Foundations of Yield Improvement in Watermelon

Taken as a group, the studies indicate the presence of heterosis in watermelon and the importance of GCA High yield is a major goal for watermelon (Citrullus lanatus(Thunb.) in the choice of parents for hybrid production. Ferreira Matsum. & Nakai) breeders. The objective of this study was to mea- sure yield in a diverse set of watermelon cultivars to identify high- et al. (2002) substantiated these conclusions, testing yielding germplasm for use in breeding programs. Phenotypic varia- sevenintercrossingpopulationswithevaluationofrecip- tion for fruit yield in a diverse set of 80 watermelon cultivars was rocal crosses. There were significant GCA, SCA, and re- studiedinthefieldinNorthCarolina.Yieldwasevaluatedinreplicated ciprocal combination effects, along with additive effects experiments of three environments (combinations of 2 yr and two lo- for all yield traits, except for the number of days to first cations), and two to four replications per environment. Plots were har- female flower and number of seeds per fruit. A second vested one to two times, depending on the average maturity of the study evaluated GCA and SCA for tetraploid females fruits at the first harvest. The highest yield overall was obtained from crossed with diploid males for the production of triploid 'Mountain Hoosier' and 'Starbrite'. Some of the new, elite hybrid cul- seeds (Souza et al., 2002). This study confirmed a higher tivars were in the top-yielding group, but there were old, inbred culti- magnitude of GCA effects than SCA effects and strong vars in the top group as well. Consistent and significant yield differ- ences among the 80 cultivars across environments indicates genetic additive effects for yield components, except for earli- variation for the trait. In addition, high-yielding cultivars for use in nessand somequalitative indexes(i.e., hollowheartinci- breeding programs were identified. Watermelon breeders interested dence). Today, watermelon breeders are less interested in developing new, high-yielding cultivars should make use of top in studying heterotic effects and combining ability as performers in this study in their breeding programs. reasons to prefer hybrids to inbreds for cultivar release. Hybrids have proven their advantage for protection of valuable parent lines. Furthermore, seedless cultivars are H igh yield is a major goal for watermelon breeders in high demand and can be produced only as triploid hy- (Mohr,1986).Earliereffortsinwatermelonbreed- brids. However, in the future it might be possible to de- ing involved development of new cultivars of different velop transgenic diploid seedless watermelons. In that typeswithhighfruitqualityandearlymaturityinthelate case, the question of the advantage in using heterotic 1800s.By1900,'Angeleno','Chilean','FloridaFavorite', hybrids vs. inbred cultivars will still be important. 'Georgia Rattlesnake', 'Cole Early', 'Kleckley Sweet', Overall, watermelon yield in the USA has been in- and other open-pollinated cultivars had been on the mar- creasing during the last 4 yr from 24 000 Mg ha1 in 1998 ket for many years (Whitaker and Jagger, 1937). In the to29 000Mgha1in2002(USDA-ARS,2003).Partofthe 20th century, high-yielding cultivars became a major increase in yield might be due to more reliable produc- goal for public and private breeders. Hybrids were pop- tion practices and to the availability of more effective ular among private breeders for protection of intellec- pesticides (Maynard, 2001). The impact of environmental tual property and because of the results of many studies, factors such as irrigation or general water availability on mainly in the 1950s and 1960s, showing heterosis in yield was important in contrasting inbred cultivars vs. watermelon. The studies measured heterosis as well as hybrids in Florida in 1985. The hybrids outyielded inbred general (GCA) and specific (SCA) combining ability in cultivars only in irrigated fields, while in dry conditions watermelon (Brar and Sidhu, 1977; Brar and Sukhija, yield was the same for both groups, although fruit quality 1977; Nandpuri et al., 1974, 1975; Sidhu and Brar, 1977, was higher among the inbred cultivars (Rhodes, 1985). 1985; Sidhu et al., 1977a, 1977b). Major problems with Many watermelon yield trials are run each year in the those studies were that heterosis was inconsistent across USA,andoftenfewdifferencesamongtheexperimental experiments, and that results were based on diallel or entries in the trial are observed. Our question was whether top crosses of elite inbreds, not on a random set of lines that was due to a lack of genetic variation for yield in from a population. More recent studies of the effects of the crop species, or a lack of genetic variation for yield reciprocal crosses on yield components in watermelon among the new experimental entries being tested. Ge- have been contradictory (Gill and Kumar, 1988; Rajen- netic diversity among currently grown watermelon culti- dran and Thamburaj, 1993; Sachan and Nath, 1976). vars in the USA appears to be narrow, with many de- Often, the experiments included only a small number rived from 'Allsweet'. Therefore, a diverseset ofobsolete (Nmax 10) of nonrandomly chosen elite cultivars as inbred cultivars that do not trace to Allsweet and that parents, so the results are valid only for those specific represents as wide an array of cultivars as possible was crosses and are not generally applicable. included in this study. The objective of this study was tomeasureyieldinadiversesetofwatermeloncultivars.