The use of biostimulants with plant growth-promoting properties, but without significant input of nutrients, is discussed as a strategy to increase stress resistance and nutrient use efficiency of crops. However, limited reproducibility under real production conditions remains a major challenge. The use of combination products based on microbial and non-microbial biostimulants or microbial consortia, with the aim to exploit complementary or synergistic interactions and increase the flexibility of responses under different environmental conditions, is discussed as a potential strategy to overcome this problem. This study aimed at comparing the efficiency of selected microbial single-strain inoculants with proven plant-growth promoting potential versus consortium products under real production conditions in large-scale tomato cultivation systems, exposed to different environmental challenges. In a protected greenhouse production system at Timisoara, Romania, with composted cow manure, guano, hair-, and feather-meals as major fertilizers, different fungal and bacterial single-strain inoculants, as well as microbial consortium products, showed very similar beneficial responses. Nursery performance, fruit setting, fruit size distribution, seasonal yield share, and cumulative yield (39–84% as compared to the control) were significantly improved over two growing periods. By contrast, superior performance of the microbial consortia products (MCPs) was recorded under more challenging environmental conditions in an open-field drip-fertigated tomato production system in the Negev desert, Israel with mineral fertilization on a high pH (7.9), low fertility, and sandy soil. This was reflected by improved phosphate (P) acquisition, a stimulation of vegetative shoot biomass production and increased final fruit yield under conditions of limited P supply. Moreover, MCP inoculation was associated with selective changes of the rhizosphere-bacterial community structure particularly with respect to Sphingobacteriia and Flavobacteria, reported as salinity indicators and drought stress protectants. Phosphate limitation reduced the diversity of bacterial populations at the root surface (rhizoplane) and this effect was reverted by MCP inoculation, reflecting the improved P status of the plants. The results support the hypothesis that the use of microbial consortia can increase the efficiency and reproducibility of BS-assisted strategies for crop production, particularly under challenging environmental conditions.

This study investigated the drought protection effects of six fungal and bacterial inoculants and ten consortia thereof on vegetative growth, nutritional status, and tuberization of potato under controlled and field conditions. It was hypothesized that microbial consortia offer improved drought protection as compared with single strains, due to complementary or synergistic effects, with differential impacts also of N fertilization management. Under NO₃⁻ fertilization, a 70% reduction in water supply over six weeks reduced shoot and tuber biomass of non-inoculated plants by 30% and 50%, respectively, and induced phosphate (P) limitation compared to the well-watered control. The P nutritional status was significantly increased above the deficiency threshold by three single-strain inoculants and eight consortia. This was associated with the presence of the arbuscular mycorrhizal fungus (AMF) inoculant Rhizophagus irregularis MUCL41833 (five cases) and stimulation of root growth (five cases). Additionally, Bacillus amyloliquefaciens FZB42 and AMF + Pseudomonas brassicacearum 3Re2-7 significantly reduced irreversible drought-induced leaf damage after recovery to well-watered conditions. However, the microbial inoculants did not mitigate drought-induced reductions in tuber biomass, neither in greenhouse nor in field experiments. By contrast, NH₄⁺-dominated fertilization significantly increased tuber biomass under drought stress (534%), which was further increased by additional AMF inoculation (951%). This coincided with (i) improved enzymatic detoxification of drought-induced reactive oxygen species (ROS), (ii) improved osmotic adjustment in the shoot tissue (glycine betaine accumulation), (iii) increased shoot concentrations of ABA, jasmonic acid, and indole acetic acid, involved in drought stress signaling and tuberization, and (iv) reduced irreversible drought-induced leaf damage. Additional application of bacterial inoculants further improved ROS detoxification by increasing the production of antioxidants but stimulated biomass allocation towards shoot growth at the expense of tuber development. The results demonstrated that microbial consortia could increase the probability of drought protection effects influenced by the form of N supply. However, protective effects on vegetative growth do not necessarily translate into yield benefits, which can be achieved by adequate combination of inoculants and fertilizers.

The interaction between arbuscular mycorrhizal fungi (AM fungi) and Pseudomonas sp. has received considerable attention. The presence of biochar may affect these microorganisms, with subsequent modification of the phosphorus uptake and root morphology, and plant biomass accumulation. This research sought to identify, in the presence or absence of biochar, the effects of the interactions of mycorrhizal fungi and Pseudomonas sp. on the responses of phosphorus (P) and nitrogen (N) uptake and the root length, surface area, and volume of celery plants with low and high P fertilization under different substrate and soil conditions. The results indicate that strong growth responses of celery plants were observed due to the combination of AM fungi, Pseudomonas sp., and biochar with low P fertilization. A strong linear relationship was found between the plant root length and P accumulation in the shoot fraction in the present study. Increased P and N uptake occurred in treatments combining these microorganisms rather than alone, and this increase especially occurred in the presence of biochar. The low availability of P was substantially recovered by the association of these three aspects. The root morphology was greatly influenced by the biochar additives and in combination with AM fungi and Pseudomonas sp. The root colonization rate of AM fungi was increased by the combination of the inoculation of Pseudomonas sp. and biochar rather than AM fungi and/or Pseudomonas sp. These results indicate an accumulating effect of AM fungi, Pseudomonas sp., and biochar exists on the plant growth response and nutrient uptake because of the increasing root length, surface area, and volume, rather than root biomass.

Due to shallow root systems, potato is a particularly drought-sensitive crop. To counteract these limitations, the application of plant growth-promoting microorganisms (PGPMs) is discussed as a strategy to improve nutrient acquisition and biotic and abiotic stress resilience. However, initial root colonization by PGPMs, in particular, can be affected by stress factors that negatively impact root growth and activity or the survival of PGPMs in the rhizosphere. In this study, perspectives for the use of commercial silicate-based soil conditioners (SCs) supposed to improve soil water retention were investigated. The SC products were based on combinations with lignocellulose polysaccharides (Sanoplant^® = SP) or polyacrylate (Geohumus^® = GH). It was hypothesized that SC applications would support beneficial plant–inoculant interactions (arbuscular mycorrhiza, AM: Rhizophagus irregularis MUCL41833, and Pseudomonas brassicacearum 3Re2-7) on a silty loam soil–sand mixture under water-deficit conditions (6–12 weeks at 15–20% substrate water-holding capacity, WHC). Although no significant SC effects on WHC and total plant biomass were detectable, the SC-inoculant combinations increased the proportion of leaf biomass not affected by drought stress symptoms (chlorosis, necrosis) by 66% (SP) and 91% (GH). Accordingly, osmotic adjustment (proline, glycine betaine accumulation) and ROS detoxification (ascorbate peroxidase, total antioxidants) were increased. This was associated with elevated levels of phytohormones involved in stress adaptations (abscisic, jasmonic, salicylic acids, IAA) and reduced ROS (H₂O₂) accumulation in the leaf tissue. In contrast to GH, the SP treatments additionally stimulated AM root colonization. Finally, the SP-inoculant combination significantly increased tuber biomass (82%) under well-watered conditions, and a similar trend was observed under drought stress, reaching 81% of the well-watered control. The P status was sufficient for all treatments, and no treatment differences were observed for stress-protective nutrients, such as Zn, Mn, or Si. By contrast, GH treatments had negative effects on tuber biomass, associated with excess accumulation of Mn and Fe in the leaf tissue close to toxicity levels. The findings suggest that inoculation with the PGPMs in combination with SC products (SP) can promote physiological stress adaptations and AM colonization to improve potato tuber yield, independent of effects on soil water retention. However, this does not apply to SC products in general.