Microbial Consortia and Biochar Enhance Photosynthesis, Water Relations, and Leaf Thermal Regulation in Vigna radiata L

Plant growth-promoting microorganisms (PGPMs) and biochar are increasingly recognized as sustainable strategies to enhance crop performance. However, their combined effects on photosynthesis, water relations, and leaf temperature under hydroponic conditions remain insufficiently explored. Here, we investigated the physiological responses of Vigna radiata L. to a bacterial biostimulant (BB), arbuscular mycorrhizal fungi (AM), their combination (BA), and a triple treatment with biochar (BiB). All treatments significantly improved plant performance compared with the control, though with differing mechanisms and magnitudes. BB markedly reduced leaf temperature (LT) through enhanced stomatal conductance and transpiration, resulting in higher net photosynthesis. AM primarily improved plant water status (RWC = 0.89) and intrinsic water-use efficiency, moderating LT via hydraulic and osmotic regulation rather than evaporative cooling. The BA treatment integrated these complementary functions, achieving the lowest LT (25.66°C), highest transpiration (4.03 mmol m⁻² s⁻¹), and maximum shoot and root biomass. Incorporation of biochar (BiB) further increased photosynthetic rate and water-use efficiency, although total biomass was slightly lower than in BA. These findings reveal a functional trade-off: bacterial inoculation promotes carbon assimilation through stomatal regulation, while AM fungi enhance hydraulic stability; their co-inoculation harmonizes both processes to optimize growth. By actively lowering leaf temperature, PGPMs mitigate heat-induced photoinhibition and sustain higher photosynthetic efficiency, offering a practical approach to improve crop resilience and productivity under controlled or resource-limited conditions.