New insights into the responses of phosphite, as a plant biostimulator, on PSII photochemistry, gas exchange, redox state and antioxidant system in maize plants under boron toxicity.

This study focused on boron (B), an essential micronutrient for plant development that becomes toxic at high concentrations, adversely affecting plant growth and yield. Phosphite (PHI) is recognized for its easy absorption by plant leaves and roots and its well-documented positive effects on plant growth. The effects of phosphite (PHI-1, 2 g L⁻; PHI-2, 4 g L⁻) under boron stress (B, 2 mM) were evaluated in Zea mays. Under B stress, a 58% reduction in growth was observed in maize leaves. However, PHI applied at both concentrations positively influenced growth parameters and regulated water relations in the leaves of stressed plants. Under B stress, gas exchange was restricted, the photochemical quantum efficiency of PSII (Fv/Fm) was suppressed, and non-photochemical quenching (NPQ) values increased. Treatments with B + PHI-1 and B + PHI-2 enhanced carbon assimilation rates (A) by 37% and 23%, respectively. In OJIP transition parameters, it was observed that PHI-1 and PHI-2 treatments supported photochemical reactions by reducing the dissipated energy flux (DIo/RC). Additionally, high levels of H₂O₂ accumulation and lipid peroxidation occurred under B stress However, PHI treatments increased the activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX), mitigating oxidative damage caused by B stress. Furthermore, PHI effectively preserved ascorbate regeneration and enhanced the ascorbate-glutathione cycle, contributing to the reduction of reactive oxygen species (ROS) accumulation. Consequently, PHI treatment demonstrated its effectiveness in mitigating boron toxicity by improving the antioxidant defense system, reducing ROS accumulation, and enhancing photosynthetic efficiency, thereby increasing stress tolerance in maize plants.