Die-back and healthy stands of Phragmites australis (Cav.) Trin. ex Steud in the UK and Hungary were compared in terms of stand architecture, plant morphology and anatomy, sediment redox potential and sulphide levels, plant resistance to internal Poiseuille gas flow, and convective flow characteristics of culms. Compared with healthy sites, die-back sites were characterized by lower sediment redox potentials, and in Hungary by very high sulphide levels. Plants at die-back sites showed a markedly clumped distribution, with fewer new shoots and fewer old flower heads than those at healthy sites, a high incidence of dead and decaying rhizomes and roots, and dead ‘overwintering’ and ‘growing season’ buds, and occasional prematurely senesced culms. Dead and stunted adventitious roots with short peg-like laterals were very common, and there was abnormal lignification and suberization within the cortex and epidermis/hypodermis of adventitious root apices, and in the epidermis of lateral roots. Callus was found blocking the internal aeration pathways in root aerenchyma, root-rhizome junctions, rhizome nodes and the bases of buds and culms. Blockages of the vascular elements of both xylem and phloem in rhizomes and roots were also common. At the die-back sites, callus accounted for a greater resistance to gas flow from the dead aerial culms into the rhizome system. Compared with healthy culms, prematurely senesced culms from die-back sites exhibited smaller potentials for aerating the rhizome system by humidity-induced convection.
It is suggested that die-back in Phragmites might be brought about and perpetuated at least partly by phytotoxins which induce (a) blockages within the aeration pathways due to callus development, (b) stunting of roots and the development of abnormal root wall lignification and suberisation causing interference with water and mineral absorption, and (c) internal blockages causing interference with vascular transport. A tentative scheme for Phragmites die-back is presented.