Watermelon Quality Traits as Affected by Ploidy

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.