Photoprotection mechanisms in desiccation tolerance of two Hymenophyllaceae Family (Pteridophyte) species contrasting in their vertical distribution :morpho-anatomical variations of fronds and structural and physiological changes at the chloroplasts level

Abstract

Hymenophyllum dentatum Cav. and Hymenoglossum cruentum C. Presl (Pteridophyta: Hymenophyllaceae) are desiccation tolerant ferns. They are poikilohydric epiphytes, with fronds composed by a single layer of cells and without true mesophyll cells and stomata. In Chile, these species belong to the temperate rain forest, and they usually inhabit shady and humid environments. However, as epiphytes, their vertical distribution varies along the trunk of the host plant. Some species inhabit drier sides with higher irradiance, as H. dentatum Cav., while others inhabit wetter and shadier sides. The aim of this work was to characterize the morphological variations in H. dentatum Cav. and H. cruentum C.Presl fronds during hydrated, desiccated, and rehydrated states. Both species were subjected to desiccation and rehydration kinetics to analyze the frond morphological variations in different hydration states by a scanning electron microscope. It was observed in both species, a change in the shape of desiccated cells from convex to concave. Fully hydrated fronds showed cells with an irregular polygon shape; a disorganized disposition of cells, and different sizes of them. Glandular multicellular hairs with a distal secretory cell were observed only in H. dentatum Cav. throughout the vascular system of the lamina. We conclude that, although cells decreased their sizes, there was not cells collapse in the desiccated state of both species, which helps to their fast recovery and functioning. The presence of wax glands in H. dentatum Cav., appears to be a different strategy to cope with desiccation tolerance in the higher zones of the host trunk.