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Sudahono, D.H. Byrne, and R.E. Rouse

Eighteen citrus rootstock seedling lines were tested for their tolerance to Fe chlorosis using sand culture. Potassium carbonate was used to induce Fe-deficiency chlorosis. Chlorosis was quantified by 1) visual ratings, 2) SPAD-502 chlorophyll meter readings, 3) leaf chlorophyll concentration, 4) leaf active Fe, and 5) leaf total Fe. The first four criteria were well correlated among each other but not with leaf total Fe. Although any of the first four measurements could be used to quantify chlorosis, visual ratings and SPAD-502 readings were more convenient. The rootstock that have been reported to be tolerant or very susceptible to Fe chlorosis in calcareous soils were rated similarly for tolerance to bicarbonate-induced Fe chlorosis. Nontrifoliate types such as Texas sour orange (C. aurantium L.), Cleopatra mandarin (C. reticulata Blanco), Vangasay lemon (C. limon Burro.), and Ridge pineapple x Milam 1578-201 (C. sinensis L. Osbeck x C. jambhiri) were tolerant to moderately tolerant. Although most of the trifoliate hybrids tested were moderately susceptible to very susceptible, Smooth Seville x Argentine trifoliate {[C. grands (L.) Osbeck x C. aurantium] x Poncirus trifoliata (L.) Raf.} and F-81-12 citrange (C. sinensis x P. trifoliata) exhibited relatively high tolerance to lime-induced Fe chlorosis.

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Mark Rieger

Growth, gas exchange, root hydraulic conductivity, and drought response of seedling and rooted cuttings of Lovell and Nemaguard peach [Prunus persica (L.) Batsch], and Carrizo (Poncirus trifoliata × Citrus sinensis) and sour orange (C. aurantium L.) citrus rootstocks were compared to determine the influence of propagation method on these characteristics. Rooted peach cuttings had a higher proportion of root biomass in fibrous roots (≤ mm in diameter) and lower root: shoot ratios than seedlings, although this did not occur in citrus. Net CO2 assimilation (A) was higher for peach seedlings than for cuttings, but similar for `Redhaven' (RH) scions on either seedling- or cutting-propagated rootstocks, suggesting that leaf-associated factors were responsible for differences. As in peach, A was higher for Carrizo seedlings than for cuttings, but A was not affected by propagation method in sour orange. Peach seedlings maintained higher A than cuttings as water potentials declined during short-term soil drying, although in citrus this occurred only for Carrizo. RH scions on either root type exhibited similar declines in A as soil dried, indicating the lack of a rootstock effect. Root hydraulic conductivity (Lp) was similar between seedlings and cuttings of all cultivars when expressed on a length basis. Leaf conductance and osmotic adjustment were similar for RH scions on seedling- or cutting-propogated rootstocks during 45 days of drought stress, indicating the lack of a rootstock effect on long-term stress response.

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Kim D. Bowman and Frederick G. Gmitter Jr.

Chinotto is a selection of sour orange (Citrus aurantium L.) with short internodes and small leaves and fruit. Mature fruiting trees of Chinotto grafted on standard rootstocks produce healthy, but dwarf, trees. Most seedlings recovered from Chinotto fruit are of nucellar (maternal) origin and will faithfully grow to reproduce the Chinotto genotype and phenotype. Vigorous greenhouse-grown nucellar seedlings of Chinotto have internodes 5 to 10 mm in length and leaves 30 to 40 mm in length, about 30% the dimensions of the corresponding organs on standard sour orange nucellar seedlings. Sexual hybrids with Chinotto have been produced by controlled crosses with several other parents. Some hybrids with shortened internodes and small leaves were recovered among all hybrid progenies, regardless of whether Chinotto was used as seed or pollen parent. In some cases, segregation among Chinotto hybrids was about 1 normal: 1 dwarf. In other progenies, some intermediate forms were recovered along with normal and dwarf plants.

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Amelia Camprubí and Cinta Calvet

The selection of the most effective arbuscular mycorrhizal (AM) fungi for growth enhancement of citrus cultivars used as rootstocks was the first step toward development of an AM inoculation system in citrus nurseries in Spain. AM fungi were isolated from citrus nurseries and orchards in the major citrus-growing areas of eastern Spain. The most common AM fungi found in citrus soils belonged to Glomus species, and G. mosseae (Nicol. & Gerd.) Gerdemann & Trappe and G. intraradices Schenck & Smith were the AM fungi most frequently associated with citrus roots. The most effective fungus for growth enhancement of citrus rootstocks was G. intraradices. Significant differences in mycorrhizal dependency among rootstocks were confirmed. Sour orange (Citrus aurantium L.) and Cleopatra mandarin (C. reshni L.) were more dependent than Troyer citrange [C. sinensis (L.) Obs. × Poncirus trifoliata (L.) Raf.] and Swingle citrumelo (C. paradisi Macf. × P. trifoliata). Moreover, several inoculation systems for plant production were evaluated for their effectiveness in promoting root colonization of the rootstock cultivars.

