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  • Author or Editor: Amnon Levi. x
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Whiteflies [Bemisia tabaci (Gennadius)] and aphids [Aphis gossypii Glover and Myzus persicae (Sulzer)] are serious threats to watermelon by direct feeding and by transmitting viruses of important virus diseases. The desert watermelon Citrullus colocynthis (L.) has been shown to exhibit resistance to these insect pests and could be a useful source for breeding resistance into watermelon [Citrullus lanatus var. lanatus (Thunbs) Matsum & Nakai]. Using high-performance liquid chromatography (HPLC), we found differences among the chemical profiles of two U.S. PIs of C. colocynthis, one PI of C. lanatus var. citroides, and two heirloom watermelon (C. lanatus var. lanatus) cultivars (‘Charleston Gray’ and ‘Mickey Lee’). Flavonoid and caffeic acid derivatives were identified in the leaf extracts by a combination of ultraviolet (UV) and mass spectrometry (MS) spectral analyses. Four phenolic derivatives of caffeic and/or ferulic acid were found to be essentially unique to C. colocynthis. Total flavonoid content was found to be approximately four to 18 times higher in C. colocynthis accessions and seven to nine times higher in C. lanatus var. citroides as compared with watermelon cultivars. Caffeoyl-glucose was also identified in the leaves of watermelon cultivars for the first time. Leaf sugar concentrations (198 to 211 mg·dL−1), read from a glucometer, were statistically the same among the various germplasm entries. These results will help in the development of pest-resistant watermelon.

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Root traits are an important component for productive plant performance. Roots offer immediate absorptive surfaces for water and nutrient acquisition and are thus critical to crop growth and response to biotic and abiotic stresses. In addition, roots can provide the first line of defense against soilborne pathogens. Watermelon crop performance is often challenged by inclement weather and environmental factors. A resilient root system can support the watermelon crop’s performance across a diverse range of production conditions. In this study, 335 four-day-old watermelon (Citrullus spp.) seedlings were evaluated for total root length, average root diameter, total root surface area, and total root volume. Total root length varied from 8.78 to 181 cm (20.6-fold variation), total surface area varied from 2 to 35.5 cm2, and average root diameter and total root volume had an 8- and 29.5-fold variation, respectively. Genotypes PI 195927 (Citrullus colocynthis) and PI 674448 (Citrullus amarus) had the largest total root length values. Accessions PI 674448 and PI 494817 (C. amarus) had the largest total root surface area means. Watermelon cultivars (Citrullus lanatus) had a relatively smaller root system and significantly fewer fibrous roots when compared with the roots of the other Citrullus spp. Positive genetic correlations were identified among total root length, total root surface area, and total root volume. This genetic information will be useful in future breeding efforts to select for multiple root architecture traits in watermelon. Germplasm identified in this study that exhibit superior root traits can be used as parental choices to improve watermelon for root traits.

Open Access

A genetic linkage [randomly amplified polymorphic DNA (RAPD)-based] map was constructed for watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] using a BC1 population [PI 296341-fusarium wilt resistant × New Hampshire Midget (fusarium susceptible)] × `New Hampshire Midget'. The map contains 155 RAPD markers, and a 700-base pair sequenced characterized amplified region (SCAR) marker that corresponds to a fragment produced by the RAPD primer GTAGCACTCC. This marker was reported previously as linked (1.6 cM) to race 1 fusarium wilt resistance in watermelon. The markers segregated to 17 linkage groups. Of these, 10 groups included nine to 19 markers, and seven groups included two to four markers. The map covers a genetic linkage distance of 1295 cM. Nine of the 10 large linkage groups contained segments with low (or no) level of recombination (0 to 2.6 cM) among markers, indicating that the watermelon genome may contain large chromosomal regions that are deficient in recombination events. The map should be useful for identification of markers linked closely to genes that control fruit quality and fusarium wilt (races 1 and 2) resistance in watermelon.

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Citrullus colocynthis (CC) is a viable source of genes for enhancing disease and pest resistance in common cultivated watermelon [Citrullus lanatus var. lanatus (CLL)] cultivars. However, there is little information about genetic diversity within CC or the relationship of CC accessions to C. lanatus. In this study, we examined genetic diversity and relationships among 29 CC accessions collected in northern Africa, the Middle East, and Asia, and their relationships to 3 accessions and 3 cultivars of CLL, 12 accessions of citron melon [C. lanatus ssp. lanatus var. citroides (CLC)], and 1 accession representing the desert perennial Citrullus ecirrhosus (CE). Twenty-three high-frequency oligonucleotides-targeting active gene (HFO–TAG) primers were used to produce a total of 431 polymorphic fragments that target coding regions of the genome. Cluster and multidimensional scaling plot analysis, separated the CC into five groups, in general agreement with their geographic origins. CC genotypes admixed with CLL and CLC also were identified. Major reproductive barriers resulted in significantly reduced fertility in CC × CLL hybridizations. However, several of the U.S. PIs of CC were successfully crossed with watermelon cultivars using traditional breeding procedures, and the seeds produced from these crosses were viable. This suggests that CC can be a viable source to introduce biotic and abiotic stress resistance genes into cultivated watermelon.

Free access

A genetic linkage map was constructed for watermelon based on a testcross population and an F2 population. The testcross map includes 312 markers (RAPD, ISSR, AFLP, SSR, and ASRP). This map covered a genetic distance of 1385 cM, and identified 11 large (50.7-155.2 cm), five intermediate (37.5-46.2 cm), and 16 small linkage groups (4.2-31.4 cm). Most AFLP markers are clustered in two linkage regions, while all other markers are randomly dispersed throughout the genome. Many of the markers in this study were skewed from the classical (Mendelian) segregation ratio of 1:1 in the testcross or 3:1 in the F2 population. The order of the markers within linkage groups was similar in the testcross and F2 populations. Additionally, a cDNA library was constructed using RNA isolated from watermelon flesh 1 week (rapid cell division stage), 2 weeks (cell growth and storage deposition stage), 4 weeks (maturation stage), and 5 weeks (mature fruit) after pollination. More than 1020 cDNA clones were sequenced, and analyzed using the basic local alignment search Tool (BLAST). The sequenced cDNA clones were designated as expressed sequenced tag (EST). The ESTs were searched for simple sequence repeats. About 7% of the ESTs contained SSR motifs. The ESTs containing SSRs are being used to design PCR primers and the putative markers are being tested for polymorphism among the parental lines of the mapping populations. Polymorphic markers will then be mapped using the mapping populations.

Free access