Malic Acid: Improving Photosynthesis and Stress Tolerance

Plants are constantly exposed to various types of biotic stresses, including pathogen attacks and pest infestations, as well as abiotic stressors such as drought, salinity, temperature extremes, and environmental pollution like heavy metals. These stresses cause significant and damaging effects on plant growth, development, and their physiological, biochemical, and molecular processes. Their impacts include harm to the photosynthetic system, metabolic disruption, impaired uptake of essential nutrients, and decreased crop yields. To combat these stresses, plants activate both enzymatic and non-enzymatic antioxidant defense systems; however, additional methods are necessary to further enhance stress tolerance. In this context, applying organic acids, such as malic acid, to leaves has proven to be an effective way to boost stress tolerance in vulnerable plants. Malic acid (MA), a vital part of the Krebs cycle, is essential for ATP production. When applied externally, it can enhance photosynthesis, preserve chlorophyll, and stabilize cell membranes. It also functions as an osmolyte, aiding in water regulation in plants under salinity and drought stress conditions. Research has shown increased antioxidant enzyme activity, reduced levels of stress-related metabolic markers, and improved mineral absorption under various stress scenarios. Therefore, applying MA externally is an economical and eco-friendly strategy for supporting sustainable agriculture and increasing crop yields. Nonetheless, existing research on its external use is limited, and further studies are needed to determine optimal application rates, plant-specific responses, stress conditions, and underlying molecular mechanisms.