Physiological responses of Spartina alterniflora to varying environmental conditions in Virginia marshes

Abstract

Physiological measurements were used to investigate the dependence of photosynthesis on light, temperature, and intercellular carbon dioxide (CO2) levels in the C4 marsh grass Spartina alterniflora. Functional relationships between these environmental variables and S. alterniflora physiological responses were then used to improve C4-leaf photosynthesis models. Field studies were conducted in monocultures of S. alterniflora in Virginia, USA. On average, S. alterniflora exhibited lower light saturation values (~1000 μmol m−2 s−1) than observed in other C4 plants. Maximum carbon assimilation rates and stomatal conductance to water vapor diffusion were 36 μmol (CO2) m−2 s−1 and 200 mmol (H2O) m−2 s−1, respectively. Analysis of assimilation-intercellular CO2 and light response relationships were used to determine Arrhenius-type temperature functions for maximum rate of carboxylation (Vcmax), phosphoenolpyruvate carboxylase activity (Vpmax), and maximum electron transport rate (Jmax). Maximum Vcmax values of 105 μmol m−2 s−1 were observed at the leaf temperature of 311 K. Optimum Vpmax values (80.6 μmol m−2 s−1) were observed at the foliage temperature of 308 K. The observed Vpmax values were lower than those in other C4 plants, whereas Vcmax values were higher, and more representative of C3 plants. Optimum Jmax values reached 138 μmol (electrons) m−2 s−1 at the foliage temperature of 305 K. In addition, the estimated CO2 compensation points were in the range of C3 or C3–C4 intermediate plants, not those typical of C4 plants. The present results indicate the possibility of a C3–C4 intermediate or C4-like photosynthetic mechanism rather than the expected C4-biochemical pathway in S. alterniflora under field conditions. In a scenario of atmospheric warming and increased atmospheric CO2 concentrations, S. alterniflora will likely respond positively to both changes. Such responses will result in increased S. alterniflora productivity, which is uncharacteristic of C4 plants.