Plant growth-promoting native rhizobia isolated from red clover (Trifolium pratense L.) nodules: potential for...

Red clover (Trifolium pratense L.) is widely cultivated as forage crop. Due to the ability to accumulate potentially toxic elements from the soil, red clover is proposed as a bioindicator plant, but this potential is still being explored. Rhizobium leguminosarum biovar trifolii forms endosymbiosis with red clover which provides nitrogen to the plant, but also produces different plant growth-promoting compounds contributing to alleviation of different stress factors. The aim of this study was to characterize the native rhizobial strains associated with red clover plants and to evaluate in vitro the capacity of nitrogen fixation, plant-growth promotion potential and tolerance to nickel (Ni) as well as the effect of selected strains on elemental composition and growth of red clover in soil with elevated Ni concentration.

More than 60% of tested strains nodulated red clover when re-inoculated in in vitro culture and induced significant increase in height, shoot dry weight and shoot nitrogen content up to 140.53%, 1026.27% and 196.85%, respectively, over the non-inoculated plants. The strains were characterized as Rhizobium sp. Plant growth-promoting traits such as indole-3-acetic acid (22.18-135.32 µg/mL in the presence of 2 g/L L-tryptophan) and 1-aminocyclopropane-1-carboxylate deaminase production and phosphate solubilization were detected in some strains. Inoculation of red clover in soil with increased Ni concentration showed that inoculated treatments had increased shoot dry weight (up to 16.13%) and nitrogen content (up to 16.21% in shoots and 18.62% in roots) compared to non-treated plants. Inoculation increased Ni, Cr and Pb concentrations (up to 52.33, 145.88 and 218.03%, respectively) compared to the control. Generally, red clover plants accumulated more metals in their roots than in shoots, and shoot concentrations were mainly within common range for plants.

Inoculation of red clover with effective rhizobial strains showed potential for plant growth improvement and phytostabilization of Ni in metal-burdened soils.