The Influence of Rhizobium tropici Produced EPM Biopolymer on Green Bush Bean Root and Plant Growth

With the increase in severe weather events due to climate change, plant development both aboveground (providing vegetation cover) and belowground (enhancing root systems) is crucial in limiting soil erosion. Traditional chemical fertilizers do allow for these developments but have environmental consequences from polluting waterways to disturbing the natural biodome. Hence, much effort is currently being invested in finding alternative means to enhance plant growth and mitigate soil erosion through the application of environmentally sustainable materials. A Rhizobium tropici (R. tropici) derived biopolymer has been reported as an effective, biodegradable additive to reduce soil erosion. We, therefore, chose to study its effectiveness for root and stem/leaf growth of plants in the early stages of development, starting from the germination of the seeds through the initial growth of leaves and shoots. The plants used were green bush beans due to their relatively short germination time of 7-12 days which allowed us to see the effects of the fertilizers within a short period of time.<br/>Two biopolymer fertilizers, both ethanol precipitable materials (EPM) derived from R. tropici bacteria ATCC strain under the same cultivation environment and procedure but grown in two laboratories, were tested. Green bush beans were divided into five groups: control (tap water) and four EPM treatments. The EPM treatments consisted of the combinations of 50 mg/L EPM1 (from the first lab), 50 mg/L EPM2 (from the second lab), 100 mg/L EPM1, and 100 mg/L EPM2. The beans underwent 10 days of germination and then were transplanted into soil for 3 weeks of further watering with tap water or EPM solutions daily. Mass measurements were used to investigate whether the EPM biopolymer influenced the growth of the roots versus the stems and leaves differently. As such, the green bush bean plants were removed from the soil for roots and plant analysis after 3 weeks of transplantation. The soil was washed off and the plants were dried completely to ensure that the mass was only of the intended roots and stems/leaves. The effects of each treatment were recorded by measuring the masses and lengths of the roots and stems/leaves individually. Mass measurements were used to investigate whether the EPM biopolymer influenced the growth of the roots versus the stems and leaves differently.<br/>Watering bush bean seeds with EPM concentrations as low as 50 mg/L and 100 mg/L produced a small increase in the germination rate: from 87% to 93% for seeds grown for 10 days in a moist environment. The root mass of the plants treated with the EPM solutions increased compared to the control by 29% to 71% (for EPM1) and by approximately 14% (for EPM2). The plants treated with either of the EPM solutions have a mass of stems and leaves that is higher than the control by 45% and 58% for EPM1 and EPM2, respectively. Furthermore, the result shows that there was a 21% to 43% increase in plant height for plants treated with EPM solution. EPM1 and EPM2 solution treated plants showed enhancement in both root and stem/leaf mass compared to the control, but the EPM1 solution was better for root development and EPM2 solution was better for stems and leaves development—consistent with plants typically preferring either root or leaf production. These results suggest that the EPM biopolymer stimulates bush bean plants to develop a denser root structure and a larger leaf and shoot system, which enhances plant survival. Taken together, these results may explain the reported increase in resistance to soil erosion when biopolymer was used to irrigate different berm slope sites.<br/>We would like to thank the US Army Corps of Engineers (ERDC) for their support (W912HZ-20-2-0054) in this research, Dr. Michael Sadowsky for providing the Rhizobium tropici CIAT 899 (UMR 1899) strain, and the Morin Charitable Trust for the funding.