Interactive effects of rootstock and rhizobacteria on fruit yield, evapotranspiration, and the crop water stress index (CWSI) in watermelon under water deficit stress

Aims: In this study, we investigated the activities of ACC (1-aminocyclopropane-1-carboxylic acid) deaminase-secreting plant growth-promoting rhizobacteria (PGPRs) in watermelon grafted onto different rootstocks exposed to water stress. Methods: The effects of two PGPR species (P. aurescens-B<inf>1</inf> and P. polychromogenes-B<inf>2</inf>) on yield, actual evapotranspiration (ETa), and CWSI values of the Crimson Tide (CT) watermelon cultivar grafted onto different rootstocks [A<inf>0</inf>, ungrafted; A<inf>1</inf>, CT grafted onto wild watermelon; A<inf>2</inf>, CT grafted onto hybrid TZ-148] were investigated for two years (2023 and 2024) under different water deficit conditions. Results: While rhizobacteria did not significantly affect ETa in ungrafted plants, they increased ETa by about 30 mm in plants grafted on wild watermelon plants under severe water stress compared to their corresponding controls. The findings indicated that watermelon should be irrigated at CWSI values close to zero in semiarid areas to obtain maximum yield, and a 0.10 unit increase in the CWSI decreased fruit yield by about 10%. PGPRs significantly increased yield in plants grafted onto wild watermelon (A<inf>1</inf>) under stress-free (full irrigation), mild, and severe water stress conditions. Conclusions: The yield response factor (ky) of the plants grafted on wild watermelon was less than 1.00, and the significant increase in WUE (water use efficiency) under water stress indicated that wild watermelon might be a drought-tolerant potential watermelon rootstock. Thus, the selection of appropriate rootstocks and PGPRs in water-limited environments is a key step in supporting sweet watermelons under water stress.