LAUSR

BACKGROUND AND AIMS For a comprehensive understanding of the mechanisms of changing the plant photosynthetic capacity during the plant evolutional history, leaf gas exchange and optical properties are two essential factors, both of which strongly relates to mesophyll anatomy. Although ferns are suitable for the investigation of the evolutional history of photosynthetic capacity, comprehensive research of fern species is yet to be undertaken. METHODS We investigated leaf optical properties, gas exchange, and mesophyll anatomy of fern species with a wide range of divergence time, using 66 ferns from natural habitats and eight glasshouse-grown ferns. We used a spectroradiometer and an integrating sphere to measure light absorptance and reflectance by the leaves. KEY RESULTS The fern species of a newer divergence time had a thicker mesophyll, a larger surface area of chloroplasts facing the intercellular airspaces (Sc), thicker cell walls and large light absorptance. Although no trend with divergence time was obtained in the leaf photosynthetic capacity on a leaf-area-basis, when the traits were expressed on a mesophyll-thickness-basis, the trends in the leaf photosynthetic capacity became apparent. On a mesophyll-thickness-basis, the species with a newer divergence time had a low maximum photosynthesis rate, which was accompanied by a low Sc. CONCLUSIONS We obtained a strong coordination among the light capture, mesophyll anatomy, and photosynthesis rate in fern species for the first time. The thick mesophyll of the ferns of a newer divergence time does not necessarily relate to the high photosynthetic capacity on a leaf-area-basis. Rather, the thick mesophyll that is accompanied by thick cell walls allowed the ferns to adapt to wider environments through increasing the leaf toughness, which would contribute to the diversification of fern species.