The role of the “overlooked” elements magnesium and chloride for improving photosynthesis and crop water-use efficiency in plants

Abstract Background Enhancing water-use efficiency (WUE) is a central requirement for sustainable crop production in Mediterranean-type climates under ongoing warming. Nutrient management can improve photosynthetic performance and water economy, yet two physiologically relevant nutrients, magnesium (Mg 2 + ) and chloride (Cl¯), remain comparatively under-considered in fertilization strategies. Scope We synthesize evidence that Mg 2 + availability is vital for photosynthesis through impacts on chlorophyll biosynthesis, Rubisco activation and other enzymatic reactions, photoprotection, and carbon partitioning, highlighting its role as cofactor for the primary nitrogen assimilation enzyme glutamine synthetase; Mg 2 + limitation is prevalent in sandy and calcareous Mediterranean soils where leaching and cationic competition reduce plant-available Mg 2 + . Mg 2 + -containing fertilisers are effective, but little is known about their relationship with WUE. We also review the shift in the perception of Cl¯ from a salinity-related risk factor and a nitrate (NO 3 - ) antagonist, particularly in coastal Mediterranean soils, to a beneficial macronutrient when accumulated at typical macronutrient concentrations, improving photosynthetic capacity (notably during early vegetative development), tissue water status, drought resistance, NO 3 - assimilation, and nitrogen-use effi ciency. While many soils provide suffi cient Cl¯ for micronutrient requirements, inland and strongly leached soils may fall below thresholds associated with macronutrient benefi ts, and agronomic responses to Cl¯-containing fertilizers have been reported across crops and regions. Conclusions Targeted management of Mg 2 + and Cl¯ offers a complementary route to increase crop productivity while reducing water consumption. Priorities include defining robust soil–plant diagnostic thresholds and translating mechanistic understanding of potential synergisms (particularly for N assimilation) into precision fertilization that delivers “more crop per drop”.