2018 ASHS Annual Conference

Triploid (seedless) watermelon is a fruit of great economic importance. However, seedless watermelon cultivars tend to be more susceptible than diploid (seeded) watermelon cultivars to internal defects such as hollow heart (HH). HH is characterized as an internal crack or void developing in the center of the fruit. Watermelon are required as pollinizer plants in seedless watermelon production. Incomplete pollination plays a crucial role in the development of HH. Watermelon cultivars that have less dense flesh also tend to be more prone to HH; however, the effect of rootstock (rs) on HH is unknown. Experimental objectives were to induce HH formation in watermelon, assess rootstock influence, and determine quality. Liberty, prone to HH, was the scion grafted onto four treatments; no graft (control), Carnivor and Kazako (interspecific hybrid rootstock , C. moschata x C. maxima), and Emphasis (Lageneria siceraria, bottle gourd rs). Fruit were cut longitudinally and rated for HH incidence, void airspace and tissue firmness. Firmness was measured with a FDIX penetrometer with 0.8 cm diam. probe in heart and interlocular flesh tissue spaces. Flesh firmness was increased by 1 N in fruit of interspecific rs and fruit with no HH. Sugar content (%fructose, %glucose and %sucrose) of heart tissue was strongly and positively correlated to pH. Fruit from Emphasis rs had the highest %glucose and total sugar content (p<0.05). HH incidence and graft rs had no effect on lycopene, citrulline, or arginine content. Confocal micrographs were used to count the number and area (μm²) of cells in fruit rated no, moderate, and severe HH. At least 750 cells were measured in fruit using ImageJ software. The number of cells was not affected by grafting or HH severity; the average cell area was largest for fruit of Emphasis rs with a average cell area at 110,805 μm². Fruit with moderate HH also had a larger cell area (102,873 μm²). Fruit of Emphasis rs showed decreased tissue firmness and increased in total sugar content. Although fruit with HH tended to have higher total sugar content, no significant differences were seen compared to fruit with no HH. Interspecific hybrid rs increased watermelon tissue firmness and cell density and decreased HH incidence. Fruit were more susceptible to HH when total sugars were higher and firmness/cell density were lower. HH can be field-induced in susceptible watermelon cultivars, and in contrast, HH may be reduced with rs of some interspecific hybrids.