Molecular, physiological, and growth responses to sodium stress in C-4 grasses from a soil salinity gradient in the Serengeti ecosystem

The concentration of soil sodium (Na) is an important factor that influence s species distribution in the Serengeti short-grass plains, Tanzania. Exper iments were conducted to characterize physiological (growth, photosynthetic , nutrients, and water relations) and molecular (heat shock proteins and or ganic solutes) responses to high soil sodium in four Serengeti C-4 grasses. The species tested were Andropogon greenwayi and three species of Sporobul us, S. ioclados, S. kentrophyllus and S. spicatus. Andropogon greenwayi occ urs on locations with low soil Na concentrations, S. ioclados on low to mod erate, S. kentrophyllus moderate to high, and S. spicatus on soils with hig h Na concentration. Among all four species, short-term physiological and molecular responses to Na treatments (0, 100,: and 400 mmoI/L Na) were correlated with their fiel d soil Na concentrations. Sporobulus kentrophyllus and S. spicatus exhibite d rapid molecular induction of heat shock proteins in response to experimen tal soil Na treatments within 24 h and had increased levels of proline with in 96 h in contrast to A. greenwayi and S. ioclados. Photosynthetic rates a nd water relations were positively correlated with field soil Na concentrat ions and Hsp induction was clearly associated with photosynthetic tolerance . Long-term (6 wk) responses of the four species to Na treatment were consi stent with the short-term responses to Na. Species that occur on low Na soi ls in the field did not survive past week 1 when treated with 400 mmol/L Na and exhibited,significant reductions in biomass when treated with 100 mmol /L Na. Reduced biomass was associated with increased shoot tissue Na concen trations, and thus Na tolerance correlated with the Na concentrations of fi eld leaf tissue. The results demonstrate that the community distribution of these species reflects their Na tolerance and that the observed physiologi cal and molecular responses in tolerant species may have adaptive significa nce.