4%). In a second study, `Rio Red' trees grown on `Carrizo' rootstock [C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.] produced a significantly higher percentage of sheepnosed fruit (59%) than did those on `Swingle' (C. paradisi × P. trifoliata) (48%). In a third experiment, trees irrigated by microjet had a significantly higher percentage of sheepnosed fruit (53%) than did those that were flood-irrigated (43%). Although sheepnosed fruit had significantly greater peel thickness and a lower juice content, fruit quality was better than that of normal fruit because of a higher soluble solids: titratable acidity ratio. In 1999, the significant irrigation and rootstock effects were less than that due to growing location. Effects of location, rootstock and irrigation varied between years. The interaction between factors and years was mainly due to a lack of low amount of sheepnosing in 1998.

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Mirko Siragusa, Fabio De Pasquale, Loredana Abbate, and Nicasio Tusa

A collection of 18 accessions of sour orange (Citrus aurantium L.) coming from Sicily and other countries was investigated by two polymerase chain reaction (PCR)-based DNA marker technologies. Ten inter-simple sequence repeat (ISSR) primers and fifteen randomly amplified polymorphic DNA (RAPD) primers were used to identify and to evaluate the genetic variability and relationship of accessions. A total of 111 ISSR and 145 RAPD amplified fragments were used to estimate the Dice's coefficient of similarity for cluster analysis using a unweighted pair-group method using an arithmetic averaging (UPGMA) algorithm. The genetic relationships identified using ISSR and RAPD markers were highly concordant, such that the correlation between ISSR and RAPD genetic distance (GD) estimates was r = 0.93. The ISSR and RAPD analysis of 18 sour orange accessions found a high grade of genetic diversity in foreign accessions, while a low variability was detected in local accessions. Sicilian accessions could be grouped in two distinct clusters, including indistinctly plants from three origin regions. Some markers could be linked to the different growing areas. The ISSR and RAPD molecular reference system seems to be suitable for a fine identification of tightly related plants and the obtained results can form the basis for future setting up of Citrus rootstock genetic improvement projects.

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Fabio De Pasquale, Salvatore Giuffrida, and Francesco Carimi

Minigrafting was used for rescue of tissue culture regenerants of the following four species of Citrus: sour orange (C. aurantium L. `AA CNR 31'), sweet orange [C. sinensis (L.) Osb. `Valencia Late'], lemon [C. limon (L.) Burm. `Femminello Comune'] and mandarin (C. deliciosa Tenore `Tardivo di Ciaculli'). The grafting was carried out with different scion types including shoots, roots, inverted roots and somatic embryos. This material was obtained in vitro from embryogenic style-derived callus. Seedlings of open-pollinated sour orange (C. aurantium L.), Cleopatra mandarin (C. reshni Hort. ex Tan.) and `Troyer' citrange [C. sinensis Osb. × Poncirus trifoliata (L.) Raf.] were used as rootstocks. Minigrafting of shoots, roots, inverted roots and embryos regenerated in vitro allowed successful rescue of these four species. Percentages of successful minigrafts ranged from 100% (shoots) to 2.5% (inverted roots). The probability of successful graft unions increased with the age of the rootstock. The final mean canopy leaf area (120 days after grafting) ranged from 5.2 cm2 (`Tardivo di Ciaculli' mandarin grafted on 6-month-old Cleopatra mandarin) to 157.9 cm2 (`Valencia Late' sweet orange grafted on 18-month-old Cleopatra mandarin). In this work we examined some of the variables which influenced minigrafting and we determined the efficacy of this method for rescue of in vitro regenerants of Citrus. This method is also suggested as a technique to produce a high percentage of viable plants from in vitro regenerants difficult to root.

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Eliezer S. Louzada and Chandrika Ramadugu

Grapefruit ( Citrus × aurantium ) belongs to the family Rutaceae, subfamily Aurantioideae. Grapefruit is unique among citrus ( Citrus sp.) fruit due to its distinctive flavor, health benefits, and recent Caribbean islands’ origin less than 300

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Dariusz Swietlik and Linsen Zhang

Chelator-buffered nutrient solutions were used to study the effect of different levels of Zn activity in the rhizosphere on growth and nutritive responses of various tissues of sour orange seedlings. The seedlings were grown for 3 months in a growth chamber in a hydroponic culture containing from 5 to 69 μm and 5 to 101 μm total Zn in Expts. 1 and 2, respectively. Zn+2 activities were calculated with a computerized chemical equilibrium model (Geochem-PC), and buffered by inclusion of a chelator, diethylenetriamine pentaacetate (DTPA), at 74 and 44 μm in excess of the sum of Fe, Mn, Zn, Cu, Ni, and Co in Expts. 1 and 2, respectively. The use of DTPA-buffered solutions proved successful in imposing varying degrees of Zn deficiency. The deficiency was confirmed by leaf symptomatology, leaf chemical analyses, i.e., <16 mg·kg-1 Zn, and responses to foliar sprays and application of Zn to the roots. Growth parameters varied in their sensitivity to Zn deficiency, i.e., root dry weight < leaf number and white root growth < stem dry weight < leaf dry weight < shoot elongation and leaf area. The critical activities, expressed as pZn = -log(Zn+2), were ≈10.2±0.2 for root dry weight, 10.1±0.2 for leaf number and white root growth, 10.0±0.2 for stem dry weight, 9.9±0.2 for leaf dry weight, and 9.8±0.2 for shoot growth and leaf area. Increases in growth were observed in response to Zn applications even in the absence of visible Zn-deficiency symptoms. Seedlings containing >23 mg·kg-1 Zn in leaves did not respond to further additions of Zn to the nutrient solution. Zinc foliar sprays were less effective than Zn applications to the roots in alleviating severe Zn deficiency because foliar-absorbed Zn was not translocated from the top to the roots and thus could not correct Zn deficiency in the roots.

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Kenneth R. Summy and Christopher R. Little

), sour orange ( II ; Citrus aurantium ), ‘Valencia’ orange ( III ; C. sinensis ), ‘Bo’ tree ( IV ; F. religiosa ), grapefruit ( V ; C. paradisi ), and muskmelon ( VI ; Cucumis melo ) plants. ( I ) Trifoliate orange saplings: uninfested control (a