Rhizosheath occurrence in South African grasses

Rhizosheaths are the sandy coatings which cover the entire length of each root of many of the indigenous grass species growing in South Africa. The results of an extensive herbarium survey showed that rhizosheaths occur on more than 80% of the grass species studied, irrespective of the environmental conditions to which the individuals are exposed. Only 23 species did not have any sheath occurrence. The herbarium survey, together with growth experiments using Anthephora pubescens Nees, Digitaria eriantha Steud and Eragrostis pallens Hack, revealed that the extent of the rhizosheaths (the thickness and consolidation of the sheaths) varies not only between but also within species. The within-species variation is a function of soil texture. The higher the sand content in the soil the greater the number of epidermal hairs produced and the greater the extent of the sheaths. A. pubescens, D. eriantha and E. pallens individuals in soil with 80% sand had 75, 11 and 100 root hairs per centimetre of root length respectively. In comparison, the individuals in soil with only 30% sand had 55, 5 and 45 root hairs per cm of root length respectively. This relationship indicates that while species have a genetic predisposition to sheath development, the extent to which they develop is a facultative response to soil texture.