PHRAGMITES DIEBACK - BUD AND ROOT DEATH, BLOCKAGES WITHIN THE AERATION AND VASCULAR SYSTEMS AND THE POSSIBLE ROLE OF PHYTOTOXINS

Die-back and healthy stands of Phragmates australis (Cav.) Trin. ex St eud. in the UK and Hungary were compared in terms of stand architectur e, plant morphology and anatomy, sediment redox potential and sulphide levels, plant resistance to internal Poiseuille gas flow, and convect ive flow characteristics of culms. Compared with healthy sites, die-ba ck 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 se ason' buds, and occasional prematurely senesced culms. Dead and stunte d adventitious roots with short peg-like laterals were very common, an d there was abnormal lignification and suberization within the cortex and epidermis/hypodermis of adventitious root apices, and in the epide rmis of lateral roots. Callus was found blocking the internal aeration pathways in root aerenchyma, root-rhizome junctions, rhizome nodes an d the bases of buds and culms. Blockages of the Vascular elements of b oth xylem and phloem in rhizomes and roots were also common. At the di e-back sites, callus accounted for a greater resistance to gas flow fr om the dead aerial culms into the rhizome system. Compared with health y culms, prematurely senesced culms from die-back sites exhibited smal ler potentials for aerating the rhizome system by humidity-induced con vection. 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 lignificat ion and suberisation causing interference with water and mineral absor ption, and (c) internal blockages causing interference with vascular t ransport. A tentative scheme for Phragmites die-back is presented.