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Jennifer L. Waters and Stephen R. King

Carotenoids are important phytochemical components of our diet and have gained recent attention as important nutritive compounds found mainly in fruits and vegetables with red, orange, and yellow hues. Lycopene is often cited as being inversely correlated with the occurrence of various cancers, in lowering rates of cardiovascular disease, and improving other various other immune responses. Antioxidant activity, specifically oxidative radical quenching power, is the putative rationale for carotenoids' involvement in disease risk reduction. It is unlikely, however, that carotenoid content and antioxidant capacity are directly correlated in the whole food since there are other antioxidants present in watermelon, such as various free amino acids. A total measure of antioxidant potential may prove to be a useful tool for measuring watermelon nutritional value and implementing pursuant breeding goals. One assay that has gained recent popularity is the oxygen radical absorbance capacity (ORAC) assay. ORAC includes two assays that separate lipophylic and hydrophilic antioxidants. Currently, most ORAC protocols use isolated compounds or freeze-dried fruit or vegetable samples. Here, the application of a standard hexane-type extraction method, which is more amenable to whole food carotenoid-containing samples, was investigated as a candidate extraction method for the ORAC assay. Variants of this method as well as of the standard ORAC extraction were compared for extraction efficiency. Finally, ORAC values were correlated with carotenoid content and shown to hold a loose negative correlation. Possible reasons for this are considered and discussed.

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Stephen R. King, Angela R. Davis, Wenge Liu and Amnon Levi

The primary purpose of grafting vegetables worldwide has been to provide resistance to soilborne diseases. The potential loss of methyl bromide as a soil fumigant combined with pathogen resistance to commonly used pesticides will make resistance to soilborne pathogens even more important in the future. The major disease problems addressed by grafting include fusarium wilt, bacterial wilt, verticillium wilt, monosporascus root rot, and nematodes. Grafting has also been shown in some instances to increase tolerance to foliar fungal diseases, viruses, and insects. If the area devoted to grafting increases in the future, there will likely be a shift in the soil microbial environment that could lead to the development of new diseases or changes in the pathogen population of current diseases. This shift in pathogen populations could lead to the development of new diseases or the re-emergence of previously controlled diseases. Although grafting has been demonstrated to control many common diseases, the ultimate success will likely depend on how well we monitor for changes in pathogen populations and other unexpected consequences.

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Angela R. Davis, Penelope Perkins-Veazie, Richard Hassell, Amnon Levi, Stephen R. King and Xingping Zhang

Vegetable grafting began in the 1920s using resistant rootstock to control soilborne diseases. This process is now common in Asia, parts of Europe, and the Middle East. In Japan and Korea, most of the cucurbits and tomatoes (Lycopersicon esculentum Mill.) grown are grafted. This practice is rare in the United States, and there have been few experiments to determine optimal grafting production practices for different geographical and climatic regions in America. This is beginning to change as a result of the phase out of methyl bromide. The U.S. cucurbit and tomato industries are evaluating grafting as a viable option for disease control. Because reports indicate that type of rootstock alters yield and quality attributes of the scion fruit, some seed companies are investigating grafting as a means to improve quality. It has been reported that pH, flavor, sugar, color, carotenoid content, and texture can be affected by grafting and the type of rootstock used. Reports vary on whether grafting effects are advantageous or deleterious, but it is usually agreed that the rootstock/scion combination must be carefully chosen for optimal fruit quality. Additionally, it is important to study rootstock/scion combinations under multiple climatic and geographic conditions because many rootstocks have optimal temperature and moisture ranges. This report gives an overview of the effect of grafting on vegetable quality.

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Haejeen Bang, Angela R. Davis, Sunggil Kim, Daniel I. Leskovar and Stephen R. King

Two loci, C and i-C, were previously reported to determine flesh colors between canary yellow and red watermelon (Citrullus lanatus). Recently, lycopene β-cyclase (LCYB) was found as a color determinant gene for canary yellow (C) and a codominant cleaved amplified polymorphic sequence (CAPS) marker was developed to identify canary yellow and red alleles. The inhibitor of canary yellow (i-C), as reported in a previous work, was not detected in our original family derived from a cross between canary yellow and red parents. To identify additional genetic determinants such as i-C, we prepared a new family using ‘Yellow Doll’ (canary yellow) and ‘Sweet Princess’ (red), which was reported to carry the inhibitor gene i-C as parents. A new distinct class of flesh color, pale yellow, was identified in the progeny from the new canary yellow × red cross. The predominant carotenoid in canary yellow and pale yellow phenotypes was neoxanthin, followed by violaxanthin and neochrome; pale yellow contained less total carotenoids, but had more minor carotenoids compared with canary yellow. The chi-square goodness-of-fit test indicated that there are two genes involved in determining flesh color among canary yellow, pale yellow, and red, but the segregation pattern did not fit the pattern as reported for an i-C gene. When the genotype of the family ‘Yellow Doll’ × ‘Sweet Princess’ was analyzed with our LCYB CAPS marker, the flesh color of every individual perfectly cosegregated with the marker. The new pale yellow phenotype also cosegregated with the marker linked to the C allele, indicating that the recessive py phenotype (pale yellow) must carry at least one of the C alleles for expression. Therefore, we propose to designate py for a pale yellow determinant along with C as a canary yellow determinant. A homozygous recessive py gene resulted in pale yellow flesh color in the presence of a dominant C.

