Combined Spirulina and Sulfate-Reducing Bacteria Enhance Drought Tolerance in Lettuce, with Sulfate-Reducing Bacteria Excelling Under Severe Stress

Drought stress hinders plant growth by reducing water availability, leading to stunted plants growth, lower photosynthesis rates, and decreased yields. This research explored the impact of the combination of Spirulina and sulfate-reducing bacteria on the growth and resilience of lettuce plants under three drought levels (80%, 60%, and 40%). Overall, drought reduced germination percentages, but at 40% level, Spirulina significantly improved germination rates. At higher drought levels, Spirulina and sulfate-reducing bacteria positively influenced germination compared to the control, with SRB showing the most pronounced effect. Root growth increased significantly under 60% drought, though no root elongation under different stress levels was impacted. Leaf area decreased with increasing drought, but sulfate-reducing bacteria significantly enhanced it, especially at 80% drought. Control plants’ relative water content decreased with increasing drought severity. However, the combination of Spirulina and sulfate-reducing bacteria at 60% drought mitigated this effect. Notably, under 60% and 80% drought, hydrogen peroxide levels increased, though the Spirulina and sulfate-reducing bacteria combined consistently elevated hydrogen peroxide levels in both 60% and 80% conditions. Superoxide dismutase activity rose by 80% in non-treated plants, while Spirulina and sulfate-reducing bacteria individually enhanced superoxide dismutase activity across moderate and high drought. Catalase activity decreased at 80% drought in control plants; however, an increase was observed with sulfate-reducing bacteria in 80% stress level. The Fv/Fm ratio and PiABS declined as drought intensified, but sulfate-reducing bacteria improved these parameters at both 60% and 80% stress levels. ABS/RC and ET0/RC ratios responded positively to sulfate-reducing bacteria under severe drought. These findings suggest that while Spirulina and sulfate-reducing bacteria enhance drought tolerance in lettuce, sulfate-reducing bacteria are especially effective under higher drought stress conditions.