Senescence, and phytotoxin, insect, fungal and mechanical damage: Factors reducing convective gas-flows in Phragmites australis

Premature senescence, insect bore holes and callus development in the gas-transport pathways were each shown to impair convective (pressurised) gas-flows in Phragmites australis (Cav.) Trin. ex Steudel. Premature senescence resulting from aphid and fungal attack, and possibly from the uptake of soil-borne phytotoxins, was found on average to have reduced the gas-flow potential of living culms by at least 70%. This was attributed chiefly to a reduction in humidity-induced convection resulting from a loss of healthy leaf-sheath area. Insect bore holes, found in living culms, reduced potential humidity-induced convective flows by varying amounts, by causing the culms to be leaky. It was shown that the degree of leakiness of culms should be dependent upon the position of the bore holes along the culm. Irrespective of their position and the wind speed, bore holes in old dead culms reduced Venturi-induced convection to almost zero. However, such holes could also increase convective flows by reducing resistance to venting from the underground parts, basal holes conferring lower resistance than more apical ones. In addition, it was concluded that callus development within gas pathways of the plant is stimulated by insect damage, by mechanical damage/flooding and by uptake of phytotoxins such as acetic acid and sulphide, and that extensive callus production might completely impede all pressurised gas flows. It is suggested that each of the above factors has the potential to contribute to Phragmites die-back by adversely affecting rhizome, root and rhizosphere aeration.