Grafting and Its Effect on Watermelon Tissue Firmness and Incidence of Hollow Heart

— Written By Mary Lorscheider
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Marlee A. Trandel is a PhD Candidate in the Department of Horticultural Sciences from North Carolina State University. She completed her BS in Animal Science from the College of Agricultural Sciences and BA in Chemistry from the College of Biochemistry and Chemistry in 2014 and her MS in Horticulture Sciences in the College of Agricultural Sciences in 2016 from Southern Illinois University Carbondale. She is currently studying for her PhD in postharvest physiology under the guidance of Prof. Dr. Penelope Perkins-Veazie. Ms. Trandel has completed all of her grafting watermelon field research and is currently focusing on finishing up laboratory work and writing her thesis.

In the U.S., seedless (triploid) watermelon cultivars make up 95% of the market. A disorder found mostly in seedless watermelon genotypes is hollow heart (HH), which causes an internal split or void in the placental tissue. Currently there is no definitive cause of HH but researchers have suggested multiple factors induce the disorder, such as watermelon genetics, pollination/pollen viability, flower bloom time, decreased fruit tissue firmness and environmental stressors.

Grafting watermelon onto squash rootstocks (RS) has provided a means of combating soil borne diseases/nematodes and for changing the fruit quality attributes of tissue firmness, rind thickness and phytonutrient qualities. Grafting onto interspecific hybrid RS has been shown to increase tissue firmness in harvested fruit and may decrease the susceptibility of HH formation in triploid watermelon. Fruit with larger cells generally have lower tissue firmness and tissue density is related to the number of fruit cells in the heart tissue and the subsequent cell size.

Marlee’s research follows grafting and its effect on fruit tissue firmness and incidence of hollow heart. She used a susceptible triploid watermelon cultivar, Liberty, grafted onto interspecific hybrid RS (Cucurbita maxima x C. moshata) ‘Carnivor’ and ‘Kazako’, bottlegourd rootstock (Laginaria sciaria) ‘Emphasis’ and non-graft (control). Diploid pollenizers were limited by planting 6, 9 and 12 m apart in row (tier 1, 2 and 3). Fruit were harvested 64 to 78 days after transplant. Fruit were taken to the lab and fruit weight, length x width (cm) was determined. Fruit were cut longitudinally and assessed for the incidence and severity (1 to 5 scale, 1= no or minor and 5= severe) of HH. Rind and heart firmness (N), pH, soluble solids, lycopene, arginine and citrulline were measured. Subsamples were analyzed with a Zeiss LSM 880 Confocal workstation for cell number and cell size (mm2).

Distance from the pollinizer (6, 9 or 12 m) did not affect the incidence or severity of HH and data were pooled to determine RS effects. The total percentage of fruit with HH differed with RS. Fruit from ‘Carnivor’ and ‘Kazako’ had the lowest incidence of HH, with 32 and 38%, respectively. Fruit weight and volume were lowest for ‘Carnivor’ compared to other RS. Fruit volume was significantly increased in all watermelons with HH compared to no HH fruit (7.53 and 6.96 L), respectively. Heart tissue firmness was lower in fruit with HH compared to those without HH, averaging 16N and 18N, respectively. Heart tissue firmness in watermelons without HH was highest in fruit from interspecific squash hybrids ‘Carnivor’ and ‘Kazako’ compared to non-graft or ‘Emphasis’ RS. Soluble solids content was lowest in fruit from ‘Carnivor’ fruit and was slightly reduced in fruit with HH. No differences were seen in compositional assays (e.g., phytonutrients). Cell number did not differ significantly among RS or with level of HH within RS. Cell number was lowest in fruit from ‘Carnivor’ with moderate to severe HH and in non-graft fruit with moderate HH, and was highest in fruit from ‘Emphasis’ with severe HH. Cell size was largest in fruit from ‘Emphasis’ (mean of 108.2 x 103 mm2). Fruit with severe HH from ‘Carnivor’ and non-graft RS had larger cell areas, yet cell area was smaller in fruit from ‘Emphasis’. Cell number was inversely correlated to cell size (R2= -0.8598, p<0.001). Marlee is currently exploring the cell wall architecture, specifically pectin type and amount, in graft and non-grafted fruit and the incidence of HH.

Ms. Marlee Trandel won an ISHS Young Minds for the best oral presentation: Grafting Effects on Fruit Quality I at ISVG2019 in Charlotte, NC in July, 2019.

View a YouTube video with details of Marlee’s preparation for this award winning presentation.