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The citron ( Citrus medica L.) is believed to be one of the ancestors of the citrus group ( Barrett and Rhoads, 1976 ), but its study was somewhat neglected during the 20th century, presumably because of its limited commercial use. However, renewed

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To assess genetic diversity in the UC Riverside Citrus Variety Collection, all accessions of pummelo (59), citron (24), and trifoliate orange (48) were studied for RFLP variation using 11-18 cDNA probes that had previously been shown to reveal polymorphism in a broad range of citrus germplasm. Inheritance studies have shown that these probes hybridize to at least 20 loci. The taxa studied are believed to represent biological species rather than hybrids. Citrons were nearly monomorphic and most appeared homozygous at all of the loci studied. Pummelos were very polymorphic and highly heterozygous. Trifoliate orange, an important source of disease resistance in rootstock breeding, was nearly monomorphic but moderately heterozygous (17% of loci). Most accessions of trifoliate orange have evidently differentiated only by mutation. One multilocus probe separated trifoliate orange accessions into 3 groups. Two new trifoliate orange accessions had novel alleles at some loci. The use of genetic markers to recognize hybrid accessions classified as members of species will be discussed.

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percentages for each seed parent (pedigrees were not determined for individual seedlings) from each of the progenitor Citrus species that gave rise to all cultivated citrus: citron ( C . medica ), pummelo ( C . maxima ), mandarin ( C. reticulata ), and

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derived from AFLP data are presented in Table 3 . When interspecific mean distances within genus Citrus were compared, C. medica and C. paradisi appeared to be the most distant pair (0.0592, Table 3 ), while the least interspecific distance was

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in five population groups. The five groups are depicted using the following color codes: red, Citrus maxima ; green, Citrus reticulata ; blue, Citrus medica ; yellow, Poncirus trifoliata ; and purple, Fortunella . Interspecific introgressions

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citrus types are citron ( C. medica ), pummelo ( C. maxima ), and mandarin ( C. reticulata ). This argument was made four decades ago by several scholars ( Barrett and Rhodes, 1976 ; Scora, 1975 ) based on morphological and biochemical characteristics

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Abstract

Tests of citrus seedlings exposed to a series of hardening temperatures showed that kumquat, Fortunella hindsii (Champ.) Swing., acquired more hardiness at 21°/10°C than did ‘Redblush’ grapefruit, Citrus paradisi Macf., or citron, C. medica L. After 8 weeks’ hardening kumquat was the most cold hardy; citron, the least. Leaf photosynthetic CO2 uptake decreased, and leaf diffusion resistance (sec/cm) increased with hardening in all cultivars, but did not reflect the degree of hardening attained. Stomatal closure during hardening was not caused by moisture stress. Ethylene evolution from leaves did not change during hardening of kumquat, mandarin, C. reticulata Blanco, or grapefruit, but did increase from hardened citron leaves.

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Chloroplast DNA (cpDNA) restriction fragment length polymorphisms were used to study the parentage of lemons [Citrus limon (L.) Burm. f.] and several other putative hybrids. The 30 citrus accessions studied included nine lemons, three pummelos [C. maxima (Burm.) Merrill], three citrons (C. medica L.), three mandarins (C. reticulata Blanco, C. clementina Hort. Ex. Y. Tan., and C. sunki Hort. ex. Tan.), and single representatives of 13 other taxa. Four different fragments of citrus cpDNA were amplified by polymerase chain reaction using four universal chloroplast primers and amplification products were digested with four endonucleases, Rsa I, Dra I, Hae III, and Mbo I. A total of 104 different restriction fragments were scored and used for phylogenetic analysis by parsimony. The three taxa which have been proposed as ancestral Citrus L. sp., C. medica, C. maxima, and C. reticulata, had distinct cpDNA patterns. Pummelo contributed the chloroplast genome to lemons, sweet orange [C. sinensis (L.) Osbeck], and Bergamot orange (C. bergamia Risso and Poit.), and mandarin contributed the chloroplast genome to rough lemons (C. jambhiri Lush.), Rangpur lime (C. limonia Osbeck), and `Mexican' lime [C. aurantifolia (Christm.) Swing.]. Data suggest that the particular accessions of these ancestral species that were studied were not directly involved in the hybridization events that created these hybrids, since they have similar but not identical cpDNA restriction fragments. Using inter-simple sequence repeat markers that amplified from nuclear genomic DNA, a set of samples including pummelos, citrons, mandarins, sour oranges (C. aurantium L.), and `Samuyao' papeda (C. micrantha Wester) were tested as possible parents of lemons. Sour orange and citron together had all nuclear and chloroplast fragments found in lemon and are therefore proposed to be the maternal and paternal parents, respectively, of many commercial lemon cultivars, including `Lisbon', `Eureka', `Villafranca', and `Monachello'.

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Inter-simple sequence repeats (ISSR), simple sequence repeats (SSR) and isozymes were used to measure genetic diversity and phylogenetic relationships among 95 Citrus L. accessions including 57 lemons [C. limon (L.) Burm. f.], related taxa, and three proposed ancestral species, C. maxima (Burm.) Merrill (pummelo), C. medica L. (citron), and C. reticulata Blanco (mandarin). The ancestry of lemons and several other suspected hybrids was also studied. Five isozyme and five SSR loci revealed relatively little variation among most lemons, but a high level of variation among the relatively distant Citrus taxa. Eight ISSR primers amplified a total of 103 polymorphic fragments among the 83 accessions. Similarity matrices were calculated and phylogenetic trees derived using unweighted pair-group method, arithmetic average cluster analysis. All lemons, rough lemons, and sweet lemons, as well as some other suspected hybrids, clustered with citrons. Most lemons (68%) had nearly identical marker phenotypes, suggesting they originated from a single clonal parent via a series of mutations. Citrons contributed the largest part of the lemon genome and a major part of the genomes of rough lemons, sweet lemons, and sweet limes. Bands that characterize C. reticulata and C. maxima were detected in lemons, suggesting that these taxa also contributed to the pedigree of lemon.

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