Frond and mesophyll traits related to photosynthetic capacity and water-use efficiency in ferns with different life-forms ex situ

Leaf traits are known as indicative of species functional properties in angiosperms. Fern fronds are little studied in this concern, especially ex situ. We studied leaf mesophyll structure, pigments and gas exchange in four fern species grown in a glasshouse—Sphaeropteris cooperi (terrestrial tree fern), Phlebodium aureum (semi-epiphytic herbaceous), Asplenium australasicum (epiphytic shrubby) and Platycerium bifurcatum (epiphytic herbaceous). Saturated assimilation rate (A_sat) decreased and intrinsic water use efficiency (iWUE) increased from a terrestrial tree fern to epiphytic ferns. A_sat positively correlated with chloroplast number (N_chl/A) and their surface area per leaf area (A_chl/A). iWUE negatively related to N_chl/A and chlorophyll/carotenoid ratio. Most differences between species were found in the mesophyll thickness (MT) and mesophyll cell volume (V_cell) with the smallest values in S. cooperi and the largest ones in P. bifurcatum. We found that photosynthetic limitations in ferns were related to the chloroplast photosynthetic activity rather than to N_chl/A or pigment content which did not differ from most angiosperms. Epiphytic ferns showed larger values of V_cell per cell and per chloroplast compared to angiosperms and tree ferns. We concluded that an increase in MT and V_cell in the studied ferns was not associated with photosynthetic performance, but was related to volumetric cytoplasm–chloroplast ratio meaningful to light absorption and the water-storage function of fern fronds.