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Gene E. Lester, John L. Jifon and Gordon Rogers

Muskmelonfruit[Cucumis melo L. (Retiulatus Goup)] sugar content is related to potassium (K)-mediated phloem loading and unloading of sucrose into the fruit. During fruit growth and maturation, soil fertility is often inadequate (due to poor root uptake) to satisfy the demand for K. Potassium uptake also competes with the uptake of Ca and Mg, two essential minerals needed for melon fruit membrane structure, function and postharvest shelf-life. Supplemental foliar-applied K could alleviate this problem especially during the critical fruit growth/maturation period. We conducted experiments to determine the effects of timing of supplemental foliar K applications on fruit quality and health attributes of orange-flesh muskmelon `Cruiser'. Plants were grown in a greenhouse and fertilized with a regular soil-applied N–P–K fertilizer throughout the study. Entire plants, including the fruit were sprayed with a solution of a novel glycine amino acid-complexed potassium (Potassium Metalosate, 24% K), diluted to 4.0 mL·L-1, 3 to 5 d after anthesis (fruit set) and up to 3 to 5 d prior to abscission (full-slip). Three sets of plants were either sprayed weekly, or bi-weekly or not sprayed (control). Fruit from plants receiving supplemental foliar K matured on average 2 days earlier, and had significantly higher fruit K concentrations, soluble solids, total sugars, ascorbic acid (vitamin C), beta-carotene, and were firmer than fruit from control plants. In general, there were few differences in fruit quality aspects between bi-weekly or weekly treatments. The data demonstrate that fruit quality and marketability as well as some of the developmentally induced K deficiency effects can be alleviated through foliar nutrition.

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Gene E. Lester, Luis Saucedo Arias and Miguel Gomez-Lim

Muskmelon [Cucumis melo L. (Reticulatus Group)] fruit sugar content is the single most important consumer preference attribute. During fruit ripening, sucrose accumulates when soluble acid invertase (AI) activity is less then sucrose phosphate synthase (SPS) activity. To genetically heighten fruit sugar content, knowledge of sugar accumulation during fruit development in conjunction with AI and SPS enzyme activities and their peptide immunodetection profiles, is needed. Two netted muskmelon cultivars, Valley Gold a high sugar accumulator, and North Star a low sugar accumulator, with identical maturity indices were assayed for fruit sugars, AI and SPS activity, and immunodetection of AI and SPS polypeptides 2, 5, 10, 15, 20, 25, 30, 35, or 40 (abscission) days after anthesis (DAA). Both cultivars, grown in the Fall, 1998 and Spring, 1999, showed similar total sugar accumulation profiles. Total sugars increased 1.5 fold, from 2 through 5 DAA, then remained unchanged until 30 DAA. From 30 DAA until abscission, total sugar content increased, with `Valley Gold' accumulating significantly more than `North Star'. During both seasons, sucrose was detected at 2 DAA, which coincided with SPS activity higher than AI activity, at 5 through 25 DAA, no sucrose was detected which coincided with SPS activity less than AI activity. At 30 DAA when SPS activity was greater than AI activity, increased sucrose accumulation occurred. `Valley Gold' at abscission had higher total sugar content and SPS activity, and lower AI activity than `North Star'. `North Star' had AI isoforms at 75, 52, 38, and 25 kDa (ku) that generally decreased with maturation, although the isoform at 52 ku remained detectable up to anthesis (40 DAA). `Valley Gold' had the same four AI isoforms, all decreased with maturation and became undetectable by 20 DAA. Both `Valley Gold' and `North Star' had one SPS band at 58 ku that increased with DAA, and coincided with SPS activity. `Valley Gold' had a more intense SPS polypeptide band at abscission than `North Star'. Thus, netted muskmelon fruit sugar accumulation may be increased, either by genetic manipulation or by selecting for cultivars with a specific number of down-regulated AI isoforms, and higher SPS activity during fruit ripening.

