Relief from Salt Stress by Plant Growth-Promoting Bacteria in Hydroponic Leaf Lettuce (Lactuca sativa L.)

Indonesia is a large archipelago with a long coastline. Climate change and saltwater intrusion both endanger freshwater availability as a source of drinking water and agricultural resources. Therefore, the saline and brackish water demand for irrigated farm goods should be considered a method for producing salt-resistant crop products. This study aimed to monitor Indole-3-acetic acid (IAA) and biofilm by selected bacterial isolates and its impact on the growth of lettuce (Lactuca sativa L.) during salt stress. Several isolates are employed such as Streptomyces hygroscopicus GGF4-i18, Streptomyces sp. AB8, Micrococcus luteus, Serratia marcescens MBC1, and Streptomyces hygroscopicus subsp. Jinggangensis InaCC A497. A 1.0 percent L-tryptophan is used as a precursor of IAA. The impact of IAA and biofilm produced by bacteria was demonstrated by inoculating bacterial isolates on Lactuca sativa (lettuce) plants in the hydroponic system. The nutrient film technique (NFT) is the selected hydroponic cultivation technique. The result shows that Streptomyces genera were able to maintain higher IAA hormone production within a week. On the other hand, Serratia marcescens MBC1 and Micrococcus luteus tended to have stable IAA levels. The administration of MBC1 strain bacteria increased lettuce growth as measured by the number of leaves and leaf length, compared to isolate AB8, i18, and Micrococcus luteus. However, the growth was not greater than the control treatment, which grew in fresh water, when it was observed in root length. The ability of bacterial isolates to tolerate salt levels is needed, in addition to the capacity to produce IAA to help plants survive and grow in brackish water media. In nature, the interaction between plants and microbes does not take place alone. In this study, the treatment was still given in the form of a bacterial monoculture to plant. In the future, it is necessary to develop how the treatment is given in the form of a consortium and consider other growth-supporting factors. In addition, the use of simple technology in the form of adding bacterial inoculum to a simple NTF reactor is easy to apply to developing countries.