Guo, R., Zhou, J., Hao, W., Gong, D., Zhong, X., Gu, F., Liu, Q., Xia, X., Tian, J. and Li, H. 2011. Germination, growth, photosynthesis and ionic balance inSetaria viridis seedlings subjected to saline and alkaline stress. Can. J. Plant Sci. 91: 1077-1088. Setaria viridis seeds and seedlings were subjected to saline and alkaline stress to determine their effects on germination and growth indices, and physiological parameters. The treatments consisted of solutions of NaCl:Na₂SO₄ for saline stress and NaHCO₃:Na₂CO₃ for alkaline stress, at different concentrations. Germination, growth, photosynthesis, chlorophyll fluorescence and ionic balance were determined to elucidate the difference in the mechanism of saline and alkaline stress damage to S.viridis seedlings, and their physiological adaptive mechanism to stress. Germination indices decreased and, germ and root growth were severely inhibited with increased concentrations, especially under alkaline stress. We also found that alkaline stress had a more severe effect on S.viridis seedlings than saline stress, which was demonstrated by greater reduction in relative growth rate, water content and root system activity. Similarly, the reduction in photosynthesis and chlorophyll fluorescence was greater under alkaline than saline stress. Alkaline stress also appeared to exacerbate the ionic imbalance generated by saline stress; alkaline stress caused a heavy precipitate of phosphate and metal ions, excluding Na and K , and the loss of NO₃^- and H₂PO₄^-, which caused a sharp decrease in ionic activity and free concentrations of various ions. Our results suggest that damage caused by alkaline stress might come from greater influx of Na than caused by saline stress. Setaria viridis responds by synthesizing greater amounts of organic acids than are required to counter salinity to address the intracellular ion imbalance; this process is at the cost of the plant's already diminished energy resource.