Grafting watermelon onto interspecific hybrid squash reduces hollow heart disorder

M.A. Trandel, P. Perkins-Veazie, J.R. Schultheis, C.C. Gunter, E. Johannes
Hollow heart (HH) fruit disorder develops predominantly in triploid (seedless) watermelon. Incidence of HH can range from 0 to over 60%, depending upon the season, making it difficult to predict or control. Fruit genetics, pollination, and other environmental factors are thought to contribute to the onset and development of HH. Previous research indicated that watermelon cultivars with lower tissue firmness have a higher incidence of HH. Grafting a watermelon scion to a hybrid squash rootstock has been shown to increase tissue firmness and may decrease HH in susceptible cultivars. In this study, ‘Liberty’, a HH susceptible cultivar, was non-grafted or grafted onto the interspecific hybrid C. maxima × C. moschata rootstocks (RS) ‘Carnivor’ and ‘Kazako’, or to ‘Emphasis’ (Lageneria siceraria, bottle gourd RS). Fruit HH incidence was reduced by 28% when grafted onto ‘Carnivor’ RS compared to non-grafted fruit. Flesh firmness was increased by 2 N in fruit from ‘Kazako’ or ‘Carnivor’ RS. The soluble solids content was lowest in fruit from the ‘Carnivor’ RS (11.7) or from fruit with HH (11.9), respectively. Fruit pH was slightly higher in those without HH and those from ‘Emphasis’ or non-grafted RS. Lycopene, total sugars, citrulline, or arginine content among fruit from grafted plants were not significantly different. Confocal micrographs from ripe heart tissue showed fewer fruit cells in watermelon exhibiting HH from non-grafted or with ‘Carnivor’ RS plants. Cell area was largest in fruit without HH grafted to ‘Emphasis’ (117,100 μm2). Fruit without HH grafted to ‘Carnivor’ had the smallest cell area. Our results show that grafting onto interspecific hybrid RS produced fruits with higher firmness and lower incidence of HH. These results indicate grafting may be a useful technique to reduce HH disorder in seedless watermelon cultivars susceptible to HH.
Trandel, M.A., Perkins-Veazie, P., Schultheis, J.R., Gunter, C.C. and Johannes, E. (2021). Grafting watermelon onto interspecific hybrid squash reduces hollow heart disorder. Acta Hortic. 1302, 225-232
DOI: 10.17660/ActaHortic.2021.1302.30
grafting, fruit disorder, tissue firmness, confocal microscopy

Acta Horticulturae