USDA, ARS maintains 134 U.S. PIs of the wild-type watermelon Citrulluslanatus var. citroides collected in Africa, which is considered the possible center of origin of watermelon ( Jarret et al., 1997 ). In previous studies, we identified a
Five hundred ninety accessions of Citrullus lanatus, obtained from 42 countries, were exposed to natural infection with Sphaerotheca fuliginea (Schlecht) Poll. Only one line, P.I. 269677 from Belize, was highly susceptible. The fungus attacked all aerial parts of plants of this line, including fruits. All other lines, including those from areas of the world where powdery mildew of watermelon commonly occurs, were resistant or tolerant to the race of the fungus prevailing in NY.
Root-knot nematodes (Meloidogyne incognita, M. arenaria, and M. javanica) cause severe damage to watermelon and resistance has not been identified in any watermelon cultivar. In greenhouse tests, we evaluated 265 U.S. plant introductions (PIs) for nematode resistance (based on root galling and nematode reproduction), and identified 22 PIs of Citrullus lanatus var. citroides as moderately resistant to M. arenaria race 1. In subsequent tests, these 22 PIs exhibited low to moderate resistance to M. incognita race 3 and M. arenaria race 2. Three watermelon (C. lanatus var. lanatus) cultivars (Charleston Gray, Crimson Sweet, and Dixie Lee), three C. colocynthis PIs, and four C. lanatus var. citroides PIs, all previously shown to be susceptible to M. arenaria race 1, were susceptible to M. incognita race 3 and M. arenaria race 2. The C. lanatus var. citroides PIs that are most resistant to both M. incognita and M. arenaria should be useful sources of resistance for developing root-knot nematode resistant watermelon cultivars.
Watermelon (Citrullus lanatus Thumb. Matsum. and Nakai) flower petals usually are yellow, but in watermelon line Kw-695, light-green flowers were detected. To study the inheritance of light-green flower color, Kw-695 plants were crossed with yellow-flowered Korean cultures `SS-4' and `Dalgona'. The resulting F1, F2, and reciprocal backcross generations were analyzed for flower color. Segregation ratios in the F2 and backcross to Kw-695 were 3 yellow: 1 light green and 1 yellow: 1 light green, respectively. Backcross generations to the yellow-flowered parents showed yellow flowers only. These results indicate that inheritance of the light-green flower character in Kw-695 is governed by a single recessive gene. We propose the gf gene symbol for the green flower trait. Kw-695 plants have large vines with large, light-green leaves. The plants are andromonoecious, have large, oval, bright yellow-green fruit with irregular dark-green stripes, bright yellow-orange, inedible flesh with very low sugar content (about 3.2 °Brix), and light-yellow seeds. The trait should be useful as a marker in watermelon breeding programs. Linkages between this trait and other genetic markers in watermelon will be investigated.
Carotenoids are plant compounds that serve a variety of essential functions in the plant and have also been found to have several health-promoting activities in humans. Carotenoids found in watermelon (Citrullus lanatus) flesh are responsible for the various colors such as red, yellow and orange. Previous inheritance studies of flesh color revealed that six genes were involved in color determination. The relationship and interaction of these genes suggests that some color-determining genes may be the result of mutations on the structural genes encoding enzymes in the carotenoid biosynthesis pathway. In this study we were able to isolate and sequence six genes encoding enzymes involved in the carotenoid biosynthetic pathway, and determine their expression in different colored watermelon fruit. The cDNA was synthesized from total RNA using RACE (Rapid Amplification of cDNA ends) kit (SMART RACE cDNA Amplification Kit; Clontech, Palo Alto, Calif.). Degenerate primers were designed based on published homologous genes from other species and were used to isolate gene fragments and full-length cDNAs of phytoene synthase, phytoene desaturase, _-carotene desaturase, β-cyclase, β-carotene hydroxylase and zeaxanthin expoxidase. RT-PCR was carried out to examine any differential expression of cloned genes in white, yellow, orange and red-fleshed watermelon. All cloned enzyme-encoding genes were expressed regardless of flesh colors. These results indicate that carotenoid biosynthesis may be regulated at the post-transcriptional level. One interesting feature supports this hypothesis. In case of β-cyclase, a 229-bp leader intron was identified, and an unspliced mRNA with this leader intron existed dominantly in cDNA pool of all samples.
