Ground Cover North : Ground Cover 073 March-April 2008 - North
Promiscuous legumes to 'sex-up' nitrogen fixation THE NATIONAL RHIZOBIUM PROGRAM (NRP) IS WORKING TO DEVELOP EFFECTIVE RHIZOBIAL INOCULANTS TO IMPROVE THE NITROGEN FIXATION BY CROP AND PASTURE LEGUMES. THE NRP IS ALSO SEARCHING FOR LEGUMES THAT FIX NITROGEN WITH THE RHIZOBIA POPULATIONS THAT ARE ALREADY ESTABLISHED IN MANY AGRICULTURAL SOILS BY EMMA LEONARD n Rhizobia are the most successful of introduced soil organisms and are now found in large numbers in many Australian agricultural soils. However, the gradual replacement or evolution of the applied inoculant strains has seen some soil populations become less effective at nitrogen fixation as the time from inoculation lengthens. This is of particular concern with pasture legumes such as medics and subclovers, which are sown infrequently and allowed to regenerate year-on-year or after a cropping phase. An example of the suboptimal nitrogen fixation capacity that has developed for some key legume species is shown in Figure 1. The National Rhizobium Program (NRP) is now directing attention towards the management of the populations of soil rhizobia. Research by the Plant and Soil Health Group of the South Australian Research and Development Institute (SARDI) has identified pasture lines that are better able to fix nitrogen with background populations of soil rhizobia. Pasture lines that form effective symbiotic relationships have been called 'promiscuous' legumes. The research by Dr Ross Ballard, Dr Liz Drew and Nigel Charman focuses on the ability of different lines of medic and subclover to form effective symbiotic relationships with the soil populations of rhizobia. "Often pasture species form symbiotic relationships with the background rhizobia but the nodules are poor at fixing nitrogen for the plant," explains Dr Ballard. "At SARDI, we are developing a promiscuous line of strand medic. This has involved screening several hundred medic lines for effective nodulation when inoculated with soils gathered from regions where regenerating medics are important. Some outstanding lines were identified and retested with 33 different soil populations. The best promiscuous line performed well with most of these and has since been crossed with the popular cultivars HeraldA and AngelA. In glasshouse trials, using 15 soils from the Eyre Peninsula and the Mallee, the new crosses are more compatible with background rhizobia populations than the original cultivars. Eighth-generation progeny of the promiscuous crosses are ready to be assessed in field trials. For the past two years, Dr Drew has been searching for promiscuous cultivars of subclover, a major contributor of nitrogen to cropping systems. A focus of her work is to understand some of the mechanisms that control symbiotic promiscuity. While it is early days, already promiscuous subclover lines have been identified. In a screening trial of 50 subclover lines from the many thousands that are available, the levels of nitrogen fixation with soil populations of rhizobia ranged from 0 to 100 per cent of the commercial inoculant strain. Two factors, which could be contributing to the better performance, are being investigated. Some subclover lines are simply more compatible (able to fix nitrogen) with the array of rhizobia that reside in the soil. Other subclover lines may have the ability to select or exclude strains from the soil community to enhance their symbiotic capacity. "If we can understand the mechanisms that control the relationships between subclovers and rhizobia, this will provide a foundation for our understanding of this relationship in other legume species," Dr Drew says. Notwithstanding the efforts to manage the already suboptimal population of soil rhizobia through legume selection, rhizobia inoculants remain a vital tool in the success and improvement of legume nitrogen fixation, especially in sown pastures and grain legume crops. At the Centre for Rhizobium Studies at Murdoch University, WA, efforts are under way to ensure that new inoculants and legumes are less prone to developing poor symbioses. Dr Kemanthi Nandesena of Murdoch University, also under the umbrella of the NRP, is attempting to determine why ineffective populations develop. Research has shown that this is partly due to genetic exchange of key genes from the inoculant to other soil bacteria to form new root- nodulating bacteria that are less effective at nitrogen fixation. Dr Nandesena is currently working on stopping the transfer of these key genes from the inoculant strain for the new pasture legume biserrula. The importance of understanding the relationship between legume and background populations of rhizobia is not confined to regenerating pastures. For example, many field pea crops are sown into soils with large rhizobia populations. This is particularly the case for the neutral and alkaline pH soils of SA. While suboptimal symbioses are less frequent in current varieties of field pea (see Figure 1), new lines being developed with disease resistance contain diverse sources of pea germplasm. The NRP is working closely with pea breeders to ensure that new lines continue to form effective symbioses with commercial inoculants and, importantly, the soil rhizobia. International research highlights that if symbiosis is not considered in breeding programs, varieties with poorer nitrogen-fixation capacity can be the result. For this reason, Australian legume breeders and rhizobiologists are working in tandem on the evaluation of new lines of pulse and pasture legumes. GRDC Research Code UMU00003 More information: Dr Ross Ballard, SARDI, 08 8303 9388, firstname.lastname@example.org Nitrogen fixation GROUND COVER MARCH -- APRIL 2008 20 KEY POINTS n Pasture lines better able to fix nitrogen with soil populations of rhizobia have been found n Researchers are investigating why some legume species are more able to form symbiotic relationships with background rhizobia populations Dr Liz Drew is part of a team that has identified pasture lines better able to fix nitrogen with background populations of soil rhizobia, and is working to understand the mechanisms behind the performance of these lines. To prevent experimental contamination, plants are watered individually with sterile water using a sterilised syringe. PHOTOS: EMMA LEONARD Subclover (YorkA) 40 0 80 120 160 40 0 80 120 160 Group C inoculant Mean of 43 soil populations 48% Strand medic (HeraldA) Group AL inoculant Mean of 28 soil populations 56% Field pea (Parafield) 150 0 300 450 600 Group E inoculant Mean of 20 soil populations 70% Figure 1 Shoot dry matter production due to nitrogen fixation by strains of rhizobia used in commercial inoculants (orange) compared with that by the rhizobia that reside in Australian soils (red). Vertical bars indicate the range in the effectiveness of the communities of soil rhizobia.
Ground Cover 072 January-February 2008 - North
Ground Cover 074 May-June 2008 - North