Leaf Anatomical Adaptation and Chloroplast Ultrastructure Changes Upon Magnesium Foliar Application of Faba Bean (<i>Vicia faba</i> L.) Grown Under Drought Stress

ABSTRACT Background Drought stress (DS) impedes plant growth and development by impairing the uptake of nutrients, such as magnesium, which is central to many physiological processes, particularly photosynthesis. Leaf application was proposed to be an effective strategy to compensate for inadequate Mg 2+ supply from the nutrient solution. Aim The present study is designed to investigate the role of Mg 2+ leaf application in ameliorating leaf anatomy and chloroplast ultrastructure changes in faba beans grown under DS. Methods Hydroponically grown plants were subjected to DS under various levels of Mg 2+ , that is, sufficient (0.5 mM), deficient (0 mM), and leaf‐application (250 mM). Light and transmission electron microscopy (TEM) were conducted to examine leaf anatomy and ultrastructural changes. Results Mg 2+ deficiency alone and under DS significantly affected plant biomass and photosynthesis. Additionally, sucrose concentration, oxidative stress, and lipid peroxidation were increased. Accordingly, the excessive deposition of photoassimilates in source organs due to the inhibition of phloem loading results in a disruption of the thylakoid structures leading to chloroplast damage. In the current study leaf application of Mg 2+ partially ameliorated physiological functions, most notably chlorophyll concentration, photosynthesis and transpiration rate, plant biomass, and preservation of ultrastructure of the chloroplast. Conclusion Although the Mg application via roots enhanced drought tolerance, compared to Mg 2+ leaf application. However, Mg 2+ leaf application was proven to be an efficient strategy in mitigating DS in field trials. Therefore, Mg 2+ foliar application should be prioritized for further investigation under relevant environmental stress conditions.