Seeds treated with biofilm-forming microbes retain moisture during cold-dry winter, reducing temperature fluctuations...

Diurnal temperature fluctuations are common in soil during the dry winter season in the Eastern sub-Himalayan Terai region (Cwa climatic zone, according to Köppen's classification). The difference between the evening and morning temperatures of 5-6 °C results in 70-75% vigor loss due to electrolyte leakage from the seeds, thereby extending the nursery period. Therefore, it is essential to minimize soil thermal fluctuations to retain seed vigor. In this study, we utilised biofilm-forming bacterial species like Phytobacter, Priestia, and Bacillus as bioinoculants and aimed to elucidate how they buffer soil temperature fluctuations. Since the highest fluctuations in soil temperatures occur in the cold, dry winter season, it was hypothesised that using biofilm-forming microbes as seed coatings could prevent desiccation-induced water loss from seeds. We performed in vitro (changing the daily temperature) and in situ experiments (using a Randomized Block Design) to analyze the seed vigor. In vitro assessment of tomato seeds indicates the role of both duration and intensity of cold stress in seed vigor reduction. This loss was recovered with bacterial inoculation (30-100% depending upon the intensity and duration of stress). Vigor recovery was also demonstrated in fields where seeds treated with bacteria displayed better emergence (70-95%). Interestingly, the difference between morning and evening soil temperatures reduced upon bioinoculant treatment (0.8 ± 0.2 °C ) as compared to control (2.7 ± 0.9 °C) in the rhizosphere developed from seeds treated with biofilm-forming bacteria, demonstrating thermal buffering of rhizospheric soil. Since biofilm have hygroscopic properties, a relatively higher residual moisture was recorded from soils in fields raised from the bio-inoculant-treated seeds. Water, owing to its high specific heat (4.186 J g^- 1 °C^- 1), can buffer temperature fluctuations. This is the first report showing the mechanism of microbe-mediated thermal buffering of rhizosphere. Since these microbes can improve moisture retention during cold, dry winter months, they can be used in areas with comparable climates.