Employing a Plant Probiotic Actinomycete for Growth Promotion of Lettuce (Lactuca sativa L. var. longifolia) Cultivated in a Hydroponic System under Nutrient Limitation

Abstract

The consumption of lettuce is associated with an increased risk of ingesting nitrate, a naturally occurring and potentially harmful compound that can have adverse effects on human health. Hydroponic cultivation systems serve as effective tools for regulating nutrient solutions and nitrogen availability, which are essential for controlling nitrate levels. However, the techniques for reducing nutrient levels need to be appropriately calibrated based on lettuce growth responses and their interactions with the environment and growing conditions. Previous studies have demonstrated that plant probiotic actinomycetes can alleviate nutritional stress in various crops. However, there is a noticeable gap in research concerning the effects of actinomycetes on hydroponically grown lettuce, particularly under nutrient-limiting conditions. This study aimed to evaluate the effectiveness of the actinomycete Streptomyces thermocarboxydus S3 in enhancing lettuce growth in a nutrient-restricted hydroponic system. The results indicated that the detrimental effects of nutrient stress on lettuce were mitigated by the inoculation of lettuce with S. thermocarboxydus S3. This mitigation was evident in various growth parameters, including leaf count, shoot length, and the fresh and dry weights of both shoots and roots. In the presence of nutritional stress, S. thermocarboxydus S3 likely mitigated the negative effects on lettuce by reducing hydrogen peroxide levels, presumably through the synthesis of H₂O₂-scavenging enzymes. Furthermore, S. thermocarboxydus S3 successfully survived and colonized lettuce roots. Therefore, the inoculation of lettuce with S. thermocarboxydus S3 offers significant advantages for promoting lettuce growth in nutrient-limited hydroponic systems.

Keywords: hydroponic; lettuce; nutrient limitation; plant growth-promoting actinomycete; plant probiotic.

Figure 1

Growing lettuce in the nutrient film technique (NFT) system after 7 days (a) and 30 days (b).

Figure 2

Effect of surface sterilization on lettuce seeds. (a) Sterility check by incubating surface-sterilized lettuce seeds on NA media at 37 °C for 72 h; and (b) determination of seed germination after sowing surface-sterilized lettuce seeds on wet filter paper in Petri dishes at room temperature for 7 days.

Figure 3

Growth of hydroponic lettuce inoculated and uninoculated with Streptomyces thermocarboxydus S3 under (a) full-strength nutrient solution at an EC of 900 µS/cm and (b) half-strength nutrient solution at an EC of 450 µS/cm.

Figure 4

Growth promotion (leaf number (a), fresh weight (b), dry weight (c), shoot length (d), and root length (e)) of lettuce inoculated and uninoculated with Streptomyces thermocarboxydus S3 cultivated in a hydroponic system under full-strength nutrient solution at an EC of 900 µS/cm and half-strength nutrient solution at an EC of 450 µS/cm. Different lowercase letters (a, b, or c) indicate significant differences among the treatments (p < 0.05).

Figure 5

Biochemical parameters (chlorophyll a (a), chlorophyll b (b), total chlorophyll (c), carotenoids (d), and H₂O₂ production (e)) of lettuce inoculated and uninoculated with Streptomyces thermocarboxydus S3 cultivated in a hydroponic system under full-strength nutrient solution at an EC of 900 µS/cm and half-strength nutrient solution at an EC of 450 µS/cm. Different lowercase letters (a, b, or c) indicate significant differences among the treatments (p < 0.05).

Figure 6

Colonization of Streptomyces thermocarboxydus S3 in lettuce roots using a culture-based method (colony characteristics of re-isolated S. thermocarboxydus S3 from lettuce roots (a) and pure culture of re-isolated S. thermocarboxydus S3 (b)) and light microscopic examination (c).