Luffa Rootstocks Affect Fruit Quality of Grafted White-Fruited Bitter Melon

White-fruited bitter melon is a rare species of Momordica charantia L., valued for its pearl-like white pericarp. However, limited research is available regarding its supporting cultivation techniques and fruit quality. Two-season field trials were performed using ‘Kuzhongle’ as scion to evaluate the applicability of luffa rootstocks and seasonal variation in rootstock-mediated effects. Three luffa inbred lines (SS07, SS11 and SS26) were screened in spring, and the superior SS26 was further compared with the commercial rootstock ‘Yinzhen No.1’ in autumn. All rootstocks increased stem diameters, shortened internodes and promoted female flower formation in spring, but their effects on fruit quality varied among genotypes. SS26 exhibited the highest survival rate and significantly enhanced soluble solid content, water content, crude protein content and vitamin C content, while reducing crude fiber content and titratable acidity content. Seasonal comparison revealed consistent quality-modulating effects of SS26 across seasons, except for crude fiber, with a comprehensive performance comparable to the commercial rootstock. Overall, luffa rootstocks consistently modulate vegetative and flowering traits within a season, whereas their impacts on fruit quality show genotypic differences. SS26 exhibits stable and superior grafting performance, providing important material support for bitter melon rootstock breeding research.