Background: Low soil temperature in spring is a major constraint for cultivation of tropical crops in temperate climates, associated with impaired seedling development, inhibition of root growth and root activity. In this study, potential cold-stress protectants, such as supplemented micronutrients (Zn, Mn), seaweed extracts, and rhizobacteria with plant growth-promoting potential (PGPRs) were tested in order to improve the tolerance of maize to low root zone temperatures (RZT) during early growth.Methods: Maize (v. Colisee) was cultivated in a root cooling system for adjustment of the RZT. In three independent experiments, after germination at 20 °C, the cold-stress phase (12-14 °C) started at 14 days after sowing to simulate a cold period in spring. Micronutrients, seaweed extracts, and PGPRs were supplied by fertigation (experiment 1), fertigation and seed dressing (experiment 2), and nutrient seed priming (experiment 3). At the end of the experiments, scoring of oxidative leaf damage, biomass production, chlorophyll status (SPAD), root length density, superoxide dismutase activities in leaf and root tissues, and the shoot mineral-nutritional status were determined. Results:Positive effects on plant growth and particularly on root development at low RZT were detected exclusively for seaweed extracts with high Zn/Mn contents and similar growth promotions were induced by Zn and Mn application in comparable amounts. This finding suggests that the selected seaweed extracts were mainly acting via improved Zn and Mn supply to the plants. It was essential that the cold-stress protectants were present during seed imbibition. The beneficial effect of Zn/Mn treatments and sea weed extracts was associated with increased superoxide dismutase activity in the root and leaf tissue, with key functions in antioxidative stress defense, depending on Zn, Mn, Cu, and Fe as enzymatic co-factors. Accordingly, leaf damage, shoot and root growth inhibition in cold-stressed plants was associated with a low Zn-nutritional status, mitigated by application of the cold-stress protectants. Conclusions:Since micronutrients are effective already at low concentrations, starter applications of Zn/Mn or the respective seaweed extracts may offer an economic option for cold-stress prophylaxis in crops.

Soil microbial communities interact with roots, affecting plant growth and nutrient acquisition. In the present study, we aimed to decipher the effects of the inoculants Trichoderma harzianum T-22, Pseudomonas sp. DSMZ 13134, Bacillus amyloliquefaciens FZB42 or Pseudomonas sp. RU47 on the rhizosphere microbial community and their beneficial effects on tomato plants grown in moderately low phosphorous soil under greenhouse conditions. We analyzed the plant mass, inoculant colony forming units and rhizosphere communities on 15, 22, 29 and 43 days after sowing. Selective plating showed that the bacterial inoculants had a good rhizocompetence and accelerated shoot and root growth and nutrient accumulation. 16S rRNA gene fingerprints indicated changes in the rhizosphere bacterial community composition. Amplicon sequencing revealed that rhizosphere bacterial communities from plants treated with bacterial inoculants were more similar to each other and distinct from those of the control and the Trichoderma inoculated plants at harvest time, and numerous dynamic taxa were identified. In conclusion, likely both, inoculants and the rhizosphere microbiome shifts, stimulated early plant growth mainly by improved spatial acquisition of available nutrients via root growth promotion. At harvest, all tomato plants were P-deficient, suggesting a limited contribution of inoculants and the microbiome shifts to the solubilization of sparingly soluble soil P.

Here we describe the effects of the well-characterized, commercial plant growth-promoting rhizobacteria (PGPR) strain Pseudomonas sp. DSMZ 13134 (Proradix®) on plant growth, root morphology, and nutrient acquisition of a maize mutant (rth2) with impaired root hair production as compared with the corresponding wild type line, to study the importance of root hairs for the interaction of the PGPR strain with the host plant. The study was conducted in rhizobox culture with a sand–soil mixture and moderate P supply. Root hair development of the mutant was clearly impaired, reflected by slower growth and limited elongation as compared with the wild type line. This defect was compensated by more intense root growth and fine root production of the mutant which was particularly expressed after inoculation with Proradix®. By contrast, PGPR inoculation had no effect on root hair length. The beneficial effects of Proradix® on root growth were reflected in higher shoot contents of the macronutrients P and K. Interestingly, negative effects on shoot accumulation of the micronutrients Zn and Cu were observed. These findings support proposed PGPR effects of this strain but also show limitations that may be explained by additional strain-specific properties. Possible implications of these findings are discussed.