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Angela R. Davis, Amnon Levi, Sungil Kim, Stephen R. King and Alvaro Hernandez

RNA isolation from ripe fruit can be complicated by high concentrations of sugar and water. These sugars interfere with RNA extraction often resulting in low RNA quality and quantities, and high water concentrations dilute the RNA, making isolation difficult. We report a simple but novel method by which the majority of the excess sugar and water in mature fruit of tomato (Lycopersicon esculentum Mill.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], and muskmelon (Cucumis melo L.) can be easily removed from tissue before RNA extraction. This method produced quality RNA in a shorter time than the currently accepted method for fruit tissue RNA isolation and does not require liquid nitrogen or a freeze dryer.

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Angela R. Davis, Todd C. Wehner, Amnon Levi and Stephen R. King

Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past nine years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.

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Angela R. Davis, Todd C. Wehner, Amnon Levi and Stephen R. King

Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past 9 years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.

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Haejeen Bang, Sunggil Kim, Daniel I. Leskovar, Angela Davis and Stephen R. King

Gene identification and characterization can be utilized for the identification of respective functions and their relationship to flesh color inheritance. Phytoene synthase (PSY), which converts two molecules of GGPP into phytoene, is the first committed step of the pathway. Previous phylogenetic analysis of PSY has indicated that PSY duplication is common in Poaceae, but rare in dicots. Degenerate PCR and RACE were used for PSY cloning. Three members of PSY gene family (PSY-A, PSY-B and PSY-C) were identified. PSY-A shared higher identity with PSY-C than PSY-B. PSYC shared 96% identity with melon PSY. PSY-C also showed a high homology with tomato PSY1, even higher than PSY-A and PSY-B. It showed a similar gene expression pattern, so we propose that PSY-C is a homologue to PSY1. RT-PCR analysis indicated that PSY-B has a different transcriptional behavior from PSY-A, similar to tomato PSY2. Therefore, PSY genes appear to be under different regulatory mechanisms. Deduced protein sequence of PSY1 or PSY2 between species has higher homology than between PSY1 and PSY2 within species. Phylogenetic analysis indicated that watermelon PSY gene family is very distantly related. Watermelon and carrot PSY gene families did not appear to cluster as closely as in Poaceae or tomato. This indicates that watermelon and carrot PSY genes are not conserved as much as PSY in tomato or Poaceae. There was no particular pattern in phylogenetic relationship of dicots. Poaceae PSY genes showed a clustering into a PSY1 group and PSY2 group. PSY duplication in watermelon provides additional evidence that PSY duplication may be a common phenomenon in dicots. They are likely to be duplicated evolutionarily a long time ago, possibly even prior to the evolution of monocot and dicot divergence.

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Angela R. Davis, Charles L. Webber III, Wenge Liu, Penelope Perkins-Veazie, Amnon Levi and Stephen King

High-quality, high-phytonutrient watermelons [Citrullus lanatus (Thumb.), Matsum & Nakai] have strong market opportunities. To produce highly nutritious fruit in a seedless triploid market, the nature of phytonutrient accumulation as affected by ploidy must be understood. The present study performed on six field-grown watermelon diploid (2n) inbred lines, their induced autotetraploids (4n), and autotriploids (3n) determined the importance of ploidy on quality and nutritional content. Lycopene, total soluble solids (TSS), L-citrulline (hereafter referred to as citrulline), glutathione (GSH), weight, width, and length were measured in ripe fruit from one location. Our findings contradict some previous manuscripts, which did not use diploid inbred lines and their induced autoploidy relatives. Of the traits we analyzed that did not have a family-by-ploidy interaction (citrulline, GSH, weight, and width), we determined citrulline levels were not significantly affected by ploidy in five of six families nor was there a significant correlation when all family’s citrulline values were averaged. Previous studies on field-grown fruit that did not use autoploidy lines suggested triploid fruit had more citrulline than diploid fruit. GSH was higher in autotriploid than in diploid or autotetraploid (95.0 vs. 66.9 or 66.7 μg·g−1 GSH, respectively). Additionally, we found an association with higher GSH in larger fruit. Autotriploid fruit were, in general, heavier and wider than diploid and autotetraploid fruit, and autotetraploid fruit were generally smaller than diploid fruit. Of the traits we analyzed that had a family by ploidy interaction (lycopene, TSS, and length), we determined within four families, ploidy affected lycopene concentration, but whether this interaction is positive or negative was family-dependent. These data suggest the triploid state alone does not give fruit higher lycopene concentrations. The mean TSS was higher in autotetraploid than in autotriploid, which was again higher than in diploid fruit averaged across families (10.5%, 10.2%, and 9.5% TSS, respectively); there was a family × ploidy interaction so the significance of this increase is affected by the triploid’s parents. Lycopene and TSS had a slight positive correlation. Four of six families showed no statistical correlation between ploidy and length, and although mean length across family demonstrated smaller tetraploid fruit, the family-by-ploidy interaction demonstrates that this observation is family-dependent. Length and width correlate well with weight when combining data for all ploidy levels and when analyzing each ploidy separately. Length correlates more closely with width in autotriploid fruit than in diploid or autotetraploid fruit.