Variation in leaf mesophyll anatomy of fern species imposes significant effects on leaf gas exchange, light capture, and leaf hydraulic conductance.

The mesophyll anatomical traits are essential factors for efficient light capture, CO₂ diffusion, and hydraulics in leaves. At the same time, leaf hydraulics are governed by the xylem anatomical traits. Thus, simultaneous analyses of the mesophyll and xylem anatomy will clarify the links among light capture, CO₂ capture, and water use. However, such simultaneous analyses have been scarcely performed, particularly on non-seed plants. Using seven fern species, we first showed that fern species with a large mesophyll thickness had a high photosynthetic rate related to high light capture, high drought tolerance, and low leaf hydraulic conductance. The chloroplast surface area (S_c) per mesophyll thickness significantly decreased with an increase in mesophyll thickness, which may increase light diffusion and absorption efficiency in each chloroplast. The photosynthetic rate per S_c was almost constant with mesophyll thickness, which suggests that ferns enhance their light capture ability via the regulation of chloroplast density.