IN INOCULANTS Nodulaid - 17th International Nitrogen Fixation ...

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Wednesday 30 November 2011
Concurrent Session 10 – Ecology of RNB
1100 – 1230
Authors: Macarena Gerding 1,2 , Graham O‟Hara 1 , Lambert Bräu 1 & John Howieson 1
1 Centre for Rhizobium Studies, Murdoch University, Murdoch, 6150, Western Australia.
2 Facultad de Agronomía, Universidad de Concepción, Casilla 537, Chillán, Chile.
Presentation Title: Mesorhizobium strains from the South African herbaceous legume Lessertia spp.
Presentation Time: 1120 – 1140
differ in their competitive ability in Australian soils
Legumes of the South African genus Lessertia, along with their microsymbionts, were introduced into different
sites of the Western Australia wheatbelt during 2007-2009. Four Lessertia spp. achieved only poor establishment
followed by weak summer survival. This was caused in part by low levels of nodulation with the inoculant strains,
and by competitive ineffective nodulation from naturalized strains of Rhizobium leguminosarum bv. trifolii. To
improve the adaptation of the legume and to assess the competitive ability of inoculant strains, two field
experiments were set up at Karridale, Western Australia. The aims were to assess the effect of increased doses
of an effective inoculant strain (WSM3565) with L. herbacea, and to study the competitive ability and symbiotic
performance of different Mesorhizobium strains nodulating L. diffusa.
Increasing the inoculation dose of L. herbacea with WSM3565 did not improve establishment and survival of the
legume. Although WSM3565 nodule occupancy improved from 28 to 54% with higher doses of inoculation, none
of the treatments increased L. herbacea yield over the inoculated control.
The inoculation of L. diffusa with the strains WSM3598, 3636, 3626 and 3565 resulted in greater biomass
production than the uninoculated control. These strains were able to outcompete resident rhizobia and to occupy
a high (>60%) proportion of lateral root nodules. Low rainfall in 2009 caused an early senescence of tap root
nodules and plants had to rely on nitrogen fixation from nodules formed later in the season (i.e. lateral root
nodules). This possibly explained the increased dry weight achieved by these strains.
The high numbers of resident rhizobia in Western Australian soils, and their ability to rapidly nodulate Lessertia
spp. are likely to be the main reasons for the low nodule occupancy achieved by some effective inoculant strains
with L. diffusa and L. herbacea. The naturalised strains that achieved nodulation were identified as R.
leguminosarum bv. trifolii, whereas in the South African soils Lessertia is nodulated by mesorhizobia. This
phenomenon represents a barrier to the successful introduction of the exotic legume genus Lessertia to WA
soils.
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