Phragmites australis: effects of shoot submergence on seedling growth and survival and radial oxygen loss from roots

Laboratory experiments indicated that any degree of permanent submergence +40–+120 mm) of young seedling shoots of Phragmites (height: 20–30 mm) greatly reduced (75–100%) the chances of their becoming emergent. With +40 mm submergence and with an absence of algae, about 25% of shoots emerged and grew normally; where algal growth developed during the experiment, 15% of shoots emerged, but where the submerging water was particularly rich in P and algae were present from the onset, no shoots emerged. Seedling leaves which became covered in epiphytic algae lost their chlorophyll. The roots of submerged plants tended to develop more extensive aerenchyma than those of controls whose shoots were emergent. When, after 25 days, water tables were lowered to the soil surface in sand treatments, seedlings recovered quickly except for those from a treatment with dense algal growth. In more mature seedlings (shoot height = 300–400 mm) gradual submergence of shoots in either light or darkness reduced radial oxygen loss (ROL) from adventitious root apices. At 23°C there was no ROL at complete submergence in darkness; at 18°C the values were from 7–23% of the dark, fully emergent condition. When the lights were switched on, ROL increased almost immediately due to photosynthetic oxygen generation and entrapment in the submerged shoots. At 18°C the values attained, relative to the dark emergent condition, were from 42–59%, but at 23°C they were much lower: 13–26%. These effects are discussed in relation to the regeneration of Phragmites by the establishment of seedlings, possibly after die-back.