EFFECTS OF SOIL HYPOXIA AND SALINITY ON GAS-EXCHANGE AND GROWTH OF SPARTINA-PATENS

Laboratory experiments were conducted to evaluate the effects of increased soil salinity on gas exchange functioning and growth of the brackish-marsh grass Spartina patens (Ait) Muhl. under controlled soil redox potentials (Eh) which were maintained at either aerobic (+489 mV) or hypoxic (-40 mV) conditions, Plants were subjected to salinities of 0, 5, 15 and 25 ppt gradually over a 7 d period and leaf tissue chlorophyll content, gas exchange and growth responses were measured. Leaf tissue chlorophyll content was not affected by the treatments. Gas exchange of S. patens was adversely influenced by increases in salinity. Leaf conductance was reduced by 36 % and net photosynthesis by 23 % of control plants under aeration plus 25 ppt salinity, and water use efficiency increased to 161 % of control plants. When increased salinity accompanied hypoxia, leaf conductance and net photosynthesis were reduced by 60 and 44 % of control plants, respectively. The interaction between salinity and hypoxia was not significant. Salinity of 25 ppt reduced root dry weight, leaf dry weight and height growth significantly. Salinity and hypoxia combined affected various biomass components while hypoxia primarily affected root dry weights. Elevated salinity and hypoxia also adversely affected gas exchange functions and growth. Results presented suggested that saltwater intrusion is a major factor contributing to the rapid rate of marsh deterioration in coastal Louisiana (USA) brackish marshes which are dominated by S. patens.