Kombucha Microbiome as a Potential Microbial Biostimulant in Tomato: Formulation-, Dose-, and Application Route-Dependent Effects on Growth, Flowering, and Redox Metabolism

Abstract Kombucha microbiome is a multi-component system containing bacteria, yeasts, extracellular matrix constituents, and fermentation-derived metabolites, and may therefore represent a potential source of microbial biostimulants. This study compared the effects of Kombucha microbiome (KM) on tomato with those of the commercial microbial fertilizer Bac-OMET (MF) and an equal-dose KM+MF combination. The three inoculants were tested in liquid and pellet formulations at four dose levels using seed, soil, and foliar application routes in a full factorial design. Germination, vegetative growth, biomass, flowering, photosynthetic pigments, soluble protein, antioxidant enzyme activities, proline, and malondialdehyde (MDA) were evaluated. The main effects of inoculant, formulation, dose, and application route were significant for 18, 7, 13, and 14 of the 21 response variables, respectively, while pervasive interactions demonstrated strong context dependence. Low-dose MF treatments accelerated germination, whereas 20 mL liquid KM achieved 100% final germination. The highest dose delayed germination and reduced final germination to 20% in liquid treatments. MF produced the greatest fresh and dry biomass, whereas KM was associated with greater plant height, root and stem length, chlorophyll, proline, and CAT, APX, GR, and GST activities. KM+MF produced the highest protein concentration and SOD activity but did not show consistent synergy in growth traits. Foliar application advanced flowering and increased flower number. Concurrent increases in proline and MDA at the highest dose indicated that strong defense activation was accompanied by cellular stress. Overall, KM emerged as a dose-, formulation-, and route-dependent microbial biostimulant candidate for tomato, with low-to-moderate doses offering the most balanced responses.