Functional roles of plant growth-promoting rhizobacteria in ungrafted and grafted watermelons under various deficit irrigation strategies

Drought is the central abiotic stressor limiting agricultural sustainability in arid and semi-arid regions. Plant growth-promoting rhizobacteria (PGPR) can play a key role in drought resistance in many vegetables including watermelon. This study examined the effects of two distinct PGPRs (Pseudarthrobacter polychromogenes and Paenarthrobacter aurescens) secreting 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase on yield, yield traits, fruit quality, and crop water stress index (CWSI) in watermelons of ungrafted and grafted (onto wild watermelon rootstock) under various deficit irrigation methods over the course of 2-year field trials in 2023 and 2024. This study results showed that increased water stress caused significant fruit yield loss. In both grafted and ungrafted plants, continuous severe water stress (CSWS) conditions led to yield loss of approximately 40 % by comparison to non-water stress (NWS). PGPRs produced varied results in grafted and ungrafted plants. Under CSWS, the wild watermelon rootstock interacted with rhizobacteria, particularly P. aurescens, increasing yield by up to 20 % compared to plants without PGPR. In grafted plants, P. aurescens increased irrigation water use efficiency (IWUE) by approximately 21 % under CSWS irrigation strategy. PGPRs were more effective under severe water stress than mild and moderate water stress, and they significantly protected watermelon from the damaging effects of water stress. PGPRs did not significantly affect the physical properties of watermelon fruits, including weight, width, length, rind thickness, and flesh color. However, they increased the soluble solids content in ungrafted plants and total phenolic compounds in grafted plants. Conversely, P. aurescens reduced the sucrose content in the fruit, resulting in a drop in the total sugar content of fruits produced in the presence of this bacterial species. The mean CWSI values of watermelon varied considerably under different deficit irrigation strategies, and increasing water stress caused CWSI values to rise. CWSI threshold were found higher in grafted plants (0.20) than in ungrafted plants (0). This study provides a theoretical basis for rootstock-PGPR synergistic regulation for efficient water-saving in cultivation of watermelon in water stressed environments.