2019 ASHS Annual Conference

Hollow heart (HH), a disorder where a void appears in the placental tissue in triploid watermelon, develops randomly with growing season. This internal defect negatively plagues growers and industry as fruit with moderate to severe cases cause load rejection. Previous research has indicated increased tissue density (the number and size of cells) in watermelon cultivars increases fruit firmness and decreases incidence of internal disorders. The goal of this work was to induce HH formation in triploid watermelon in the field, assess tissue density via confocal micrographs, and characterize cell wall polysaccharide content in fruit with and without HH. ‘Liberty’, a watermelon cultivar susceptible to HH, was transplanted in Clayton NC with diploid pollenizers (SP-6) planted at 6, 9 and 12 m down the tiers to induce HH. This system yielded 52% HH. Fruit were harvested, cut longitudinally and resistance to puncture measured in heart tissue. Roughly 100 g of heart tissue was saved for confocal microscopy and cell wall polysaccharide assays. The number and cross sectional area size (μm³) of cells in control fruit and fruit with moderate to severe HH were counted in confocal micrographs using ImageJ software. Frozen watermelon tissue was homogenized and washed with acetone, ethanol and methanol to yield alcohol insoluble residues (AIR). Water soluble and water insoluble pectins were isolate from AIR. Alcohol insoluble residues were also reduced, derivatized, hydrolyzed, methylated for linkage assembly, and acetylated for a GC-MS based compositional analysis of monosaccharide building blocks. While the number of fruit cells were not affected by incidence or severity of HH, fruit with moderate HH had the largest cells at 102,873 μm³ (P < 0.001). In control (no HH) watermelon, highest levels of galactose, glucose, xylose, arabinose, and rhamnose cell wall monosaccharides are expected. We hypothesize that fruit with HH have less complex pectins causing separation in fruit cells and a decrease in cell density leading to the disorder. Therefore, we predict that the AIR pectins from watermelon flesh in fruit found with no HH will contain higher amounts of xylose and arabinose monosaccharides and total pectins.