Mycorrhizal and bacterial inoculants enhance photosynthesis, water relations, and leaf thermal regulation in Vigna Radiata L. under soilless substrate culture

Plant growth-promoting microorganisms (PGPMs) and biochar are increasingly recognized as sustainable strategies to enhance crop performance; however, their comparative and integrative effects on carbon-water coordination and leaf thermal regulation under soilless substrate conditions remain poorly resolved. We investigated the physiological responses of Vigna radiata L. to bacterial biostimulant (BB), arbuscular mycorrhizal (AM) fungi, their co-inoculation (BA), and BA combined with biochar (BiB). All treatments enhanced biomass relative to the uninoculated control, but through distinct physiological mechanisms. BB reduced leaf temperature (LT) primarily via increased stomatal conductance and transpiration, supporting higher net photosynthesis. In contrast, AM enhanced plant water status, as reflected by higher relative water content, and maintained intrinsic water-use efficiency comparable to the control but greater than in the other inoculation treatments. The reduction in LT under AM, despite unchanged transpiration, may reflect differences in plant water relations rather than purely evaporative cooling. BA integrated these complementary functions, resulting in the lowest LT, and maximum biomass. BiB further enhanced photosynthetic rate and maintained intrinsic water-use efficiency, although biomass was slightly lower than in BA. Collectively, the results demonstrate that the distinct physiological roles of bacterial and mycorrhizal inoculants, stomatal-driven carbon acquisition versus hydraulic stabilization, become functionally complementary under co-inoculation, enabling coordinated regulation of carbon assimilation, water balance, and leaf thermal dynamics to maximize biomass production. By reducing leaf temperature, PGPMs could contribute to maintaining photosynthetic efficiency under potential heat stress.