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Amnon Levi, Claude E. Thomas, M. Newman, O.U. K. Reddy, X. Zhang, and Y. Xu

Wide phenotypic diversity exists among American heirloom cultivars of watermelon (Citrullus lanatus var. lanatus). However, in published studies, low or no polymorphism was revealed among those heirlooms using isozyme or randomly amplified polymorphic DNA (RAPD) markers. In this study, experiments with inter-simple sequence repeat (ISSR) [also known as simple sequence repeat-(SSR-) anchored primers] and amplified fragment-length polymorphism (AFLP) markers produced high polymorphisms among watermelon heirloom cultivars. ISSR (111) and AFLP (118) markers (229 total) identified 80.2% to 97.8% genetic similarity among heirloom cultivars. The phylogenetic relations based on ISSR and AFLP markers are highly consistent with the parental records available for some of the heirloom cultivars, providing confidence in the dendogram constructed for heirlooms based on similarity values. As compared with RAPD markers, ISSRs and AFLPs are highly effective in differentiating among watermelon cultivars or elite lines with limited genetic diversity.

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S. Alan Walters

Honey bees (Apis mellifera L.) are important pollinators of triploid watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai]. Pistillate (or female) watermelon flowers require multiple honey bee or other wild bee visitations after visiting staminate (or male) flowers for fruit set, and pollination is even more of a concern in triploid watermelon production since staminate flowers contain mostly nonviable pollen. Six honey bee visitation treatments—1) no visitation control, 2) two visits, 3) four visits, 4) eight visits, 5) 16 visits, and 6) open-pollinated control—were evaluated to determine the effectiveness of honey bee pollination on `Millionaire' triploid watermelon fruit set, yield, and quality utilizing `Crimson Sweet' at a 33% pollenizer frequency. `Millionaire' quality characters (hollow heart disorder or percent soluble solids) did not differ (P > 0.05) between honey bee pollination treatments. The open-pollinated control provided the highest fruit set rate (80%) and the greatest triploid watermelon numbers and weights per plot compared to all other honey bee visitation treatments. Fruit set, and fruit numbers and weights per plot increased linearly as number of honey bee visits to pistillate flowers increased from 0 (no visit control) to the open-pollinated control (about 24 visits). This study indicated that between 16 and 24 honey bee visits are required to achieve maximum triploid watermelon fruit set and yields at a 33% pollenizer frequency, which is twice the number of honey bee visits required by seeded watermelons to achieve similar results. This is probably due to many honey bees visiting staminate triploid watermelon flowers (that are in close proximity) before visiting pistillate flowers thus providing mostly nonviable pollen that is useless for fruit set and development. Therefore, more honey bee visits to pistillate triploid watermelon flowers would be required to achieve maximum fruit set and subsequent development compared to seeded watermelons.

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S.J. Locascio and G.J. Hochmuth

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] were grown with three rates each of lime, gypsum, and K during two seasons to evaluate their effects on fruit production and mineral concentration. The first experimental site was a recently cleared Sparr fine sand with an initial water pH of 5.0 and Mehlich I extractable K of 8 mg·kg-1 (very low) and 20 mg·kg-1 Ca (very low). The second site was a virgin Pomona fine sand with a water pH of 4.8, 28 mg·kg-1 K (low), and 612 mg·kg-1 Ca (high). `Crimson Sweet' fruit yields were reduced 10% with an increase in lime rate from 0 to 4.48 t·ha-1 in the first season. In the second season, lime rate had no effect on yield. In both seasons, fruit yields were reduced 14% with an increase in Ca from gypsum from 0 to 1.12 t·ha-1. On the soil testing very low in K, yield increased with an increase in K rate from 90 to 224 kg·ha-1 with no lime or gypsum. On the soil testing low in K, greatest yields were obtained with 90 kg·ha-1 K with no lime and gypsum. Application of lime and gypsum increased Ca and decreased K in seedlings but not consistently in older leaf and fruit tissues. An increase in K application increased leaf K in the first season but not in the second. Fruit firmness and soluble solids content were not consistently affected by treatment during the two seasons. Thus, on soils low in toxic elements (Mn and Al) such as used in this study, watermelon will grow well and tolerate a wide range of soil pH values without additional Ca from lime or gypsum.

