**5. Conclusions**

Fruit morphometric characteristics were affected least by the choice of rootstock. However, the more vigorous interspecific rootstock delivered higher and more stable yield across seasons than gourd rootstock, which highlights the superior crop performance imparted by interspecific rootstocks, provided the absence of physiological rootstock–scion incompatibility. Interspecific rootstock also induced thicker rind and higher pulp firmness, both of which improve shelf-life. Pulp firmness was higher in seedless triploid cultivars compared to diploid ones, which might relate to higher density of parenchymatic cells. Current results indicate that scion genotype but not ploidy level is the most determining factor for carotenoid composition of watermelon fruit. Moreover, interspecific rootstock induced higher fruit lycopene content than gourd rootstock. Ambient postharvest storage for 10 days increased pulp lycopene, corroborating previous reports that lycopene synthesis continues and lycopene levels peak postharvest although watermelon is a non-climacteric fruit. Diploid cultivars attained overall higher TSS and total sugars than seedless cultivars; storage, however, reduced both, which renders the postharvest handling of triploid cultivars particularly prone to loss of sensory quality. Reducing sugars decreased during storage while sucrose increased. This observation in conjunction with the postharvest accumulation of lycopene sheds new light onto the postharvest physiology of watermelon, otherwise considered a non-climacteric fruit. Minimal reduction in sugars by interspecific rootstock was related to higher fruit load and possible slight delay in ripening compared to gourd, further supported by the higher hexoses-to-sucrose ratio observed with the former. The marginal rootstock effect on sugars renders interspecific rootstock advantageous over gourd on account of higher yield, firmness and lycopene content.

**Author Contributions:** Conceptualization, G.A.S., M.C.K. and Y.R.; methodology, G.A.S. and M.C.K.; formal analysis, G.A.S. and M.C.K.; investigation, G.A.S. and M.C.K.; writing—review and editing, G.A.S., M.C.K. and Y.R.; supervision, M.C.K. and Y.R.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
