Na stress and grazing: Molecular, physiological and growth responses on four Serengeti C4 grasses

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Samuel J. McNaughton


Grazing, Growth responses, Serengeti, Soil sodium, Andropogan greenwayi, Sporobulus

Subject Categories

Botany | Ecology and Evolutionary Biology | Life Sciences | Plant Sciences


The concentration of soil sodium (Na) is an important factor that influences species distribution in the Serengeti short-grass plains, Tanzania. Experiments involving soil Na treatments were conducted to detect physiological (photosynthetic and water relations) and molecular (heat shock proteins and organic solutes) adaptations. The species tested were Andropogon greenwayi and three species of Sporobulus, S. ioclados, S. kentrophyllus and S. spicatus . Each species grows on different concentrations of soil Na that vary over two orders of magnitude. Clipping experiments were conducted to investigate the trophic interaction of grasses and herbivores with respect to Na, a required micronutrient in C4 grasses, and an essential nutrient for herbivores.

Comparisons of all four species detected short-term physiological and molecular responses to Na treatments (0, 100 and 400 m.M Na) that correlated with their field soil Na concentrations. S. kentrophyllus and S. spicatus exhibited rapid molecular induction of heat-shock proteins (Hsp) in response to experimental soil Na treatments (24 hr). Photosynthetic tolerance to Na was positively correlated with field soil Na concentrations, and Hsp induction was clearly associated with photosynthetic tolerance.

Long term (6 weeks) responses of the four species to Na treatment and clipping supported trends observed in the short term responses to Na. Species that occur on low soil salinity in the field did not survive past week one when treated with 400 m.M Na and exhibited significant reductions in biomass when treated with 100 MM Na. Reduced biomass, increased shoot tissue Na concentrations and Na tolerance correlated with the Na concentrations found in field soil Na concentrations. Analysis of rinse samples from leaf tissue showed that S. spicatus exudes Na salts. Clipping reduced the amount of root and shoot produced by S. spicatus and increased the total amount of Na available for herbivore consumption in all species. The results suggest that Na tolerant specie exhibit molecular responses that protect physiological processes and therefore sustain growth. Soil Na and grazing influence each species differently and these differences maintain the observed species distribution. This suggests that Na and grazing tolerance may have coevolved in the Serengeti ecosystem through a grass-grazer trophic interaction.