Predicting Hollow Heart Incidence in Triploid Watermelon (Citrullus lanatus)

Abstract

In triploid (seedless) watermelon [Citrullus lanatus var. lanatus (Thunb. Matsum. and Nakai)], hollow heart (HH) is a disorder that is expressed as a crack in the center of the fruit that expands to an open cavity. Although HH incidence and severity is part of a screening process for marketable watermelon fruit during cultigen evaluations, HH incidence is highly variable with growing season, even when the best cultural practices are used. Placental tissue firmness is also measured because firmness is related to the marketability of watermelon and may be related to HH. Genetic and environmental factors can influence watermelon HH development, including plant genetics, pollen amount and viability, pollinator activity, and temperature and rainfall fluctuations. We used seedless watermelon cultigen evaluation data collected over 3 years (2012–14) to determine the relationship between germplasm HH and tissue firmness. Transplanted watermelon representing 30 to 44 cultigens per year were grown at the Central Crops Research Station, Clayton, NC, and interplanted with pollenizers ‘Ace’ and/or ‘SP-6’. Harvested fruit were cut length-wise and rated for HH incidence and severity. Flesh firmness was determined by a handheld penetrometer at five locations in the flesh (stem end, top side, ground spot, blossom end, and heart). A common cultigen subset, consisting of 13 cultigens that were grown in all three experiments, was used for analysis of HH severity and incidence, and placental firmness. The presence of HH was negatively correlated with tissue firmness in both the large multiyear cultigen set (R2 = −0.32; P = 0.0001) and the common cultigen set (R2 = −0.78; P = 0.0001). Cultigens with lower watermelon tissue firmness values had higher HH incidence and severity. By using multiyear cultigen studies and logistic regression, we were able to detect trends for cultigen susceptibility to this highly variable disorder. Using logistic regression, the probability of HH development was highest for ‘Bold Ruler’, ‘Liberty’, and ‘Affirmed’, and lowest for ‘Maxima’ and ‘Captivation’. The identification of cultigens with a tendency for higher or lower rates of HH will be useful for further research of the causes of HH. Measurements of placental flesh firmness may be useful indicators of susceptible cultigens.

Cell Wall Polysaccharide Composition of Grafted ‘Liberty’ Watermelon With Reduced Incidence of Hollow Heart Defect

“…Yuan et al (2016) speculated that increased flesh firmness was due to the increased root size and plant vigor associated with the interspecific hybrid rootstock as mature watermelon fruit in their study also had higher amounts of calcium, magnesium, iron, and zinc. Watermelon cultivars with lower tissue firmness are generally more susceptible to internal fruit disorders (Trandel et al, 2020a). Increased watermelon firmness has been related to more cells/unit area in the placental tissue (Kano, 1993;Trandel et al, 2021).…”

“…Samples were ground in 80% ethanol and further extracted with acetone and methanol to yield AIR. Total pectin methods follow Trandel et al (2020b) .…”

“…Triploid watermelon do not produce viable pollen, thereby requiring diploid (seeded) pollenizer plants at correct ratios (25–33% of field) to avoid the onset of HH ( Freeman et al, 2007 ). Inadequate pollination from reduced bee visits or limited pollen release due to cold or wet weather also causes HH in watermelon ( Johnson, 2015 ; Trandel et al, 2020a ). HH disorder is thought to develop at the epidermal layers within the carpels, ovule and septum (mesocarp or placental tissue) ( Kano, 1993 ).…”

“…This internal fruit disorder has been associated with textural changes including decreased tissue firmness and cell density (e.g., increased cell size) ( Kano, 1993 ). In a multi-year, large cultivar study of triploid selections, cultivars that produced fruit with lower tissue firmness had a higher incidence of HH ( Trandel et al, 2020a ). However, grafting of triploid watermelon onto squash rootstock shows promise for both increasing watermelon flesh firmness and reducing the incidence of HH disorder ( Trandel et al, 2020b ).…”

“…This suggests that differences in watermelon textural characteristics may be from de-esterified pectins and matrix glycans ( Brummell and Harpster, 2001 ). Since fruit firmness has been associated with incidence of HH ( Trandel et al, 2020a ) and differences in pectic substances, a closer look into the cell wall components of watermelon fruit is warranted.…”