Leaf diffusional capacity largely contributes to the reduced photosynthesis in rice plants under magnesium deficiency

Numerous studies have clarified the impacts of magnesium (Mg) on leaf photosynthesis from the perspectives of protein synthesis, enzymes activation and carbohydrate partitioning. However, it still remains largely unknown how stomatal and mesophyll conductances (g_s and g_m, respectively) are regulated by Mg. In the present study, leaf gas exchanges, leaf hydraulic parameters, leaf structural traits and cell wall composition were examined in rice plants grown under high and low Mg treatments to elucidate the impacts of Mg on g_s and g_m. Our results showed that reduction of leaf photosynthesis under Mg deficiency was mainly caused by the decreased g_m, followed by reduced leaf biochemical capacity and g_s, and leaf outside-xylem hydraulic conductance (K_ox) was the major factor restricting g_s under Mg deficiency. Moreover, increased leaf hemicellulose, lignin and pectin contents and decreased cell wall effective porosity were observed in low Mg plants relative to high Mg plants. These results suggest that K_ox and cell wall composition play important roles in regulating g_s and g_m, respectively, in rice plants under Mg shortages.