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Dariusz Swietlik

The purpose of this study was to determine the effect of Ca: NH4 ratio in the rhizosphere of hydroponically grown sour orange seedlings (SO) (Citrus aurantium L.) on the plants' vegetative growth and N uptake. The experiment was prompted by our observation that application of N in the form of NH4 in conjunction with CaCl2 was more efficient in eliminating N deficiency in field-grown grapefruit trees than the same rates of N applied in the form of NH4NO3 without CaCl2. About 40-cm-tall SO were pruned back to the 4th leaf and grown for 6 weeks in nutrient solutions containing 5 mm NH4 + at CaCl2: NH4 + molar ratios of 1.0, 1.3, 1.6, 1.9, 2.2, or 2.5. In an additional treatment, NO3 was used as the sole source of N at CaCl2: NO3 ratio of 1:1. The level of Ca:NH4 ratio had no effect on new leaves number, shoot growth, total and average leaf area, specific leaf weight, as well as leaf, stem, and tap root dry weight. However, lateral root dry weigh decreased at Ca: NH4 ratio of 2.5. No growth differences were found when the plants were supplied with NH4 + vs. NO3 at Ca:N molar ratio of 1:1.

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Mukadder Kayum, N. Kemal Koç, and Veli-Matti Rokka

Laser flow cytometry was used to analyze nuclear DNA contents (2C values) of five genera (Severinia Ten., Atalantia Corrèa, Fortunella Swing., Poncirus Raf., and Citrus L.) taxonomically grouped in subtribe Citrinae (citrus fruit trees) of the Rutaceae. The genotypes analyzed had 2C values ranging from 0.67 pg for diploid Severinia buxifolia (Poir.) Ten. to 1.27 pg for tetraploid Hongkong Fortunella hindsii Swing. There was no significant difference in the 2C values within the sexually compatible diploid species of 11 “true citrus fruit trees” [Citrus aurantium L., C. grandis (L.) Osbeck, C. limon (L.) Burm. f., C. limonia Osbeck, C. paradisi Macf., C. reshni Hort. ex Tanaka, C. sinensis (L.) Osbeck, C. volkameriana Ten. & Pasq., Poncirus trifoliata (L.) Raf., and the intergeneric hybrid C. sinensis × P. trifoliata]. The species Atalantia ceylanica (Arn.) Oliv. (a “near-citrus fruit tree”), sexually incompatible with Citrus spp., had a 2C value significantly different from those of the true citrus fruit tree species. The 2C value of Severinia buxifolia (a “primitive citrus fruit tree”), another species sexually incompatible with the Citrus spp., also differed from those of some of the true citrus fruit tree species. The data largely corresponds with taxonomical differences between a) the genera Citrus and Poncirus and b) the genera Severinia and Atalantia, all assigned to subtribe Citrinae.

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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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Chunxian Chen, Jude W. Grosser, Milica Ćalović, Patricia Serrano, Gemma Pasquali, Julie Gmitter, and Fred G. Gmitter Jr

Somatic hybridization is a powerful tool for the genetic improvement of citrus rootstocks, and it is part of an efficient in vitro-based breeding system described here. An essential component of the system is the requirement of confirming tetraploidy and the combination of the two donor genomes. Expressed sequence tag–simple sequence repeat (EST-SSR) markers provide a means to accomplish both of these objectives, and their application to a population of pummelo [Citrus grandis (L.) Osbeck] + mandarin (C. reticulata Blanco) somatic hybrids developed for the specific purpose of providing alternative rootstocks for sour orange (Citrus aurantium L.) is detailed. Nineteen new somatic hybrids were produced from various mandarin and pummelo parents, and their ploidy level and the complementation of their nuclear genomes were confirmed using four EST-SSR markers. These markers were selected from markers previously mapped in sweet orange [C. sinensis (L.) Osbeck] and trifoliate orange [Poncirus trifoliata (L.) Raf.] and prescreened for suitable allelic polymorphism within the mandarin and pummelo lines used. After polymerase chain reaction amplification of sequences from the parents and putative hybrids, the products were separated on a genetic sequencer and visualized electronically. Additionally, EST-SSR markers identified the unexpected zygotic origin of a presumed nucellar embryogenic callus line. Integration of EST-SSR techniques for high-throughput genotyping with previously developed approaches to somatic hybrid creation increases substantially the effectiveness and efficiency of this in vitro-based breeding system for citrus rootstock improvement.

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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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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.