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Carl E. Motsenbocker and Ramon A. Arancibia

Triploid watermelon (Citrullus lanatus), commonly called seedless watermelon, is increasing in popularity and market share. The optimum in-row spacing of triploid watermelon has not been studied previously. Triploid watermelon `Crimson Jewel' and `Honeyheart' were grown with drip-irrigation and black plastic mulch at 1-, 2-, 4-, 6-, and 8-ft (0.3-, 0.6-, 1.2-, 1.8-, and 2.4-m) in-row spacings in 1996 and 2-, 3-, 4-, 5-, 6-, 7-, and 8-ft. (0.6-, 0.9-, 1.2-, 1.5-, 1.8-, 2.1-, and 2.4-m) spacings in 1997 to determine the effect of in-row plant spacing on fruit yield. Marketable yield of `Crimson Jewel' was not affected by in-row spacing while narrower in-row spacing resulted in greater `Honeyheart' yield both years. For both cultivars, narrower spacing resulted in the highest number of fruit per acre, but primarily more extrasmall and small fruit. Fruit number per plant, fruit weight per plant, and individual fruit weight were higher at wider spacings, and yield per acre was lower. The data suggest that triploid watermelon yield, fruit weight and number can be adjusted by in-row spacing. Narrower in-row spacing can maximize yields, depending on the specific grower's cultural practices. In wider in-row spacings, the yield of medium and large fruit is maintained with a subsequent decrease in extra small and small fruit. Gross returns per acre were only different for farmers' market prices, not wholesale, and net returns were not significantly influenced by in-row spacing.

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Jonathan R. Schultheis, John T. Ambrose, Stephen B. Bambara, and Wyatt A. Mangum

The effectiveness of two commercial bee attractants, Bee-Scent and Beeline, for enhancing pollination of cucumber (Cucumis sativus L.) and watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] was evaluated by counting the number of bee visitations to blossoms of cucumber and watermelon and their effect(s) on fruit quality, yield, and crop profitability. In 1989, Bee-Scent was tested in a commercial pickling cucumber field. In 1990, watermelon plots were sprayed with Bee-Scent and Beeline and compared with a nontreated control. The compounds did not improve bee visitations for either pickling cucumbers or watermelons. There was no significant improvement in cucumber or watermelon yield or monetary returns.

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Michael E. Compton and D.J. Gray

Adventitious shoots were obtained from watermelon [Citrullus lanatus (Thunb.) Matsun. & Nakai] cotyledons incubated on a modified Murashige and Skoog medium containing BA. Initial experiments comparing the effects of BA (0, 5, 10, or 20 μm) and IA4 (0, 0.5, or 5 μm) demonstrated that BA was required for adventitious shoot formation but its concentration in the medium was not critical. The addition of IAA to medium with BA increased callus production and inhibited shoot formation. However, the percentage of responding explants in the best treatment was <30%. Therefore, the manner in which cotyledon explants were prepared and seedling age at the time of explantation was examined to improve the organogenic response. The percentage of explants with shoots was improved by using explants that consisted of cotyledon bases (43%) or cotyledons cut in half longitudinally (39%). A lower percentage (16%) of cotyledons cut longitudinally into four pieces produced shoots. Explants taken from the apical half of cotyledons failed to regenerate shoots. Shoot formation was improved further by using explants from young seedlings. The percentage of explants with shoots was >90% for `Minilee', 64% for S86NE, and 50% for `Jubilee II' when explants were prepared from 5-day-old seedlings. Explants from nongerminated embryos or seedlings germinated for 10, 15, or 20 days produced fewer shoots. The effect of several cytokinins on shoot organogenesis was then examined using the optimized protocol. The percentage of explants with shoots and the number of shoots per explant were about two to four times higher when 5 to 10 μm BA was used compared to the most effective kinetin (20 μm) or thidiazuron (0.1 μm) concentration. The percentage of explants with shoots and the number of shoots per explant were greater for diploid (57% and 2.2, respectively) than for triploid (22% and 0.6, respectively) or tetraploid (20% and 0.8, respectively) lines. Chemical names used: N -(phenylmethyl)-1 H -purin-6-amine (BA); 6-furfurylaminopurine (kinetin); N -phenyl-N' -1,2,3-thiadiazol-5-ylurea (thidiazuron); 1 H -indole3-acetic acid (IAA).

