Effect of Different Rootstocks on the Salt Stress Tolerance in Watermelon Seedlings

The selection of salt-tolerant watermelon rootstocks requires the evaluation of its underlying mechanism. The present study investigated the effect of applying 200 mmol.L-1 NaCl on various physiological and biochemical parameters of Citrullus lanatus 'Jingxin No.2' (abbreviated as W) seedlings grafted onto W, Cucurbita moschata 'Quanneng Tiejia' (P1), 'Kaijia No.1' (P2), and Lagenaria siceraria 'Hanzhen No.3' (G), which were grown hydroponically. All the measured growth parameters were significantly inhibited by salt stress, with W/P2 exhibiting superior growth. Salt stress increased Na+ uptake, which in turn disrupted K+/Na+ homeostasis in all grafting combinations. The photosynthetic capacity, chlorophyll concentration, and photochemical efficiency of photosystem II of all grafts significantly decreased with salt stress. However, W grafted onto G and P2 showed higher resistance than W grafted onto W and P1. Electrolyte leakage (EL) superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase activity were significantly affected by both grafting and salt stress. Minimum EL was observed in W/P2, which also exhibited superior antioxidative capacity. Physiological and biochemical assessment indicated that W grafted onto the P2 rootstock displayed the greatest salt tolerance. Plants grafted onto salt-resistant P2 accumulated less Na+ and reactive oxygen species (ROS) and exhibited superior growth, photosynthesis, and ROS-scavenging capacity compared to those grafted onto the other rootstocks.