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 C-4 marsh grass Spartina alterniflora. Functional relationships between these environmental variables and S. alterniflora physiological responses were then used to improve C-4-leaf photosynthesis models. Field studies were conducted in monocultures of S. alterniflora in Virginia, USA. On average, S. alterniflora exhibited lower light saturation values (similar to 1000 mu mol m(-2) s(-1)) than observed in other C-4 plants. Maximum carbon assimilation rates and stomatal conductance to water vapor diffusion were 36 mu mol (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 mu mol m(-2) s(-1) were observed at the leaf temperature of 311 K. Optimum V (pmax) values (80.6 mu mol m(-2) s(-1)) were observed at the foliage temperature of 308 K. The observed V (pmax) values were lower than those in other C-4 plants, whereas V (cmax) values were higher, and more representative of C-3 plants. Optimum J (max) values reached 138 mu mol (electrons) m(-2) s(-1) at the foliage temperature of 305 K. In addition, the estimated CO2 compensation points were in the range of C-3 or C-3-C-4 intermediate plants, not those typical of C-4 plants. The present results indicate the possibility of a C-3-C-4 intermediate or C-4-like photosynthetic mechanism rather than the expected C-4-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 C-4 plants.