Effects of salinity on temperature-dependent photosynthetic parameters of a native C₃ and a non-native C₄ marsh grass in the Yangtze Estuary, China

The invasion of Spartina alterniflora along the coasts of China has allowed this C₄ grass to outcompete often much of the native, salt marsh vegetation, such as Phragmites australis (C₃ grass), in the Yangtze Estuary. In this study, native grass, P. australis, and non-native grass, S. alterniflora, were grown in fresh and saline water (moderate salinity of 15‰ and high salinity of 30‰) to compare the effects of salinity on photosynthetic and biochemical parameters in combination with measurement temperatures. The C₄ grass, S. alterniflora, showed a greater CO₂ assimilation rate than P. australis, across the tested temperatures. The net photosynthetic rate declined significantly with increasing salinity as a result of inhibited stomatal conductance together with a greater decrease in the maximum rate of electron transport (J_max). In P. australis, salt treatments shifted the optimum temperatures for the maximum rate of carboxylation by Rubisco (V_cmax) and J_max to lower temperatures. S. alterniflora showed a greater salt tolerance to moderate stress than that of the native grass, with lower sensitivity of V_cmax, J_max, and the maximum rate of phosphoenolpyruvate carboxylation. Both moderate and high stress decreased significantly stomatal conductance of S. alterniflora; high salinity reduced significantly photosynthetic efficiency and J_max. Our findings indicated that the combination of stomatal conductance, enzyme activity, and electron transport affected the photosynthetic performance of the plants in response to salt treatments. The success of S. alterniflora could be probably attributed to its C₄ photosynthetic pathway and the tolerance to moderate salinity. In this study, a modified parameterization of the photosynthetic model was suggested to support a more reasonable simulation of photosynthesis under salt stress.