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Bhimanagouda S. Patil

Two-year field studies at three sites in the Lower Rio Grande Valley of Texas were conducted to evaluate the effects of location, rootstock, and irrigation on sheepnosing of `Rio Red' grapefruit (Citrus paradisi Macf.,) on sour orange (Citrus aurantium L.) rootstock. Based on the equatorial/polar diameter ratio, grapefruit grown in Weslaco had significantly higher percentage of sheepnosed fruits (62.66) than fruit grown in Mission (57.32), while Bayview had a negligible percentage of sheepnosed fruit (4.07). In a second study, `Rio Red' grapefruit grown on Carrizo [C. sinensis (L.) Osbeck × Poncirus trifoliate (L.) Raf.] had significantly higher percentage of sheepnosed fruit (59.46), compared to `Rio Red' grown on Swingle (C. paradisi × P. trifoliata) (47.83). In a third experiment, grapefruit with microjet irrigation had a significantly higher percentage of sheepnosed fruit (53.40), compared to flood irrigation (42.68). Although sheepnosed fruit had significantly greater peel thickness and a lower juice content, fruit quality was better because of higher soluble solids: titratable acidity ratio compared to normal shaped fruits. While significant, the irrigation and rootstock appear have a minor effect on sheepnosing less than growing location.

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Victor Medina-Urrutia, Karla Fabiola, Lopez Madera, Patricia Serrano, G. Ananthakrishnan, Jude W. Grosser, and Wenwu Guo

No presently available rootstock combines all the available rootstock attributes necessary for efficient long-term citriculture (production and harvesting) of Mexican limes and other commercially important scions. In the present study, somatic hybridization techniques were used to combine the widely adapted Amblycarpa mandarin (also known as Nasnaran mandarin) with six different trifoliate/trifoliate hybrid selections: Benton, Carrizo, and C-35 citranges; Flying Dragon and Rubidoux trifoliate oranges; and a somatic hybrid of sour orange + Flying Dragon. The ultimate goal of this research is to generate polyploid somatic hybrids that express the complementary horticultural and disease resistance attributes of the corresponding parents, and have direct potential as improved tree-size controlling rootstocks. Somatic hybrids from all six parental combinations were confirmed by a combination of leaf morphology, flow cytometry, and randomly amplified polymorphic DNA (RAPD) (for nuclear hybridity) and cleaved amplified polymorphic sequence (CAPS) analyses (for mtDNA and cpDNA). This is the first report of citrus somatic hybridization using Amblycarpa mandarin. Unexpected hexaploid somatic hybrid plants were recovered from the fusion of Amblycarpa mandarin + C-35 citrange. Hexaploid hybrids should be very dwarfing and may have potential for producing potted ornamental citrus. Resulting somatic hybrid plants from all six combinations have been propagated by tissue culture and/or rooted cuttings and are being prepared for commercial field evaluation for their potential as improved rootstocks for Mexican lime and other important scions.

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James J. Ferguson, Fedro S. Zazueta, and Juan I. Valiente

Fungal diseases have their greatest impact on citrus in Florida by reducing tree vigor, fruit yield, and quality. Given the complex etiology of these diseases, this software was developed to facilitate diagnosis of symptoms and to explain the dynamics of Alternaria brown spot of mandarins, greasy spot, melanose, Phytophthora brown rot, post-bloom fruit drop, and sour orange scab. CITPATH includes a diagnostic key to identify symptoms of the major fungal diseases of citrus foliage and fruit in Florida and a hypertext program containing a description and graphic display of symptoms, maps of geographic occurrence, diagrams of disease development, and management strategies. Users can also consult a list of citrus cultivars susceptible to specific diseases and a reciprocal list of diseases affecting specific cultivars. Chemical control methods are discussed briefly with reference to the current Florida Citrus Spray Guide, a hardcopy of which is included with the software purchase. Developed for commercial growers, county extension programs, citrus horticulture classes, and master gardeners, this software is available on CD-ROM disks containing other citrus databases and as a separate disk for MS-DOS-based computers.

