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.

Carbohydrate source of peach [Prunus persica (L.) Batsch] embryo culture media affects embryo growth and survival. The first objective of this study was to determine the effect of five carbohydrates (fructose, glucose, maltose, sorbitol, and sucrose) in Woody Plant Medium (WPM) on the germination and survival of peach embryos in vitro. Fructose, glucose, maltose, and sucrose in WPM resulted in better embryo germination and survival than sorbitol. Fructose (2% and 3%) produced greater survival than all other carbohydrates tested in smaller embryos (<10% ovule dry weight). However, sucrose was better than all other carbohydrates tested in the larger embryos (≥10% ovule dry weight). In addition, large embryos (>10% ovule dry weight) on fructose at 1% combined with glucose, maltose, sorbitol, or sucrose at 1% had equivalent or higher survival than did those on either 1% or 2% sucrose in conjunction with the same carbohydrates. Embryo survival on different carbohydrates varied with genotype. The second objective of this study was to determine the effect of three levels of MES buffer (0.0 mm, 4.5 mm, and 9.0 mm) on medium pH stability and embryo survival. MES buffer at 0.0 mm and 4.5 mm concentration produced significantly better embryo survival than 9.0 mm. The pH stability was better at MES 9.0 mm, however survival decreased significantly. Chemical name used: [2-(N-morpholino)-ethane sulphonic acid] (MES)

Embryo culture techniques are employed to germinate seed of early ripening peach and nectarine [Prunus persica (L.) Batsch] cultivars. Generally, the embryos in these genotypes do not mature by the time the fruit matures, thus rendering normal stratification procedures ineffective. In 1998 and 1999, immature embryos from multiple peach genotypes were cultured in an embryo rescue medium (Woody Plant Medium, 3% sucrose, 0.065% agar) at 5 °C for 45 days in the dark. Embryos were then placed under lights at either a cool-temperature (18 °C in 1999 and 20 °C in 1998) or a warm-temperature (30 °C in 1999 and 28 °C in 1998) treatment with a photoperiod of 12 hours for germination and initial growth. After 2-4 weeks, embryos were rated for germination, root number, and top growth. The embryos incubated at the cool-temperature regime not only had better germination, but also had a higher rate of greenhouse survival.

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Sucrose, fructose, total sugars and soluble solids are major factors in determining mature melon fruit sweetness. Bulked segregant analysis was utilized to detect RAPD markers associated with QTL for sucrose, total sugars and soluble solids in an F2 population from the ananas melon cross of Deltex (high sugars) × TGR1551 (low sugars). Sucrose, glucose, fructose and total sugar data were obtained from 108 F2 plants by means of HPLC. Clear separations for sucrose, total sugars and soluble solids between Deltex and TGR1551 were observed, whereas slight differences for glucose and fructose were found. Continuous distributions for sucrose, total sugars and soluble solids were observed in the F2 population indicating quantitative inheritance for the sweetness traits. A significant negative correlation was observed between sucrose and glucose (r = -25) or fructose (r = -0.31). A significant positive correlation was noted between sucrose and total sugars (r = 0.80) or soluble solids (r = 0.64). Three low and high DNA bulk pairs for sucrose, total sugars and soluble solids were developed. A total of 360 primers were used to simultaneously screen between the low and high bulks, and between Deltex and TGR1551. Sixty-eight RAPD markers were polymorphic for the low and high bulks. Of the 68 markers, 24 were found to be significantly associated with sucrose, total sugars or soluble solids on the basis of single-factor ANOVA. Marker OM15.550 was consistently associated with QTL affecting sucrose, glucose, fructose, total sugars and soluble solids, and accounted for 7% to 25% of the phenotypic variation for the traits. These markers associated with the sugar synthesis QTL could be useful to transfer these genes into a low sugar cultivar to enhance the fruit sweetness.