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

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 (V_cmax), phosphoenolpyruvate carboxylase activity (V_pmax), and maximum electron transport rate (J_max). Maximum V_cmax values of 105 μmol m^-2 s^-1 were observed at the leaf temperature of 311 K. Optimum V_pmax values (80. 6 μmol m^-2 s^-1) were observed at the foliage temperature of 308 K. The observed V_pmax values were lower than those in other C₄ plants, whereas V_cmax values were higher, and more representative of C₃ plants. Optimum J_max 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.