Modulation of C:N:P stoichiometry is involved in the effectiveness of a PGPR and AM fungus in increasing salt stress tolerance of Sulla carnosa Tunisian provenances

Abstract

Sulla carnosa (Hedysarum carnosum Desf.) is an important legume grown as animal feed on salt-affected areas. Here, we assess the influence of plant-growth-promoting bacteria (PGPR) (Bacillus subtilis) with or without the mycorrhizal fungus (AMF) Rhizophagus intraradices on four S. carnosa Tunisian provenances (Sidi khlif, Thelja, Kalbia and Kerker) grown in the greenhouse under saline and non-saline conditions. Plant growth, mineral nutrition, C:N:P stoichiometry in plant shoots, and soil enzymatic activities were considered. Microbial effectiveness on plant (roots and shoots) growth ranged from insignificant in Kerker to highly positive in the provenances Sidi khlif and Kalbia. In the latter provenance, the positive effect on growth was more pronounced under dual than single inoculation. Plant growth declined significantly under salt stress compared to the control, but Sidi khlif provenance was the most severely impacted. Especially in Kalbia, shoot mineral (N, P, K, Mg, Cu, and Fe) concentrations increased significantly in salt-stressed plants inoculated with R. intraradices as compared to the salt-stressed and non-inoculated plants. This suggests that inoculation with R. intraradices during nursery establishment contributes to salt stress alleviation by maintaining a favourable nutrient profile in Kalbia provenance. Shoots of Sidi khlif plants inoculated with B. subtilis demonstrated distinct C:N:P stoichiometry as compared to salt-treated and non-inoculated plants, indicating that B. subtilis could increase plant tolerance to salt stress through nutrient allocation patterns. The efficiency of B. subtilis in Kalbia and Thelja was due to the high IAA production by this strain under stress conditions. Application of PGPR and AMF improved soil quality by increasing the activities of soil enzyme such as urease, alkaline phosphatase, β-glucosidase and dehydrogenase compared to their respective non-inoculated controls. From this study, we conclude that microbial inoculation was necessary for S. carnosa provenances to reach their optimal nutrition under saline and non-saline conditions. Thus, the use of adequate microbial inocula and provenance could be a critical issue for the successful ecological stability of this Mediterranean legume, thereby improving its suitability for the remediation of saline lands.