Leaf hydraulic architecture and water relations of three ferns from contrasting light habitats.

Leaf hydraulic architecture and water relations of three fern species were measured. The species selected were adapted either to deeply shaded (Woodwardia radicans), moderately shaded (Dryopteris affinis) or moderately sunny (Polystichum setiferum) habitats, as confirmed by microclimatic measurements performed in the field. Leaf water potential (Psi(leaf)) was lower and leaf conductance to water vapour (g(L)) was higher in P. setiferum than in the shade-adapted ferns. Leaf osmotic potential and water potential at the turgor loss point were lower in the sun-adapted species than in the other ferns. Leaf hydraulic resistance (R(leaf)) was lowest in P. setiferum and R(leaf) was correlated with g(L) across species. Low R(leaf) was coordinated with low rachis hydraulic resistance (R(rachis)). Low values of R(rachis) in P. setiferum were not due to the presence of wide xylem conduits as checked on the basis of anatomical measurements, but to increased radial permeability of vascular bundles. This was a consequence of the absence of endodermis surrounding the vascular bundles in P. setiferum, which was observed in the rachis of shade-adapted species. We conclude that hydraulic adjustment of fern fronds is a key component of adaptation of pteridophytes to contrasting light habitats.