Comparative patterns of sodium accumulation in leaves of selected savanna species growing on sodic and nonsodic soils

Abstract

Plants show differing mechanisms of tolerance to high sodium levels in soils. Physiological and biochemical mechanisms have been investigated for many crop species but little information is available for indigenous tree and grass species. Foliar concentrations of sodium, calcium, potassium, magnesium and nitrogen were measured in selected savanna species growing on and off sodic soils. Sodic soils are those defined as having a high exchangeable sodium percentage or a high sodium adsorption ratio. Sodium concentrations of up to 6% were measured in the leaf tissue of species found growing solely on sodic sites. Species that grow both on and off sodic sites did not show significantly different levels of foliar nutrients although the levels of exchangeable soil cations were quite different on the two sites. The data suggest that plants which grow specifically on sodic soils have inclusion mechanisms which allow for high levels of sodium in the leaf tissue which do not disrupt physiological functioning whereas other species have the ability to exclude sodium either at the root membrane level or by sequestration in the root tissue. In all cases there was an inverse relationship between calcium and sodium levels in the tissue suggesting competitive uptake. Although sodic-soil sites have low vegetation cover, herbivores tend to congregate at these sites. It is suggested that sodium, magnesium and potassium in the leaves are influential in attracting herbivores to sodic-soil sites.