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F.S. Davies, M.W. Fidelibusa, and C.A. Campbell

Gibberellic acid (GA) applied in late summer or fall delays subsequent loss of peel puncture resistance (PPR) and development of yellow peel color in many citrus cultivars. Our objective was to determine the optimal time to apply GA for increasing juice yield of `Hamlin' sweet orange [Citrus sinensis (L.) Osb.]. Mature trees on sour orange (Citrus aurantium L.) rootstock were sprayed with ≈24 L of a solution of GA (45 g a.i./ha) and organo-silicone surfactant (Silwet, 0.05%). Trees were sprayed on 26 Aug., 9 Sept., 2 Oct. (colorbreak), or 13 Oct. 1997, or nonsprayed (control). Peel puncture resistance, peel color, and juice yield were evaluated monthly between Dec. 1997 and Mar. 1998. Fruit from trees sprayed with GA had peels with higher PPR and less yellow color than fruit of control trees for most of the harvest season. The effect of GA on PPR and peel color lasted about 5 months. Juice yield was usually numerically greater for GA-treated fruit than for nontreated fruit. Fruit treated with GA at color break had significantly greater juice yield when harvested in late February than fruit from control trees. Thus, GA applied at color break appears to be the most effective time for enhancing peel quality and juice yield of `Hamlin' oranges.

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Richard J. Crawford and David M. Eissenstat

The relationship of genotypic variation in root hair development with root proliferation, mycorrhizal colonization, and specific root length (length / dry mass) was studied in sixteen field-grown citrus relatives. The species varied widely in hair development, root length and mass density, and specific root length. No correlation was found between hair development, mycorrhizal colonization, root proliferation, and specific root length. However, there was a significant correlation (r=.55) between the percentage of total root length with hairs and the percentage of hairs with adhered soil. In a second experiment, the phenotypic plasticity in root hair development was studied in four-citrus rootstooks: Swingle citsumelo, Sour orange, Trifoliate orange and Volkamer lemon. Roots were grow” in either mineral soil or high organic matter media. After eight weeks, root length density and percentage of root length with hairs averaged over all four rootstocks was 232 % and 85 % greater in the organic media than in the mineral soil. Similar to the first experiment the percentage of total root length with hairs was significantly correlated (r=.99) with the percentage of hairs with adhered soil.

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J.P. Syvertsen, M.L. Smith, and B.J. Boman

Effects of salinized irrigation water on tree canopy and root growth, water use, foliar nutrition, and leaching losses below the rootzone were studied during a 2-year period using single tree lysimeters. Eighteen 6-year-old `Valencia' orange trees on either Carrizo citrange (CC) rootstock or sour orange (SO) rootstock were each transplanted into 7.8 m3 drainage lysimeters and irrigated with water having an electrical conductivity of 0.3, 1.6, or 2.5 dS m-1 from a 3:1 ratio of NaCl:CaCl2. Six additional trees (3 on each rootstock) were transplanted into soil without tanks. Trees outside the tanks were smaller, but nutritionally similar to the low salinity trees in lysimeters. Trees on CC were larger, had greater root densities, and were associated with less leaching of ions and nutrients into drainage water from the tanks than trees on SO. High salinity irrigation water reduced canopy growth and ET, but increased fibrous root dry weight. Trees on CC accumulated more Cl in leaves and in fruit juice than those on SO. Leaching loss of total N varied from 2-8% of that annually applied to trees, but up to 70% of the applied N and up to 80% of the applied K were leached from the blank tank with no tree. Salinized trees lost more N and K to drainage water, especially those on SO. Tree size, root density, and irrigation water quality can influence leaching losses beyond the rootzone.