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Aimin Liu, Joyce G. Latimer, and Robert E. Wilkinson

The influence of two fungicides—captan and thiram—on growth and 45Ca absorption by roots of `Starbrite' watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] seedlings was investigated. Unilateral application of Ca+2 and Al in agar induced curvature in roots from untreated and pretreated seeds. In untreated seeds, PCMBS inhibited Ca+2- and Al-induced root curvature by 82% and 92%, respectively. In commercially pretreated seeds (captan + thiram), PCMBS inhibited Ca+2- but not Al-induced root curvature. Captan or thiram also inhibited Ca+2- or Al-induced root curvature, and the effects of captan and thiram on root curvature were additive. Serial concentration (0, 0.01, 0.1, 1, 10, or 100 mg·liter-1) tests indicated that captan inhibited 45Ca absorption the most at 100 mg·liter-1, whereas thiram inhibited 45Ca absorption the most at 0.01 mg·liter-1. The effects of captan and thiram on 45Ca absorption were statistically additive. Thiram seemed to influence Ca+2 uptake by affecting exofacial sulfhydryl groups (a mode of action similar to that of PCMBS). DTT reversed the inhibitory effect of thiram on 45Ca absorption by 34% but did not reverse the effect of captan. A field test showed that acidic soil (pH 4.55) reduced leaf number; leaf, stem, shoot, and whole-plant dry weights; and stem length of 15-day-old seedlings. Although there was no difference in root dry weights or root: shoot ratios of plants from pretreated and untreated seeds planted in soil at pH 6.26, planting commercially pretreated seeds in acidic soil produced plants with greater root dry weights and root: shoot dry weight ratios than those from untreated seeds. Seedlings showed a greater response to seed treatment in early growth stages. Captan and thiram may have influenced growth characteristics by inhibiting Al uptake of seedlings planted in acidic soil. To our knowledge, this is the first report on the influence of the fungicides captan and thiram on mineral ion uptake in roots. Chemical names used: p-Chloromercuribenzenesulfonic acid (PCMBS), dithiothreitol (DTT), N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide (captan), tetramethylthiuram disulfide (thiram).

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D.S. NeSmith

Transplanting generally results in more rapid stand establishment than direct seeding for cucurbit crops. A 2-year field study was conducted to examine the pattern of rooting of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nak.] following usage of different planting methods, and to determine subsequent effects on crop yield. Root length was assessed by obtaining soil cores three times per growing season to a depth of 75 cm. Transplanted watermelons generally had greater root length density in the upper 30 cm of soil 4 to 7 weeks after planting (WAP). However, by 11 to 12 WAP root distribution was similar over the entire 75 cm soil profile for the two planting methods. Total marketable yields were comparable for direct seeded and transplanted watermelons during 1995, but transplanted watermelon yield exceeded direct seeded yield by 40% in 1996. In both years, 90% to 100% of the marketable yield of transplanted watermelons was obtained at the first harvest, compared to 0% to 55% for direct seeded watermelons. These findings suggest that rapid root proliferation of transplanted watermelons may be an important factor in their earlier establishment and increased early yields as compared to direct seeded watermelons.

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Amnon Levi, Claude E. Thomas, Xingping Zhang, Tarek Joobeur, Ralph A. Dean, Todd C. Wehner, and Bruce R. Carle

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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A. Levi, C.E. Thomas, T. Trebitsh, A. Salman, J. King, J. Karalius, M. Newman, O.U.K. Reddy, Y. Xu, and X. Zhang

Seventy-one amplified fragment length polymorphism (AFLP), 93 sequence related amplified polymorphism (SRAP), and 14 simple sequence repeat (SSR) markers were used to extend an initial genetic linkage map for watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. The initial map was based on 151 randomly amplified polymorphic DNA (RAPD) and 30 and inter-simple sequence repeat (ISSR) markers. A testcross population previously used for mapping of RAPD and ISSR markers was used in this study: {plant accession Griffin 14113 [C. lanatus var. citroide (L.H. Bailey) Mansf.] × the watermelon cultivar New Hampshire Midget (C. lanatus var. lanatus)} × PI 386015 [C. colocynthis (L.) Schrad.]. The linkage map contains 360 DNA markers distributed on 19 linkage groups, and covers a genetic distance of 1976 cM with an average distance of 5.8 cM between two markers. A genomic DNA clone representing 1-amino-cyclopropane-1-carboxylic acid (ACC-) synthase gene, involved in ethylene biosynthesis, was also mapped. As in previous mapping studies for watermelon, a large number of AFLP and SRAP markers were skewed away from the 1:1 segregation ratio, and had to be excluded from the final mapping analysis. The stringent mapping criteria (JoinMap 3.0 mapping program) produced linkage groups with marker order consistent with those reported in previous mapping study for watermelon.