Stress Responses to Hydrogen Peroxide and Hydric Stress-Related Acoustic Emissions (MHAF) in Capsicum annuum L. Applied in a Single or Combined Manner

Hydrogen peroxide (H₂O₂) application in several plant species has been widely studied as a plant biostimulant; however, the use of acoustic emissions related to hydric stress (MHAF) in biostimulating plants has not been widely studied, including the response of plants to the interaction of different stress factors. The aim of the present work was to evaluate the stress response in some morphological, biochemical, and molecular variables of the single or combined application of H₂O₂ and MHAF in <i>C. annuum</i> L. plants. Acoustic emission frequencies were obtained in a previous study where the frequencies came from <i>C. annuum</i> plants submitted to medium hydric stress (MHAF). Our results showed that the combination of the two stressors evaluated has a possible synergistic effect on variables such as SOD activity and relative gene expressions of <i>ros1</i>, <i>met1</i>, and MAPkinases (<i>mkk5</i>, <i>mpk4-1</i>, <i>mpk6-2</i>), as well as an antagonistic effect for flavonoid content, DPPH, and ABTS free radical inhibition, and <i>def1</i> gene expression. MHAF showed increased plant height, PAL activity, and <i>mpk6-1</i> and <i>erf1</i> gene upregulation, while H₂O₂ increased POD activity and upregulated <i>pr1a</i> gene. These findings suggest possible stress response pathways that are activated and enhanced by the presence of these stress factors, both individually and in conjunction with one another, making it possible to use them as novel strategies for agricultural stress management.