Effects of magnesium deficiency on photosynthesis and carbohydrate partitioning

Magnesium nutrition is often forgotten, while its absence adversely affects numerous functions in plants. Magnesium deficiency is a growing concern for crop production frequently observed in lateritic and leached acid soils. Competition with other cations (Ca²⁺, Na⁺, and K⁺) is also found to be an essential factor, inducing magnesium deficiency in plants. This nutrient is required for chlorophyll formation and plays a key role in photosynthetic activity. Moreover, it is involved in carbohydrate transport from source-to-sink organs. Hence, sugar accumulation in leaves that results from the impairment of their transport in phloem is considered as an early response to Mg deficiency. The most visible effect is often recorded in root growth, resulting in a significant reduction of root/shoot ratio. Carbohydrate accumulation in source leaves is attributed to the unique chemical proprieties of magnesium. As magnesium is a nutrient with high mobility in plants, it is preferentially transported to source leaves to prevent severe declines in photosynthetic activity. In addition, Mg is involved in the source-to-sink transport of carbohydrates. Hence, an inverse relationship between Mg shortage and sugar accumulation in leaves is often observed. We hereby review all these aspects with a special emphasis on the role of Mg in photosynthesis and the structural and functional effects of its deficiency on the photosynthetic apparatus.