Application of biostimulants promotes growth and productivity by fortifying the antioxidant machinery and suppressing oxidative stress in faba bean under various abiotic stresses

Abstract Natural extracts and biostimulants have recently been used to enhance growth and productivity of stressed plants. A pots trial was performed to verify the impacts of maize grain (MgE, 60 g L−1) and/or propolis (PrmE, 40 g L−1) extract foliar application (three times) on growth, physio-biochemical attributes, and productivity of faba bean plants exposed to drought (40% of soil capacity), salinity (150 mM NaCl), or cadmium (2.0 mM Cd2+) stress versus absence of stress as a control. Alterations in the antioxidant system and its relationship with stress tolerance were also examined. The results indicated that each stress caused a significant decrease in yield traits, photosynthetic efficiency, pigment contents, gas exchange, relative water content, membrane stability index, and osmolyte contents compared to non-stressed plants. Otherwise, MgE and/or PrmE enhanced the plant's stress tolerance and increased the aforementioned attributes under normal or stress conditions. Moreover, MgE and/or PrmE increased enzymatic activities (SOD, CAT, POX, and APX) and antioxidant levels (proline, glutathione, ascorbate, and α-tocopherol) under the studied stresses compared to untreated controls. The combined MgE+PrmE was the most efficient treatment. Salinity considerably increased Na+ content, whereas, MgE+PrmE treatment reduced Na content by 39.6, 16.7, or 37.0% under salinity, drought, or Cd2+ stress, respectively. Under Cd2+ stress, the best treatment (MgE+PrmE) reduced root and leaf Cd2+ contents by 74.1 and 78.6%, respectively compared to untreated plants. Our findings indicated that foliarly-applied MgE+PrmE was highly effective in enhancing the antioxidant machinery, thus reducing ROS, Na+, and Cd2+ levels resulting in increased plant productivity under salinity, drought, or Cd2+ stress.