Effects of Salinity on Germination, Seedling Growth and Ecological Properties of Phragmites australis Communities in the Estuary of the Chikugogawa River, Southwestern Japan

摘要

Salt tolerance of Phragmites australis populations was investigated in natural reed habitats in the estuary of the Chikugogawa River, southwestern Japan. P. australis populations were selected along the salinity gradient in the estuary, including limnetic (salinity 0.05%), oligohaline (0.4%) and polyhaline sections (2.5%). The ratio of Total-P/Total-N of river water showed high values in the oligohaline section and the above ground biomass, population height and culm diameter of P. australis showed maximum values in this section. Sufficient phosphorus supply could sustain the high productivity of the community in the oligohaline section, irrespective of the salinity of inundated water. The seed production of P. australis was lowest in the polyhaline section. Thus, the ecological performance of P. australis was highest in the oligohaline section and the performance declined with the increasing salinity of the habitat. The effects of salinity on germination and seedling growth were evaluated by means of cultivation in 0.0% -5.0% (NaCl w/w%) salinity medium. Seeds of P. australis collected from every natural population in the estuary failed to germinate at salinity levels above 2.3%. Growth of shoot length and above-ground biomass of seedlings germinated in the fresh water medium were measured for 21 days’ exposure to constant salinity solutions ranging from 0.0% -5.0%. Although mortality was high at salinity levels 3.5% and 5.0%, elongation of shoots of some plants was evident at a salinity level of 5.0%. Seeds from the population in the limnetic section never germinated at a salinity of 3.5%, whereas seedlings obtained from the seeds of the same population germinated in freshwater conditions grew under a salinity of 3.5%. Thus, P. australis seedlings have higher salt tolerance compared to that during the germination stage of the seeds. In this study, it is clarified that P. australis has low salt tolerance during the germination stage and then it acquires salt tolerance during the stage of seedling growth.