Long Term Comparison of Talc- and Peat-Based Phytobeneficial Pseudomonas fluorescens and Pseudomonas synxantha Bioformulations for Promoting Plant Growth

Use of plant growth promoting rhizobacteria (PGPR) is an important strategy in sustainable agriculture. Among PGPR, many Pseudomonas strains are of great interest due to their abilities to colonize and thrive in the rhizosphere, in addition to displaying mechanisms in plant growth promotion and biocontrol activities. However, as Pseudomonas strains are non-spore forming micro-organisms, their development into stable bioformulations for commercial applications can be difficult. This study examined over a long term period the effect of two different carriers, peat and talc, to prepare bioformulations using phytobeneficial Pseudomonas strains belonging to two taxonomical groups of interest: P. fluorescens and P. synxantha. Each strain has previously demonstrated plant growth promotion activity when inoculated in the rhizosphere. Each bioformulation was stored at ambient temperature and their viability was measured up to 180 days. In parallel, every 30 days (up to 180 days) each bioformulation was also applied in the rhizosphere of plantlets to validate their plant growth promotion activity, and their establishment in the rhizosphere was quantified by using strain-specific quantitative polymerase chain reaction assays. The viability of both Pseudomonas strains in the bioformulations was found to decrease after the first 15 days and remained relatively stable for up to 180 days. When applying the bioformulations to Buglossoides arvensis plantlets, the expected plant growth promotion was observed when using up to 180 day-old formulations of P. fluorescens and up to 120 day-old formulations of P. synxantha, with similar results for both carriers. Establishment of both Pseudomonas strains in the rhizosphere inoculated with the peat-based carrier bioformulations stored for up to 180 days was found to be stable. While a lower establishment of P. fluorecens in the rhizosphere was observed when talc-based bioformulations were stored for 90 days or more, rhizosphere colonization by P. synxantha talc-based bioformulations remained stable for up to 180 days. In conclusion, both peat and talc appear to be suitable carriers for Pseudomonas bioformulations, however strain-specific variability exists and therefore the viability of each Pseudomonas strain and its capacity to maintain its plant growth promotion activity should be validated in different substrates before determining which formulation to use.