Lettuce growth, plant nutritional status, and yield can be improved by mixing of brassinosteroid analogue (DI-31) and plant-growth-promoting bacteria in soilless conditions

The use of plant growth-promoting bacteria (PGPB), brassinosteroids (Br), or their mixtures was evaluated to estimate the effects of these plant growth promoters on the physiological behavior (plant growth and sap nutrient content) and yield of lettuce (Lactuca sativa var.crispa L. 'Levistro') plants established under two different soilless cultivation conditions (hydroponics or substrate).Two separate experiments were conducted in the substrate and hydroponic media.In both experiments, two PGPB strains (Bacillus velezensis [Bv and Azospirillum brasilense Az]) were applied at a concentration of 110 6 cell mL -1 .The PGPB strains were applied individually, mixed with each other, or with a Br analogue (DI-31) at a concentration of 0.5 mL L -1 .Under hydroponic conditions, the mixture of Br and Bv produced higher plant yield (73.6 g fresh weight) than the control plants (33.6 g fresh weight).Regarding sap ion content, the Br and Az mixture caused a reduction in sap nitrate (2522 mg L -1 ), K (680 mg L -1 ), and Ca (157.5 mg L -1 ) content compared to the control plants (4550 mg L - 1 for nitrate, 1825 mg L -1 for K and 648.7 mg L -1 for Ca).In the substrate trial, PGPB treatments ( 56.7 g fresh weight) caused a higher plant yield than control plants (36.5 g fresh weight).In conclusion, the use of mixtures of plant growth promoters can be considered an agronomic alternative because the mixture can cause a synergic effect on the growth and yield of L. sativa in plants grown under soilless conditions.