An investigation of the ameliorative effects of supplemental potassium and rhizobium inoculation on the growth, physiology, biochemistry, and nodulation of salt-stressed Bambara groundnut

Abstract

Abiotic stressors such as salinity threaten agriculture and plant health, particularly affecting salt-sensitive crops like Bambara groundnut. Despite its potential, Bambara groundnut has received little research attention regarding strategies to mitigate salt stress. While the individual roles of potassium supplementation and rhizobium inoculation have been studied, their combined effects remain largely unexplored. The purpose of the investigation was to evaluate both sole and integrated impacts of supplemental potassium as well as inoculation of rhizobium on the growth, physiology, and biochemical properties of salt-stressed Bambara groundnut. The experiment was executed at the University of Venda during the 2024/2025 academic year using a greenhouse and laboratory setup in a completely randomised design. Treatments comprised of two NaCl (0 & 35 mM), four potassium rates (0, 8, 16, & 24 mM), and two rhizobium conditions (inoculated & uninoculated). Growth parameters were measured at intervals, while physiological and biochemical traits, including chlorophyll and proline, were assessed at 21 and 42 days after treatment. Final measurements of root length, biomass, water content, mineral nutrients, and nodulation were taken at harvest (56 days after planting). Data were analysed using ANOVA, and separation of means by LSD at a 5% significance level. Salinity significantly reduced Bambara groundnut growth, chlorophyll content, water status, and nodulation. Potassium supplementation and rhizobium inoculation alleviated these effects, with the combined application showing the greatest improvement. Plant height increased by up to 31%, leaf area and biomass improved, and chlorophyll content rose markedly under combined treatments. Sodium accumulation decreased while potassium and calcium uptake improved, reducing the Na⁺/K⁺ ratio by up to 89%. Proline levels, which are elevated under stress, declined with the administration of potassium and rhizobium, indicating a reduction in osmotic stress. Nodulation parameters responded strongly, with the number and dry weight of nodules increasing by 108% and 226%, respectively. Although some trends were statistically non-significant, the combined treatment consistently enhanced growth, physiological performance, and ion homeostasis under saline conditions. Therefore, this study suggests that supplementation of K in conjunction with B. japonicum can improve the growth, physiological and nodulation parameters of Bambara groundnut growing under saline conditions.