Co-application of microbial and non-microbial seaweed-based biostimulants improves salinity tolerance in young watermelon plants through ion homeostasis and protective metabolite responses.

Abstract

Salinity is a major constraint to watermelon seedling establishment because it reduces growth, disrupts ion balance, damages membranes, and impairs physiological and biochemical function. Biostimulants improve plant performance under abiotic stress, but the individual and combined effects of microbial and seaweed-based biostimulants on watermelon syoung plants under salinity remain insufficiently understood. Therefore, this study evaluated the effects of the microbial biostimulant (Spectrum-DS), the seaweed-based biostimulant (Kelpak), and their combined application (Kelpak+DS) on the growth, physiology, biochemistry, and mineral composition of young watermelon plants (Citrullus lanatus cv. Jubilee) under salt stress. The experiment was conducted under controlled environment (growth chambers and greenhouses) in two independent cycles using a completely randomized 4 × 3 factorial design, consisting of four biostimulant treatments (Control, Kelpak, Spectrum-DS, and Kelpak+DS) and three salinity levels (0, 30, and 60mM NaCl). In each cycle, five biological replicates were evaluated for each treatment combination. High salinity at 60mM NaCl significantly reduced total dry weight, stem dry weight, shoot length, shoot dry weight, root dry weight, and leaf number compared with the 0mM control. The combined Kelpak+DS treatment mitigated negative effect of salinity, increasing total dry weight by 23.8%, stem dry weight by 33.8%, shoot length by 33.3%, shoot dry weight by 26.3%, root dry weight by 22.2%, and leaf number by 10.2% relative to untreated control. Leaf area was also improved by Kelpak+DSrespectively. Under 60mM salt stress, Kelpak+DS increased total chlorophyll and carotenoids compared to the 60mM control in addition to improving ion homeostasis by reducing Na concentration, Na/K ratio, while increasing K concentration. PCA and correlation analyses further showed that improved growth under Kelpak+DS was associated with greater pigment retention, lower physiological injury, and better mineral balance under salinity. Overall, the combined application of Spectrum-DS and Kelpak was more effective than the individual treatments in mitigating salinity stress during early watermelon growth. These findings suggest that combined microbial and seaweed-based biostimulant application may be a useful strategy for improving watermelon seedling establishment and young plants under saline conditions.