Inoculation of watermelon weeds with Bacillus spp. to mitigate water deficit effects
Water deficit is one of the main factors limiting watermelon productivity by affecting physiological processes and plant growth. Plant growth-promoting bacteria (PGPB) have emerged as a sustainable alternative to mitigate the effects of water stress. This study evaluated the growth of watermelon plants inoculated with PGPB under different soil moisture levels (SML). The experiment was conducted in a randomized complete block design with five replications using a split-split-plot arrangement over time in a 4 × 4 × 5 factorial design. The treatments consisted of four soil moisture levels (40%, 60%, 80%, and 100% of the soil water-holding capacity), four inoculation treatments (non-inoculated control, Bacillus velezensis NB69, Bacillus subtilis NB P6.2, and co-inoculation), and five evaluation times (19, 31, 45, 59, and 72 days after sowing). Water deficit from the flowering stage onward significantly reduced watermelon plant growth. Inoculation with B. subtilis NB P6.2 mitigated the negative effects of water stress by increasing stem length and diameter, leaf number, and leaf area, regardless of soil moisture level. Co-inoculation improved root development by increasing root length and root volume under soil moisture levels between 66% and 69%, although it did not increase fresh or dry root biomass. These results indicate that specific Bacillus strains are a promising and sustainable strategy to reduce the effects of water deficit on watermelon cultivation, particularly in arid and semi-arid regions.