Enhancing Growth, Biochemical Traits, and Stress Resilience of Mentha pulegium L. in Calcareous Soil Using PGPR

Abstract

This study conducted to evaluate the influence of plant growth-promoting rhizobacteria (PGPR), specifically Paenibacillus polymyxa and Bacillus halotolerans, on the growth, biochemical, and antioxidant parameters of Mentha pulegium cultivated in calcareous soil. PGPR inoculation significantly enhanced plant growth parameters, including branch number, shoot and root lengths and fresh and dry weights. The combined inoculation of P. polymyxa and B. halotolerans achieved significant increase, with the highest number of branches (17.7), shoot length (30 cm), root length (31 cm), and fresh/dry weights (14.7/2.2 g/plant)as compared with the control plants. Chlorophyll content, nitrogen (N) and phosphorus (P) uptake, and protein content were significantly improved with inoculated treatments. Dual inoculation recorded the highest chlorophyll reading (42.2 SPAD units), N uptake (0.279%), P uptake (0.533%), and protein content (1.744%). Proline and total protein levels in leaves increased significantly, with dual inoculation achieving the highest values at 60 and 110 days. Antioxidant enzyme activities (CAT, POD, SOD, and PPO) were significantly elevated, with the combined inoculation yielding the most substantial enrichment. Additionally, the levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), markers of oxidative stress, were significantly reduced in inoculated plants. The dual inoculation treatment exhibited the lowest MDA (0.13/1.051 µM g⁻¹ FW) and H₂O₂ (0.173/0.215 µM g⁻¹ FW) levels, indicating its superior ability to mitigate oxidative stress. These findings highlight the potential of PGPR in promoting the growth and stress resilience of Mentha pulegium under calcareous soil conditions.