Brassinosteroid priming mitigates negative physiological responses of Garcina humilis to flooding and salinity

It was previously determined that G. humilis, a woody perennial species native to the Amazon and grown commercially as a tropical fruit tree, is tolerant of either flooding or high soil salinity and simultaneous exposure to flooding and high soil salinity, as evidenced by no visible signs of stress and continued vegetative growth. However, exposure to either stressor or to both stressors simultaneously, significantly reduced net CO₂ assimilation (A) and stomatal conductance (gs), which can negatively impact plant growth and yield. This study tested the effects of 24-epibrassinolide (EB) priming on reducing the negative physiological responses of G. humilis to flooding and high soil salinity. G. humilis trees that were either flooded or non-flooded were each subjected to either low or high soil salinity levels and either primed with a solution of 1.0 mg l^-1 EB or not primed prior to flooding and salinity treatments. Prior to the imposition of treatments and periodically during the treatment period, A, gs, internal CO₂ content in the leaf (Ci), the leaf chlorophyll index (LCI), and the variable to maximum chlorophyll fluorescence ratio (Fv/Fm) were determined. At the end of the treatment period, nutrient element concentrations in the leaves and roots, as well as reactive oxygen species (ROS), antioxidants, and lipid peroxidation in the leaves were measured. Epibrassinolide priming decreased the negative impacts of salinity or flooding and simultaneous exposure to both stressors on G. humilis, apparently by reinforcing antioxidant responses which decreased ROS and lipid peroxidation.