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N. Tusa, J.W. Grosser, F.G. Gmitter Jr., and E.S. Louzada

Allotetraploid somatic hybrid plants of `Hamlin' sweet orange (Citrus sinensis L. Osbeck) + `Femminello' lemon (C. limon L. Burm. f.), and Milam lemon (purported hybrid of C. jambhiri Lush) + `Femminello' lemon were regenerated via somatic embryogenesis following protoplast fusion. `Hamlin' and Milam protoplasts were isolated from undeveloped ovule-derived embryogenic callus cultures and fused using a polyethylene glycol method with seedling leaf-derived protoplasts of `Femminello' lemon. Somatic hybrids were identified on the basis of leaf morphology, root-tip cell chromosome number, and electrophoretic analyses of phosphoglucose isomerase, phosphoglucose mutase, and 6-phosphogluconate dehydrogenase leaf isozymes. The somatic hybrids will be used in interploid crosses with lemon in an effort to generate seedless triploid lemon types with improved tolerance to mal secco disease.

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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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Xiaomeng Li, Rangjin Xie, Zhenhua Lu, and Zhiqin Zhou

of C. limon and C. aurantifolia (BS 95). Within clade A, two accessions of C. medica formed a strongly supported monophyletic group (BS 100), and C. jambhiri , C. limonia ‘Guangxi’, and C. limonia ‘Rangpur’ nested in the mandarin group

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Dafna Langgut

third and second centuries BC; 5) seeds and fruit remain of Citrus , most probably of C. limon which marks its earliest appearance outside Asia, the Forum Romanum, Rome; 6) seed remains from Roman sites in the Egyptian Desert, first and fourth

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Ed Stover, Randall Driggers, Matthew L. Richardson, David G. Hall, Yongping Duan, and Richard F. Lee

results in this report. Progenies most susceptible to ACC. Progeny of Poncirus and its hybrids as well as those of C. maxima , C. limon , and related species dominated the most symptomatic group at all assessment times and in all parameters measured

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Makki A. Al-Khafaji and M. M. Musalat

Application of Phyll set (G A3 + NAA) on whole trees of local cultivars of sweet orange (Citrus sinensis Osbeck) and lemon (C. Limon Burmann) at full bloom stage was made during 1988 and 1989 seasons. All concentrations of Phyll set (12, 24 and 48 mg/l) increased fruit set and yield of sweet orange. Lemon yields were increased only at 12 mg/l Phyll set. The use of Phyll set as a new growth regulator for improved fruit quality will be discussed.

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Zenaida Viloria and Jude W. Grosser

Interploid hybridization was conducted using `Key' lime [Citrus aurantifolia (Cristm.) Swing.], `Lakeland' limequat hybrid [C. aurantifolia × Fortunella japonica (Thumb.) Swing.], Palestine sweet lime (C. limettioides Tan.), `Etrog' citron (C. medica L.), and seven lemon [C. limon (L.) Burm. F.] varieties as female progenitors and five allotetraploid somatic hybrids {`Hamlin' sweet orange [C. sinensis (L.) Osbeck] × `Femminello' lemon (C. limon)]; `Key' lime × `Valencia' sweet orange (C. sinensis); `Valencia' sweet orange × rough lemon (C. jambhiri Lush); Milam lemon (purported C. jambhiri hybrid) × `Femminello' lemon (C. limon); and `Valencia' sweet orange × `Femminello' lemon} and two autotetraploids [`Giant Key' lime (C. aurantifolia) and `Femminello' lemon] as pollen progenitors. A few tetraploid × diploid crosses were also performed. Thirty-five parental cross combinations were accomplished in 2000, 2001, and 2002. The breeding targets were seedlessness, cold-tolerance, and disease resistance. Triploid hybrids were recovered through embryo culture. Generation of triploid citrus hybrids was affected by several factors including sexual compatibility, cross direction, embryo developmental stage, pollen viability, as well as horticultural practices and climatic conditions. Efficiency of triploid hybrid production was higher in diploid × tetraploid crosses than the reciprocal. Many more triploid hybrids were generated from lemon seed progenitors compared to the other acid citrus fruit progenitors. `Todo el Año', `Lisbon', and `Limonero Fino 49' showed the highest sexual compatibility. Embryo germination rate and normal plant recovery were also higher in lemons as compared to the other seed progenitors. Low winter temperatures might have affected the hybrid production efficiency from tropical acid fruit progenitors. A total of 650 hybrids (mostly triploid) were transferred to soil. The novel genetic combinations of these progenies should be valuable for the genetic improvement of acid citrus fruit (lemons and limes).

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E. Cohen, B. Shapiro, Y. Shalom, and J.D. Klein

Water loss was found to be a nondestructive indicator before visible symptoms of chilling injury (CI) in cold-stored grapefruit (Citrus paradisi Macf.) and lemon (C. limon L. Burm. f.). The water-loss rate increased significantly after removing the fruit from cold storage and holding at 20C. Scanning electron microscopy revealed large cracks around the stomata. Changes in electrical conductivity of the flavedo tissues, total electrolyte leakage, and K+ or Ca2+ leakage were all inadequate predictors of CI, appearing only after CI was evident.

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Nicasio Tusa, Sergio Fatta Del Bosco, Franco Nigro, and Antonio Ippolito

The reaction of lemon hybrids obtained by symmetrical and asymmetrical protoplast fusion, toward “mal secco” infection caused by Phoma tracheiphila (Petri) Kanc. et Ghik. was examined. Resistance was tested in `Valencia' sweet orange [Citrus sinensis (L.) Osbeck] and `Femminello' lemon [C. limon (L.) Burm. f.] somatic hybrid and cybrids, by stem and leaf inoculation tests and by analysis of propagule number of P. tracheiphila in the xylem of stem-inoculated plants. In general, the somatic hybrid and the cybrids showed an intermediate degree of resistance, with slight differences in disease symptoms, in comparison with resistant `Monachello' lemon and susceptible `Femminello' lemon, used as controls. The lower mortality in the asymmetrical lemon cybrids suggests that specific mechanisms of resistance to the disease could be activated in these genotypes.

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S. Singh, B.K. Ray, S. Bhattacharyya, and P.C. Deka

Multiple shoots were obtained from shoot tips (2 to 3 mm) derived from mature plants (5 to 6 years old) of Citrus reticulata Blanco cv. Khasi mandarin and C. limon Burm.f. cv. Assam lemon when cultured on Murashige and Skoog (MS) medium, supplemented with (mg·liter-1) 1.0 BAP, 0.5 kinetin, and 0.5 NAA. Root induction was observed when 7-week-old single shoots (≈ 2 cm long) of both Citrus species were cultured on MS medium supplemented with (mg·liter-1) 0.25 BAP, 0.5 NAA, and 0.5 IBA. These plantlets were successfully established in the soil. Chemical names used: naphthalene acetic acid (NAA), indole 3-butyric acid (IBA), and benzylamino purine (BAP).