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 d15Nvalue of
soil-derived N. Isotopic analysis showed that wild populations
of Cyclopia were highly dependent on N2 fixation
for theirNnutrition, 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 d15N value of the reference
plants and the B value of the test Cyclopia species, where B
is the d15N 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 nutrientpoor
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 d15N values within sites, however, suggests that
genetic variability is not a major factor influencing N2
fixation rates in cultivated Cyclopia, and thatmore benefit
may be gained from soil amelioration and the selection of improved rhizobial strains
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:tut/oai:encore.tut.ac.za:d1000376 |
| Date | 07 January 2009 |
| Creators | Spriggs, AC, Dakora, FD |
| Publisher | Oxford University Press |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Text |
| Format | |
| Rights | Oxford University Press |
| Relation | Tree Physiology |
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