Effects of salinity on the growth of Phragmites australis

The field salinity tolerance of Phragmites australis was evaluated by investigating 27 natural reed habitats along the eastern and western coasts of Jutland, Denmark. Die-back took place in the lower fringe of stands, before the onset of flowering, at sites where soil water salinities were higher than 15 parts per thousand within the rooting depth. In greenhouse experiments, juvenile plants produced from seeds and rhizome-grown plants, grown over a range of salinity levers, displayed different levels of salt tolerance. Both types of plants showed low mortality at salinity levels of 15 parts per thousand and lower. A total of 75% of the rhizome-grown plants survived 22.5 parts per thousand salinity in the rooting medium, whereas only 12% of the juvenile plants survived this salinity level. All plants grown at salinity levels of 35 parts per thousand and 50 parts per thousand died. Relative growth rates of juvenile plants were negatively correlated with salinity levels. Relative growth rates of rhizome-grown plants on a wet weight basis showed an optimum at 5 parts per thousand salinity. However, rates based on leaf number and shoot height were unaffected by salinities from 0-5 parts per thousand, but decreased at higher salinities. Phragmites australis adapted to saline conditions by adjusting the level of osmotically active solutes in its leaves. In the salinity range allowing survival (0-22.5 parts per thousand) osmolality in leaves of rhizome-grown plants was approximately 200 mmol kg(-1) higher than medium osmolality. In leaves of juvenile plants, osmotic pressure and chlorinity increased exponentially at salinity levels above 10 parts per thousand in the rooting medium, indicating a lower capability of osmoregulation, However, water stress could also be responsible for the lower resistance to salinity of these juvenile plants, as was suggested by a wilted appearance of leaves exposed to high salinities. Leaf longevity was not affected by different salinity treatments. Phragmites australis did not use leaf abscission to excrete toxic salts during the growth season.