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

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Tuesday 29 November 2011
Concurrent Session 6 – Field Applications II
1100 - 1230
Authors: Ryan Farquharson 1 , Ross Ballard 2 , Nigel Charman 2 , Jeff Baldock 1
1 CSIRO Land and Water. PMB 2 Glen Osmond, South Australia 5064.
2 South Australian Research and Development Institute. GPO Box 397, Adelaide, South
Australia 5001.
Presentation Title: Symbiotic performance of a herbicide tolerant Medicago littoralis
Presentation Time: 1200 – 1220
Group B herbicides inhibit the acetohydroxyacid synthase (AHAS – also known as acetolactate synthase)
enzyme in the pathway of branched chain amino acid synthesis. These herbicides have gained widespread use
worldwide; however the potential impacts on nitrogen fixation are underappreciated. A new strand medic
(Medicago littoralis) cultivar (FEH-1) was developed by chemical mutagenesis of seed of the Herald variety
(Heap, 2000). FEH-1 displayed tolerance to various group B herbicides including the sulfonylurea class,
however this work had only analysed biomass production and yield with no consideration of nitrogen fixation
(Heap, 2000; Howie and Bell, 2005). Through a targeted approach, a single mutation in the AHAS gene was
identified as the mechanism of herbicide tolerance (Oldach et al., 2008). An experiment comparing the symbiotic
performance of FEH-1 and Herald showed that FEH-1 had useful tolerance to chlorsulfuron, with higher biomass
production, nodulation and nitrogen fixation than Herald after application of that herbicide. However it was noted
that in the absence of herbicide, Herald had greater biomass production, nodulation and nitrogen fixation than
FEH-1. Differences in seed mass and nitrogen could not explain the disparity and a „tolerance penalty‟ was
proposed. A subsequent experiment measuring the growth of Herald and FEH-1 with different strains of rhizobia
showed symbiotic compatibility had changed in FEH-1. This may be contributing to the tolerance penalty
observed when FEH-1 is grown in the absence of herbicide. Although there is scope to better understand the
complexities surrounding group B herbicides and nitrogen fixation, it is clear that where in-crop or residual
herbicides are of concern, herbicide tolerant varieties will provide a useful option to maintain inputs of biologically
fixed nitrogen.
Heap J (2000). Increasing Medicago resistance to soil residues of ALS-inhibiting herbicides. Thesis, University of
Adelaide.
Howie J & Bell CA (2005) Field performance of an annual medic tolerant of sulfonylurea herbicide residues.
Proceedings of the XX International Grassland Congress, Dublin, Ireland.
Oldach KH, Peck DM, Cheong J, Williams KJ, Nair RM (2008) Identification of a chemically induced point
mutation mediating herbicide tolerance in annual medics (Medicago spp.)
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