Changes in growth and photosynthesis linked with intensity and duration of salinity in Phragmites karka

Phragmites karka is gaining increasing attention as a biofuel crop due to high ligno-cellulosic biomass. In this study we investigated plant growth, gas exchange, chlorophyll fluorescence, photosynthetic pigments, and soluble sugar after 7, 15, and 30 days exposure to saline conditions [0 (control), 100 mM (moderate) and 300 mM (high) NaCl treatments]. Growth rate and net photosynthesis (A(NET)) were unchanged during short term (0-7 days) exposure to moderate salinity but decreased at 300 mM NaCl due to reduction in net assimilation rate, stomatal conductance (gs), and intercellular carbon dioxide concentration (Ci). However, growth rate decreased under long term (15-30 days) exposure to moderate salinity, while an increase in water use efficiency (WUE) and instantaneous carboxylation efficiency (COE) helped to maintain A(NET). Higher photosynthetic pigments, respiration, sugar content and efficiency of photosystem II (YII) appear to work together to reduce the risk of oxidative stress at 100 mM NaCl. Long term exposure at 300 mM NaCl decreased the A(NET), gs, COE, and YII. Stomatal closure improved WUE but resulted in increased ROS production (ETR/A(gross)). Photosynthesis was reduced by stomatal limitation under short-term exposure to high salinity and by both stomatal and biochemical limitation during long term exposure. These results indicate that P. karka can survive in moderate salinity for long durations by photosynthetic adaptations (photochemistry and gas exchange) that are vital for growth and biomass production in natural ecosystems.