An assessment of symbiotic N nutrition in species of the genus Aspalathus endemic to the Cape Floristic Region of South Africa

Abstract

Nitrogen fixation is important for the growth and yield of nodulated legumes. In this study, the 15N natural abundance technique was used to assess the symbiotic N nutrition of 15 Aspalathus species sampled from 13 locations. The results showed that all 15 species met part of their N requirements from N2 fixation, as shown by the different levels of symbiotic dependency. Seven species, for example, derived 54 to 77% of their N nutrition from symbiosis, nine obtained 36 to 49% from fixation, and five species derived ˂ 20% of their N from the atmosphere. More specifically, A. acuminata and A. clada both from Worcester Langerug derived 76.5% and 72.1% N, respectively, from symbiosis, A. acuminata from Rondebosch obtained 68.4% N, A. acuminata from Kalbaskraal 59.5% N, A. aculeata from Mamesbury 56.8% N, A. divericata from Penhill 56.6% N and A. cordata from Dutoitkloof 53.9% N from the atmosphere. In contrast, A. zeyheri sampled from Kokrivier derived only 4.3% of its N from symbiosis, followed by A. capensis (13.3%) and A. carnosa (14.6%) both from Silvermine. The C/N values found for the Aspalathus species in this study were unusually high for a legume, and could suggest an ecological adaptation to the low N soils of the Cape fynbos as under such N-poor plants metabolically switch from N-containing storage compounds to N-free storage molecules such as sucrose and glycerol, glucosides, galactoses and disaccharides.