Muskmelon exhibits a wide variability for vegetative traits, fruit morphology, sweetness, and climatic adaptations for yield and fruit quality (Li et al., 2006). Previous reports have attributed the lack of widely adapted cultivars of muskmelon to its extreme sensitivity to environmental variations and genotype-by-environment (G × E) interactions (Dhakare and More, 2008; Yadav and Ram, 2010).
In field evaluation trials, the performance of a genotype is determined by the genotypic main effect (G), the environment main effect (E), and the interaction between these two (G × E) (Yan et al., 2001). The term “stability” is used to characterize a genotype that shows a consistent performance across tested environments for a trait of interest. A few G × E interaction studies have been conducted in muskmelon that focused on the stability of yield performance over temporal environments (years, seasons, or both) (Dhakare and More, 2008). However, in a plant spacing by cultivar study (Kultur et al., 2001) and a generation mean analysis study (Zalapa et al., 2006) conducted at two locations (Arlington and Hancock, WI), muskmelon genotypes were reported to vary for their fruit yield as well as other yield component traits. Thus, muskmelon genotypes exhibit differential responses to both temporal and spatial environmental variation (Yadav and Ram, 2010). In spite of the importance of G × E interactions in cultivar selection, very limited information is available on quality traits (Paris et al., 2008), though some reports on yield and component traits from India are available (Dhakare and More, 2008; Yadav and Ram, 2010).
Sweetness, flavor, texture and phytonutrient levels of β-carotene and vitamin C in flesh tissue are the determinants of fruit quality in muskmelon (Lester, 2008). Orange-fleshed muskmelons are known for their unique flavor and high sugar levels. Increased awareness about the benefits of healthful foods have earned melons a reputation as an excellent source of health promoting phytonutrients (Lester, 2006), though consumer preference is still largely determined by sweetness, aroma, and texture. Thus, selecting cultivars for high productivity, acceptable sweetness, flesh color, firmness, sensory traits, and a high amount of β-carotene and vitamin C has been a great challenge for muskmelon breeders.
Soluble solids content (SSC) is a reliable indicator of quality that has been routinely used by breeders to screen germplasm for sweetness (Villanueva, 2004). Li et al. (2006) noted that soluble sugars account for more than 97% of the SSC in maturing muskmelon fruits, with sucrose accounting for nearly 50% of all sugars. As per the U.S. Department of Agriculture (USDA) standards, a high-quality muskmelon fruit should have a SSC ranging from 9% to 11% (Kultur et al., 2001). Muskmelon SSC varies with climate (Bouwkamp et al., 1978), location (Kultur et al., 2001), genotype, and crop management practices (Bhella, 1985). Edmonds and McFall (1927) observed that soluble solids were higher in a year with sunny and moderately cool conditions than in a year with cloudy days with moderately high temperatures and frequent rains. At Salisbury, MD, Bouwkamp et al. (1978) reported that high respiration rates due to increased fruit temperature through enhanced accumulated solar radiation 6 d before harvest might have resulted in decreased SSC in nine of 16 cultivars studied; whereas rainfall reduced the SSC in three cultivars.
Orange-fleshed muskmelons, cantaloupes (Cucumis melo L., reticulatus group), and honey dews (Cucumis melo L., inodorus group) are excellent sources of carotenoids (Pitrat, 2016). Crosby et al. (2007) reported that melon cultivars TAM Uvalde and Mission had more than 36 μg·g−1 of total carotenoids. Lester and Eischen (1996) observed a significant impact of genotype-by-environment interactions on β-carotene content of melon fruit and reported a decrease in β-carotene for the genotype Cruiser when grown in a fine sandy soil (15.1 μg·g−1) compared with silty clay loam (18.2 μg·g−1), while the content was similar for the genotype Primo (18.1 μg·g−1) in both soils. Thus, the amount of β-carotene in fruits may vary according to the genotype, environment (i.e., climate and soil conditions), and G × E interactions.
Muskmelon ranks in the top three among the nine most consumed fresh fruits in the United States for supplying daily requirements of ascorbic acid. Park et al. (2006) reported that accumulation of ascorbic acid is sensitive to genotype-by-environment interaction. This trend was evident from differential response of this trait when grown at different locations, such as in Weslaco in the lower Rio Grande Valley and Uvalde in the Wintergarden region of Texas. All genotypes tested produced higher levels of ascorbic acid at Uvalde than at Weslaco. The ascorbic acid content ranged from less than 15 μg·g−1 in many wild types, and some commercial cantaloupes and honeydews to 250–350 μg·g−1 in genotypes such as TAM Dulce, TAM Uvalde, Mission, and TXC 2015 (Crosby et al., 2007). From the previous studies, it can be recognized that melon quality is very sensitive to both temporal and spatial environmental variations. Thus, genotypes rich in phytonutrients and with stability over diverse environments would be of great value for breeding new, high-quality melon cultivars.
Various approaches are available to analyze multienvironment genotype evaluation data (Kang, 2002). In the G × E studies, breeders used to focus only on yield traits, due to the complexity of data analysis (Yan and Kang, 2002). However, the advances in statistical models and analysis software have facilitated multitrait analysis (Yan et al., 2000). GGE Biplot software gives a graphical representation of G and G × E effects simultaneously and thus allows researchers to concentrate mainly on the typically obscure G and G × E components. GGE Biplot can be used to evaluate the average yield and stability of a genotype as compared with others in the trial, rank environments based on ability to differentiate genotype performance, distinguish a genotype having best performance in a particular environment, and identify mega-environments within a target region based on the specifically adapted genotypes (Yan and Kang, 2002).
Due to the ubiquitous presence of G × E interaction effects and the availability of wide variation in melons for the traits of interest, we hypothesized that some genotypes would give differential responses for yield and quality traits across the different environments. The specific aim of the study was to evaluate nine melon genotypes, including five commercial cultivars and four elite breeding lines, grown in nine environments comprising three locations and 3 years. Estimates of G × E interaction should be helpful to identify breeding lines and cultivars that would be high yielding and possess good quality, including enhanced levels of phytochemicals, over different environments.
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