A ROLE FOR PHYTOTOXINS IN THE PHRAGMITES DIEBACK SYNDROME

Die-back and healthy stands of Phragmites australis (CAV.) TRIN. ex ST EUD., in the U.K. and Hungary, were compared in terms of plant morphol ogy and anatomy, sediment redox potential and sulphide levels and plan t resistance to internal Poiseuille gas flow. In laboratory experiment s rhizome cuttings were exposed to acetic acid or dissolved sulphide i n unstirred solution cultures in order to determine whether the die-ba ck symptoms found in the field could be induced by these phytotoxins. Most of the die-back symptoms, namely stunting of adventitious roots a nd laterals, bud death, callus blockages of the gas-pathways, and vasc ular blockages (both xylem and phloem), were produced by each of the p hytotoxin treatments. These symptoms were largely absent from healthy field sites and from the experimental controls. In a greenhouse experi ment, plants were grown in waterlogged sand or loam, with or without a sub-surface organic layer composed of chopped-up rhizomes and roots m ixed with the soil base. Especially during the first 70 days, redox le vels were considerably lowered, and shoot numbers and shoot growth muc h reduced by the presence of the organic layers; the effects were most pronounced in the sand plus organic matter treatment. It is suggested that accumulated phytotoxins, e.g. organic acids and/or sulphide, whe ther produced from the death and decay of the plant, or from excessive organic loading or as an indirect result of eutrophication, will perp etuate the die-back of Phragmites and prevent the recovery of the plan t in the shea term.