The Response of Photosystem II to Soil Salinity and Nutrients in Wetland Plant Species of the Northwestern Gulf of Mexico

The photosynthetic response of many wetland plant species to soil salinity and nutrients has been described in the laboratory, but less is known about the cumulative effect of these abiotic factors in the natural environment. In this investigation we correlated field measurements of chlorophyll fluorescence with simultaneous measurements of soil nitrogen and phosphorus content, soil salinity, and relative leaf nitrogen content in Spartina alterniflora (Loisel), Spartina patens ((Aiton) Muhl), Schoenoplectus californicus ((C.A. Mey.) Steud.) and Schoenoplectus robustus ((Pursh) M.T. Strong), common in brackish and salt marshes of the northwestern Gulf of Mexico. The relationship between chlorophyll fluorescence and soil N :P ratio was not significant in S. alterniflora, nonlinear in S. patens and S. robustus, and significant and positive in S. californicus. The relationship between leaf nitrogen content and effective quantum yield was significant and positive in S. alterniflora and S. robustus, but only S. alterniflora appeared to be able to increase relative leaf nitrogen content over a wide range of soil nutrient and salinity regimes. Schoenoplectus californicus had the greatest potential for photosynthetic light capture but also had the narrowest ecological distribution. Thus, the species best adapted to high levels of abiotic stress (S. alterniflora) was less dominant at lower salinities, and the species with the highest potential for photosynthetic performance (S. robustus and S. californicus) were only found in locations with favorable abiotic conditions. We found that the range of environmental conditions experienced by each species in the field is greater than what has been considered in laboratory investigations.