Cell Wall Polysaccharides in Grafted and Non-grafted ‘Liberty’ Watermelon with Hollow Heart

Marlee Trandel, Ph.D. Final Seminar

Thursday, March 19, 2020, 10:00 am

Join Zoom Meeting​: https://ncsu.zoom.us/j/164381948  Meeting ID: 164 381 948

Under the direction of Dr. Penny Perkins-Veazie, Chair of Advisory Committee

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ABSTRACT

Triploid (seedless) watermelon [Citrullus lanatus (Thunb. Mastum and Nakai)] are prone to hollow heart (HH), a disorder expressed as a crack expanding to an open cavity in the fruit.  Watermelon genetics, environmental conditions, and pollen deficit can induce HH, and fruit with lower tissue firmness are more susceptible to HH.  Grafting watermelon to cucurbit rootstocks (RS) can increase tissue firmness, possibly from increased cell density, changes in cell wall pectins, or a combination of these.  Cell wall pectins play an important role in cell wall strength, internal flesh quality and plant defense.

Because HH is sporadic, affecting 0 to 65% of fruit in a season, watermelon cultivar data from a previous three-year trial was subjected to logistic regression to identify genetic sources of HH.  Using ‘Liberty’ plus reducing diploid pollen in the field, HH was induced in a first field trial. Rootstock (RS) cultivars grafted to ‘Liberty’ were used in a second field study to follow HH. Interspecific squash hybrid (C. maxima x C. moschata) RS ‘Carnivor’ and ‘Kazako’ bottle gourd (Lageneria siceraria) RS ‘Emphasis’ and non-grafted plants were used as controls.  Flesh firmness was increased by 1 N for fruit from interspecific RS.  Using confocal microscopy, cells from fruit with HH had increased area, a decreased number of cells (less cell density), and areas of cell wall breakage. Cell density was increased in fruit grafted onto ‘Carnivor’ compared to non-grafted fruit.

In a final field study with non-grafted ‘Liberty’ or grafted to ‘Carnivor’, HH incidence was 34% less and tissue firmness was increased by 3 N in grafted fruit.  HH incidence did not affect firmness and compositional changes in fruit were not significantly different with graft or HH. Placental fruit tissue was used for extraction of alcohol insoluble residues which were then subjected to total pectin sequential extraction or were reduced, methylated, hydrolyzed and acetylated followed by GC-MS analysis.  Among pectic fractions, water soluble fractions were lowest and carbonate soluble fractions were reduced with HH. Twelve monosaccharides were identified in graft and HH watermelon. Glucose and galactose were highest in concentrations followed by xylose and arabinose. Meso-erythritol, fucose, rhamnose, arabinose and 2-deox-d-glucose were highest in fruit with HH at 0.32, 2.36, 3.38, 21.29 and 0.55 μg·mg-1, respectively. Ribose was highest in grafted fruit with no HH and non-graft with HH (1.27 and 1.20 μg·mg-1).  Allose was highest in fruit from grafted plants (0.64 μg·mg-1) and graft with no HH indicated the highest allose concentration at 0.80 μg·mg-1.  Degree of methylation or uronic acid concentrations did not differ.  Using methylation linkage assembly, 35 partially methylated alditol acetates were detected in watermelon.

Limiting the amount of pollen with a susceptible watermelon cultivar (‘Liberty’) could be used to induce HH.  Grafting ‘Liberty’ to an interspecific squash hybrid RS reduced HH incidence, increased tissue firmness and cell density.  The interaction of RS and scion graft appears to protect pectic polysaccharides from HH by increasing tissue firmness and potentially increasing cell wall adherence due to changes in monosaccharides.  Watermelon with HH had higher meso-erythritol, fucose, rhamnose, 2-deoxy-d-glucose and arabinose. Fucose and rhamnose sugars are minor constituents and relate to RG I and II side branches. Arabinose is found in arabinogalacturonan, a pectin that embeds in the cellulose/hemicellulosic matrix.  Ribose, found only in the side branch of RG II, was highest in graft with no HH and non-graft with HH. Non-grafted fruit tended to have a higher degree of cell wall degradation and lower amounts of uronic acid concentration. Grafting watermelon to interspecific rootstocks may protect the cell wall from disassembly by increasing fruit tissue firmness and changing linkages of monomeric cell wall building blocks.