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D. Mark Hodges, Gene E. Lester, Robert D. Meyer, Vivian E. Willmets and Michele L. Elliot

Consumption of phytochemicals has been associated with reduced risks of human health dysfunctions such as cancers and heart disease. Such information has led to increased sales of fruits and vegetables. For example, in the United States, an estimated 23% increase in melon consumption (up to 13.2 lbs/capita/annum) has been recorded over 16 years. However, some health issues have been attributed to cantaloupe due to bacteria such as Salmonella attaching to inaccessible sites, such as the rind netting. Honeydew melons do not have a netted rind. The purpose of this study was to compare concentrations of antioxidants between cantaloupe and both green- and orange-fleshed honeydew melons during 14 days of storage to determine if orange-fleshed honeydew melon would represent a feasible alterative to cantaloupe to the increasingly health/food safety-conscious consumer. Cantaloupe (`Cruiser'; C), green-fleshed Honeydew (`HoneyBrew'; HB), and orange-fleshed Honeydew (`OrangeDew'; OD) melons were harvested in Texas at the beginning and at the end of the production season. β-carotene content was highest in OD, followed by C; no β-carotene was detected in HB. β-carotene levels did not change during storage. Phenolic levels increased in all three melon species during storage, whereas total ascorbate content declined in OD and in early harvest HB. Ascorbate peroxidase activities were lowest in OD, perhaps due to the lower ascorbate levels; little difference between the melon species in activities of the other ascorbate-associated enzymes were observed. Based on the phytochemicals measured in this study, choosing non-netted OD over netted C in order to reduce potential exposure to pathogens would not represent a less healthy food choice.

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Jonathan W. Sinclair, Soon O. Park, Gene E. Lester, Kil Sun Yoo and Kevin M. Crosby

Our objectives were to identify randomly amplified polymorphic DNA (RAPD) markers associated with quantitative trait loci (QTL) for sucrose, total soluble solids (TSS), and sucrose percentage of total sugars (SPTS) using bulked segregant analysis in an F2 population from the melon (Cucumis melo L.) cross of `TAM Dulce' (high sugars) × TGR1551 (low sugars) in a greenhouse experiment, and confirm the associations of RAPD markers with QTL for these sugar traits in an F2 population from the different cross of `Deltex' (high sugars) × TGR1551 in a field experiment. Continuous distributions for sucrose, TSS, and SPTS were observed in the F2 populations indicating quantitative inheritance for the traits. Significant positive correlations were found between sucrose and TSS or SPTS. Nine RAPD markers were detected to be significantly associated with QTL for sucrose in the F2 population of the `TAM Dulce' × TGR1551 cross in the greenhouse based on simple linear regression. Five unlinked markers associated with QTL were significant in a stepwise multiple regression analysis where the full model explained 39% of the total phenotypic variation for sucrose. Five and seven of the nine RAPD markers associated with QTL for sucrose were also observed to be significantly associated with QTL for TSS and SPTS, respectively, suggesting that in this cross three sugar traits are controlled by the same QTL. Five RAPD markers were confirmed in the F2 population of the `Deltex' × TGR1551 cross in the field to be consistently associated with QTL for three sugar traits. Significant associations of andromonoecious (a) with both sucrose and TSS were consistently expressed in our populations under greenhouse and field environments. These RAPD and floral markers associated with the sugar synthesis QTL identified and confirmed here could be useful in melon breeding for improving the mature fruit sweetness.

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D. Mark Hodges, Gene E. Lester, Kathleen D. Munro and Peter M.A. Toivonen

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Juan Pablo Fernández-Trujillo, Gene E. Lester, Noelia Dos-Santos, Juan Antonio Martínez, Juan Esteva, John L. Jifon and Plácido Varó

Fruit cracking is an important disorder that can cause severe loss of marketable yield and revenue in the muskmelon (Cucumis melo) fruit industry. The physiological and environmental factors causing cracking are poorly understood. Although generally considered a physiological disorder caused by fluctuating environmental conditions, current evidence indicates that this disorder also has a genetic as well as a genotype × environment component. Certain cultivars are more susceptible than others, but wide fluctuations in irrigation, temperature, and nutrition during late fruit maturation stages appear to predispose fruit to cracking. This article summarizes the current state of our understanding of the causes of fruit splitting in muskmelons.