Tree Physiology Advance Access originally published online on December 9, 2008
Tree Physiology 2009 29(2):239-247; doi:10.1093/treephys/tpn021
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Field assessment of symbiotic N2 fixation in wild and cultivated Cyclopia species in the South African fynbos by 15N natural abundance
1 Department of Botany , University of Cape Town, Private Bag, Rondebosch 7701, Cape Town, South Africa
2 Department of Chemistry, Tshwane University of Technology, 175 Nelson Mandela Drive, Private Bag X680, Pretoria 0001, South Africa
3 Corresponding author (dakorafd{at}tut.ac.za)
 |
Abstract |
|---|
Nitrogen (N) derived from symbiotic fixation of atmospheric N2 in wild and cultivated populations of Cyclopia, a woody endemic genus used to make honeybush tea in the Western Cape of South Africa, was quantified by the 15N natural abundance method. Because Cyclopia species are naturally mycorrhizal, non-N2-fixing arbuscular mycorrhizal shrubs of similar phenology to Cyclopia were chosen as reference plants to provide the 
15N value of soil-derived N. Isotopic analysis showed that wild populations of Cyclopia were highly dependent on N2 fixation for their N nutrition, ranging from 70 ± 4% to 100 ± 7% (mean ± SE) at all sites, except for one. Further evidence of the high dependency of wild Cyclopia populations on symbiotic N was provided by their significantly higher foliar N concentrations compared with the non-legume reference plants. However, cultivated Cyclopia exhibited variable amounts of N2 fixation, with Cyclopia genistoides (L.) R. Br., for example, showing low amounts of N2 fixation at Sites P2 and P3 (0 ± 51% and 8 ± 46%, respectively) as a result of low D values (D is defined as the difference between the mean 15N value of the reference plants and the B value of the test Cyclopia species, where B is the 15N of an inoculated test legume grown in an N-free growth medium), whereas at Sites P1, P2, P5 and P6, up to 89 ± 2%, 94 ± 13%, 85 ± 13% and 100 ± 18%, respectively, plant N was derived from atmospheric fixation. The high symbiotic N nutrition observed for wild populations of Cyclopia suggests that these populations are major contributors to the N economy of the nutrient-poor soils of the South African fynbos. These data indicate that by breeding for high N2 fixation rates in Cyclopia cultivars and selecting more efficient rhizobial strains, this legume has the potential to achieve higher N2 fixation rates under cultivation. The low variability in Cyclopia 15N values within sites, however, suggests that genetic variability is not a major factor influencing N2 fixation rates in cultivated Cyclopia, and that more benefit may be gained from soil amelioration and the selection of improved rhizobial strains.
Keywords: natural populations, plantations
Received June 17, 2008; Accepted August 24, 2008