Mn’s Key Roles in Plant Ecophysiology—A Comprehensive Review for Unstressed and Stress Conditions

Manganese (Mn) is an essential micronutrient required for plant growth, photosynthesis and metabolic regulation. Its importance is related to the involvement in several metabolic processes that ensure proper cellular function and balanced plant development throughout the production cycle. In plants, Mn is absorbed predominantly as Mn2+, and its availability is strongly influenced by soil pH, aeration, and other mineral nutrients in the soil solution. After uptake by roots, Mn is translocated to the shoot, accumulating primarily in metabolically active organs such as stems, young leaves and flowers. Although Mn exhibits limited mobility in the phloem, adequate concentrations are necessary to sustain both vegetative development and reproductive growth. Adequate Mn concentration is directly reflected in fruit development, as well-nourished plants show improved flowering, greater assimilate translocation capacity, and better fruit filling, thereby positively influencing yield and quality. However, Mn deficiency is common in alkaline soils or soils with high organic matter, causing interveinal chlorosis in young leaves, reduced growth, and lower biomass production. Under prolonged conditions, deficiency leads to less vigorous plants with reduced metabolic efficiency. Conversely, Mn toxicity, typically associated with acidic and poorly drained soils, restricts root development and induces nutritional imbalances with other elements, such as calcium, magnesium, and iron. Therefore, proper Mn management is essential to ensure nutritional balance and optimal performance of agricultural crops. Overall, this review synthesizes advances in Mn transport, cellular compartmentalization, and metabolic regulation, emphasizing how Mn interacts with other mineral nutrients to influence plant physiology. Attention is given to the integration of Mn with redox networks, photosynthetic regulation, and reproductive development. By linking transport mechanisms with physiological outcomes, this review identifies key patterns governing Mn homeostasis and highlights implications for crop nutrition and sustainable nutrient management.