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Y. Liu, B.S. Patil, H. Ahmad, and D.T. Gardiner

Pectin is a class of complex polysaccharides that function as hydrating agents and cementing materials for the cellulose network. Pectin has various health benefits, such as decreasing serum cholesterol levels, alleviating diabetes mellitus, and preventing cancer. It has been reported that the cancer prevention effect is closely related to the structure of pectin (galactose-rich, molecular weight <10,000, and methylation degree 50% to 70%). This study was conducted to investigate the variation of grapefruit pectin content due to harvest time. `Rio Red' grapefruit on sour orange rootstock grown at Texas A&M Univ.-Kingsville Citrus Center were harvested every 2 months and analyzed for pectin content, galacturonic acid concentration, methylation degree, and neutral sugar composition. Results showed that lamella contains more pectin than flavedo and albedo. In the lamella, the edible section, the uronic acid content ranged from 85% to 90% from August to April the following year. Methylation degree increased from August (31.89%) to April (46.99%). Total neutral sugar content of lamella pectin decreased from 110.54 to 61.77% mg·g -1. Galactose, arabinose, and rhamnose are the major sugar contents of pectin (85%), and glucose content increased with the season from 3.14 to 13.34 mg·g-1. Molecular weight of pectin was also determined.

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D. M. Eissenstat and J. P. Syvertsen

The effects of elevated levels of ozone on growth, mineral nutrition and freeze resistance were studied using broadleaf-evergreen citrus and avocado trees. `Ruby Red' grapefruit (Citrus paradisi L.) trees on either Volkamer lemon (Citrus volkameriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstock and `Simmonds' or `Pancho' avocado trees (Persea americana Mill.) on the rootstock `Waldin' were exposed to ozone in open-top chambers for 4 mo in 1988 and in a second experiment in 1989 for 8 mo. Citrus tree growth, estimated by total leaf mass, was unaffected by ozone concentrations of 3 times ambient in either year but avocado growth was reduced by ozone concentration at 2 times ambient in 1989. All trees were well-fertilized and ozone had little effect on mineral nutrient concentrations in leaves. Freeze resistance, estimated by electrolyte leakage from leaf disks and survival of leaves, stems, and whole-plants following exposure to freezing temperatures, was often diminished in avocado and citrus at 3 times ambient ozone, but occasionally was increased at 2 times ambient. Thus, ozone can be related to shifts in freeze resistance that can occur prior to discernible growth effects.

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Ashok K. Alva

The aim of this study was to investigate soil pH and copper (Cu) interactions affecting Cu phytotoxicity to young citrus trees on different rootstocks. Hamlin oranges on either Carrizo citrange, sour orange, or rough lemon rootstocks were grown on Candler fine sand at varying soil pH (5.0, 5.5, 6.0, 6.5) without additional Cu or soil applied Cu (liquid form; 240 kg Cu/ha; nine pre- and five post-planting applications over a period of 43 months). Increasing soil pH increased tree height, canopy volume and trunk diameter of trees on all three rootstocks, regardless of Cu treatments. Tree growth response to an increase in soil pH was greater in Cu amended as compared to unamended treatments. Response to pH increase above 6.0 was marginal as compared to that for pH increase from 5.0 to 6.0. Leaf Cu concentrations showed negligible differences in response to Cu treatments; however, Cu concentrations in fibrous roots increased by 126 to 152% in Cu amended as compared to unamended treatments.

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Dariusz Swietlik

Root distribution of trickle–and flood-irrigated 4-year-old `Ray Red' grapefruit (Citrus paradisi Macf.) trees on sour orange (C. aurantium L.) rootstock was studied utilizing a trench method. Irrigation treatments were: flooding at 50% soil water depletion, trickle irrigation (2 drippers per tree) at 0.5 Class A Pan evaporation or at 0.02 MPa soil tension. Two trees from each treatment were studied. Five 2.5 m deep trenches positioned perpendicular or parallel to the tree row at 0.6, 2.1, or 4.3 m from the tree trunks were dug per tree. After washing off a 0.5 cm thick layer of soil from the trench wall, 0.5 cm long root sections were marked on a transparent plastic film attached to the wall. Many roots of trickle-irrigated trees grew past the trickle wetted zone and extended beyond 2.1 but not 4.3 m of the trunk. However, the roots of flood-irrigated trees were present at all distances from the trunk. From 26 to 51% of the roots of trickle–irrigated trees were found 90-230 cm deep, despite the clayey texture of the top 1 m of soil which was underlaid by a sandy clay loam. The root systems-of flood-irrigated trees were shallower and in most cases confined to the top 90 cm soil layer.