Molecular, physiological, and growth responses to sodium stress in C4 grasses from a soil salinity gradient in the Serengeti ecosystem.

The concentration of soil sodium (Na) is an important factor that influences species distribution in the Serengeti short-grass plains, Tanzania. Experiments were conducted to characterize physiological (growth, photosynthetic, nutrients, and water relations) and molecular (heat shock proteins and organic solutes) responses to high soil sodium in four Serengeti C(4) grasses. The species tested were Andropogon greenwayi and three species of Sporobulus, S. ioclados, S. kentrophyllus and S. spicatus. Andropogon greenwayi occurs on locations with low soil Na concentrations, S. ioclados on low to moderate, S. kentrophyllus moderate to high, and S. spicatus on soils with high Na concentration.Among all four species, short-term physiological and molecular responses to Na treatments (0, 100, and 400 mmol/L Na) were correlated with their field soil Na concentrations. Sporobulus kentrophyllus and S. spicatus exhibited rapid molecular induction of heat shock proteins in response to experimental soil Na treatments within 24 h and had increased levels of proline within 96 h in contrast to A. greenwayi and S. ioclados. Photosynthetic rates and water relations were positively correlated with field soil Na concentrations and Hsp induction was clearly associated with photosynthetic tolerance. Long-term (6 wk) responses of the four species to Na treatment were consistent with the short-term responses to Na. Species that occur on low Na soils 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 concentrations, and thus Na tolerance correlated with the Na concentrations of field leaf tissue. The results demonstrate that the community distribution of these species reflects their Na tolerance and that the observed physiological and molecular responses in tolerant species may have adaptive significance.