Vertical differences in Phragmites australis culm anatomy along a water depth gradient

Abstract

We reported on quantitative anatomical comparisons of reed culms grown under a wide range of water depth (from −10 to +230 cm). The study focused mainly on the above water parts of ramets, but also provided an insight into the submerged internodes. Investigated anatomical features were: radial thickness of the internode wall and that of each tissue constituting it (epidermis and subepidermal tissues, aerenchyma channels, cortical sclerenchyma and parenchyma); areas of aerenchyma channels and the summed transversal area of them; areas of the innermost vascular bundles involving the bundle sheath sclerenchyma, phloem and metaxylem vessels; the cross-sectional area of the pith cavity and that of the internode wall. After a 2–3% decrease below the uppermost internode, the thickness of internode tissues, except for cortical sclerenchyma, continuously increased downwards. Quantitative differences between the ramets grown at different water depths were observed only from the lower aerial internodes. Parenchyma taking part considerably in the mechanical stability of culms with its large extension and thick cell walls was more-layered and thicker (with the maximum thicknesses of approximately 1200 and 1000 μm) in ramets from 180–200 than from 20–30 cm water depth. Areas of the innermost vascular bundles increased downwards and decreased after a maximum value appearing in lower internodes of ramets grown in deeper than shallower water. Aerenchyma channels appeared farther up from the water surface in culms in shallower than deeper water: the top ends were found about 50 cm higher in culms from 20–30 than from 180–200 cm water depth. Along the transect at right angles to the shore, the radial thickness and area of aerenchyma channels were higher in ramets grown in medium water depth—the summed transversal area of them was greater than 4 mm 2 in internodes at the water surface, while plants from the two ends of the water depth gradient had smaller aerenchyma channels (with approximately 1 mm 2 total areas). In contrast, pith cavity area at the water surface continuously increased – up to 60 mm 2 – from shallower to deeper water. Therefore, oxygen transport in ramets grown at the open water fringe of stands may occur mostly in the pith cavity.