Secondary Metabolites, Osmolytes and Antioxidant Activity as the Main Attributes Enhanced by Biostimulants for Growth and Resilience of Lettuce to Drought Stress

Drought stress (DS) is one of the most serious abiotic stresses decreasing crop production and agricultural yield worldwide. In this study, an experiment was conducted in the controlled greenhouse to test the effect of three water regimes (75, 50 and 33% field capacity (FC)) and three biostimulants (alone or in combination): B) Bacillus sp. and Bacillus subtilis, M) native arbuscular mycorrhizal fungi and C) compost on Lactuca sativa L. DS disrupted the functioning of plant growth, physiological, biochemical, and mycorrhizal status as well as soil processes. However, the biostimulants especially B showed higher values (505%) of total dry biomass compared to the control plants under 33% FC. Under the same conditions, the higher value of water content and stomatal conductance was recorded in plant treated by M (465%) and B (60%), respectively, while sugars and protein contents were accumulated significantly by C (109%) and CM (399%), respectively compared to the control plants. In comparison to the controls under DS, B and BCM reduced the malondialdehyde and hydrogen peroxide contents and showed a strong antioxidant activity as well as high levels of flavonoids under DS. In addition, total organic matter and available phosphorus were improved by C (105%) and BCM (864%), under DS. Indeed, the application of biostimulants could help lettuce plants to tolerate water stress damages as well as the restoration of drought affected lands in arid and semi-arid regions.