photosynthetic responses in reed (phragmites australis (cav.) trin. ex steud.) seedlings induced by different salinity-alkalinity and nitrogen levels

many phragmites-dominated wetlands have been markedly salinized and alkalinized in the songnen plain, northeastern china. agricultural wastewater with high nitrogen content has been discharged into these alkalinized-salinized wetlands. to understand the effect of salinity-alkalinity on reed (phragmites australis) seedlings at various nitrogen levels, we examined photosynthesis rate, chlorophyll fluorescence characteristics, and chlorophyll content of reed seedlings using gas exchange and chlorophyll a fluorescence tests. the greatest decreases (by 82%, 15%, 82% and 98%) of net photosynthesis rate (pn), maximal efficiency of photosystem ii (psii) photochemistry (fv/fm), comprehensive photosynthesis performance index (piabs) and plant height growth rate were observed at high salinity-alkalinity (mixed with 150 mm nacl and 100 mm nahco3). stomatal limitation was the main reason for decreased photosynthesis rate at low salinity-alkalinity (mixed with 50 mm nacl and 25 mm nahco3). the activity of psii was significantly inhibited at high salinity-alkalinity. both donor and acceptor sides of psii are the target sites of high salinity-alkalinity. high n (30 mm) at low salinity-alkalinity and moderate n (15 mm) at high salinity-alkalinity mitigated the toxicity of salinity-alkalinity on reeds and promoted plant height growth, chlorophyll synthesis, and psii activity. proper levels of n partly reduced the toxicity of salinity-alkalinity on the donor and acceptor sides of psii. this suggests that agricultural wastewater containing high level of nitrogen may be helpful in restoration of phragmites-dominated salinized wetland, though the n level needed for salinity-alkalinity stressed reed varies with the salinity-alkalinity level.