Bottle gourd rootstock enhances salinity tolerance in watermelon by strengthening ionic homeostasis and antioxidant defense

Salinity stress severely limits watermelon (Citrullus lanatus L.) production by disrupting ionic balance, impairing photosynthesis, inducing oxidative damage, and reducing yield and fruit quality. Grafting onto salt-tolerant rootstocks is considered an effective strategy to enhance stress tolerance in horticultural crops. The field study evaluated watermelon grafted onto bottle gourd (Lagenaria siceraria L.) and squash (Cucurbita pepo L.) rootstocks under normal and saline conditions, focusing on morphological, physiological, biochemical, ionic, and yield-related traits. Salinity significantly (p < 0.05) reduced plant growth, photosynthetic performance, water relations, and yield in all treatments. However, grafting alleviated these negative effects, particularly with bottle gourd rootstock. Watermelon scion grafted onto the bottle gourd rootstock (GBG) showed improved shoot and root growth, biomass, vine length, fruit number, and fruit weight compared with squash-grafted (GS) and ungrafted plants (UG). It maintained a higher photosynthetic rate, stomatal conductance, and chlorophyll concentration, and showed better leaf water status under saline conditions. Enhanced accumulation of osmoprotectants (proline and glycine betaine), increased protein concentration, and elevated antioxidant enzyme activities [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT)] were observed in bottle gourd-grafted plants. Additionally, these plants shows low malondialdehyde (MDA) concentration, lower electrolyte leakage, restricted sodium and chloride accumulation, and a lower Na+:K+ ratio, indicating improved ionic homeostasis. Grafting watermelon onto bottle gourd rootstock (GBG) effectively enhanced salinity tolerance by strengthening osmotic adjustment, antioxidant defense mechanism, and ion regulation mechanisms. This strategy significantly improved growth, yield, and fruit quality under saline field conditions, making bottle gourd a promising rootstock for sustainable watermelon production in salt-affected soils. Bottle gourd rootstock enhanced watermelon salinity tolerance in field conditions. Grafting improved growth, yield, and fruit quality under saline stress. Grafted bottle guard (GBG) rootstock maintained ionic balance via lower Na+/K+ ratios. Antioxidant enzymes (SOD, POD, CAT) were upregulated in GBG-grafted plants. Reduced Na+, Cl−, MDA, and electrolyte leakage improved stress resilience.