Plant-Beneficial Streptomyces thermocarboxydus S3 Mitigates Heat Stress in Hydroponically Grown Lettuce

Climate change presents a significant threat to global agriculture by increasing abiotic stresses that negatively impact crop yields. Lettuce (Lactuca sativa), a cool-season crop, is particularly vulnerable to heat stress, which accelerates metabolism and increases respiration rates beyond photosynthetic capacity, ultimately leading to growth and yield reduction. In hydroponic systems, elevated temperatures further impair plant development by altering nutrient solubility and availability, resulting in deficiencies. Microbial bioinoculants offer a sustainable and ecofriendly strategy to mitigate heat stress and enhance plant performance in crop production. Actinobacteria, in particular, are recognized for their plant growth-promoting properties. This study evaluates the effectiveness of Streptomyces thermocarboxydus S3 in enhancing hydroponic lettuce growth under heat stress conditions. Inoculation with S. thermocarboxydus S3 significantly improved key growth parameters, including fresh weight, dry weight, number of leaves, and chlorophyll content. The strain also induced the accumulation of osmoprotective compounds, such as proline and total soluble sugar (TSS), contributing to cellular protection under thermal stress Additionally, S. thermocarboxydus S3 reduced hydrogen peroxide (H₂O₂) levels, indicating a potential role in oxidative stress mitigation and activation of plant defense responses. Root colonization assays confirmed the strain's ability to establish itself in the lettuce roots, supporting its applicability for long-term application. These findings highlight S. thermocarboxydus S3 as a promising bioinoculant for promoting hydroponic lettuce growth under heat stress, offering a sustainable approach to crop production in the context of changing climate.