The effectiveness of bumble bees, Bombus impatiens, and honey bees, Apis mellifera, on the pollination of watermelon, Citrullus lanatus (Thunb.), was compared at the individual bee level. Correlations between the number of bee visits a flower received and the resultant seed set and fruit abortion rates were established. Using `Royal Jubilee' watermelon, B-impatiens-visited flowers resulted in higher seed sets than A. mellifera when compared at equal bee visit numbers. This difference between bee types was highly significant. With respect to fruit abortion rates, no statistical difference between bee types was detected. However, bee visit count was significant. Increasing the number of bee visits received by a flower resulted in a lower percentage of aborted fruit.
Materials and Methods
Watermelon ( Citrulluslanatus Matsumu. Et Nakai, cv. Tsukuba no Kaori) was grown under a plastic film greenhouse at Ishikawa Agricultural Research Center. Plants were grown 40 cm apart in rows 300 cm wide
Watermelon varieties were evaluated for hollowheart (HH) at Bradenton, Leesburg, and Quincy, Fla. HH varied with location, variety, and season. Among icebox varieties, `Sugar Baby', `Baby Gray', and SSDL had less HH than `Tiger Baby', `Minilee', and `Mickylee'. Among standard varieties, `Sangria' and `Jubilee II' had less HH than `Crimson Sweet' and `Royal Sweet'. In 1990, HH was more severe at Quincy than at Bradenton or Leesburg, but the ranking of seedless entries was similar among the locations. HMX 7928, `Nova', `Tycoon', and `Millionaire' had least HH, and `Jack of Hearts', `Ssupersweet 4073', `Ssupersweet 5344', and `King of Hearts' had the most HH. `Jack of Hearts' and `Crimson Sweet' fruit were cut and evaluated in Spring 1993 at 5, 12, 19, 26, and 33 days after anthesis. Incidence of HH was low in 5- and 12-day-old fruit, increased in fruit that were 19 or 26 days old, and did not increase in older fruit. About one-third of fruit from both varieties had some HH. Among the seven entries in another test in Spring 1993, `Tri-X-313' had the least HH and `Crimson Sweet', `Jack of Hearts', and `Jubilee II' had the most.
. Ezura, H. 1999 Race identification in powdery mildew ( Sphaerotheca fuliginea ) on melon ( Cucumis melo ) in Japan Plant Breed. 118 259 262 Kristková, E. Lebeda, A. 2000 Citrulluslanatus —A potential host of powdery mildew in the Czech Republic Cucurbit
A linkage map was constructed of the watermelon genome using F2 and F2:3 populations segregating for resistance to race 1 and 2 of Fusarium oxysporum f. sp. niveum (FON 1 and 2). Sixty-four percent of the RAPD primers used in the parents and F1 detected polymorphism. In the F2, 143 polymorphic bands were scored, 60% of which exhibited the expected 3:1 segregation ratio. A 113 cM linkage map was constructed using Mapmaker version 3 and LOD of 4. DNA pools of Fusarium wilt resistant or susceptible F2:3 lines were created and bulked segregant analysis was used to detect molecular markers linked to FON 1 or FON 2 resistance. Four individuals per line were used to confirm linkages and construct an F2:3 linkage map. One large linkage group was detected in both generations. A large proportion of the RAPD and SSR markers were unlinked and many showed segregation distortion. Single-factor ANOVA for each pairwise combination of marker locus and resistance or morphological trait was conducted. RAPD markers with putative linkages to FON 1 and FON 2 and several morphological traits were detected.