Biochemical and histological contributions to textural changes in watermelon fruit modulated by grafting

- Flesh firmness was higher in heterografts than homeografts and non-grafted controls.
- Firmness correlated positively with cell density and negatively with cell size.
- Cell wall material and water-insoluble pectins were highest in heterografts.
- Cell wall neutral sugars and membrane permeability were not linked to firmness.

Increased watermelon fruit flesh firmness is systematically incurred with grafting on Cucurbita hybrid rootstocks (heterografting). Possible differences in mesocarp cell wall constitution and histology between heterografted, homeografted (self-grafted) and non-grafted watermelon were examined, as well as their contributions to fruit texture. Firmness correlated positively (r = 0.78, p < 0.001) with cell density (cells mm−2) which was higher in heterografts (5.83) than homeografts (4.64) and non-grafted controls (4.69). Mean cell size was smallest in heterografts and correlated negatively (r = −0.75, p < 0.001) with firmness. Cell wall material, particularly the water-insoluble pectin fractions associated with firmness, were highest in heterografts. No associations with firmness were found for cell wall neutral sugars and membrane permeability. Higher parenchymatic cell density with higher content of alcohol insoluble residue and more abundant water-insoluble pectin fractions underscore enhanced firmness in heterografts. Possible implication of osmolytes in rootstock-mediated cell pressure regulation warrants further investigation.

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