Improving N2 fixation from the plant down: Compatibility of Trifolium subterraneum L. cultivars with soil rhizobia can influence symbiotic performance

Abstract

Plant genotypes of Trifolium subterraneum L. (subterranean clover) were evaluated for differences in symbiotic N 2 fixation with soil rhizobia, with the long-term aim of using plant selection to overcome sub-optimal N 2 fixation associated with poorly effective soil rhizobia. Symbiotic performance (SP) was assessed for 49 genotypes of subterranean clover with each of four pure Rhizobium strains isolated from soil. Plants were grown in N free media in the greenhouse and their shoot dry weights measured and expressed as a percentage of dry weight with R. leguminosarm bv. trifolii WSM1325, the recommended commercial inoculant. Average SP with two Rhizobium strains (H and J) ranged from completely ineffective to 80% of potential for the subterranean clover genotypes. Two clover cultivars with high (cv. Campeda) and low (cv. Clare) SP values were investigated in more detail. Campeda typically fixed more N 2 than Clare when inoculated with 30 soil extracts (4.2 vs 2.4 mg N 2 fixed/shoot) and with 14 pure strains isolated from those soils (4.2 vs 2.2 mg N 2 fixed/shoot). The poor performance of Clare could be attributed to interruptions at multiple stages of the symbiotic association, from nodule initiation (less nodules), nodule development (small, white nodules), through to reduced nodule function (N 2 fixed/mg nodule) depending on the inoculation treatment. Through the careful use of subterranean clover genotypes by plant breeders it should be possible to make significant gains in the SP of future subterranean clover cultivars.

Rhizobial communities in symbiosis with legumes: genetic diversity, competition and interactions with host plants

“…Rhizobia are common inhabitants of diverse soil types, although population sizes can substantially differ -from a few cells to 10 5 bacteria per gram of soil [46,47]. The architecture and functioning of rhizobial genomes is a reasonable basis for intra-diversification of local rhizobial populations.…”

“…The architecture and functioning of rhizobial genomes is a reasonable basis for intra-diversification of local rhizobial populations. Naturalized rhizobial populations relatively new to an environment, such as in Australian soils, may be composed of only few strains [48], and such low-diversified populations could be sparse as well as numerous [47]. Rhizobial communities originating from regions with an endemic legume presence could contain dozens of genetically and physiologically diverse strains [49][50][51][52][53].…”

“…Surprisingly, while the symbiotic efficiency of rhizobial populations may vary greatly, the majority are considered to have a low efficiency. Consequently, the use of high-effective strains as legume inoculants is considered a promising agricultural strategy [47,48].…”

Abundance and Diversity of Soybean-Nodulating Rhizobia in Black Soil Are Impacted by Land Use and Crop Management

“…These bacteria are very important in ecology and in the economy for their great nitrogen-fixing capability inside nodules, the symbiotic organ induced by rhizobia on their host legume plants. The existence of diverse rhizobia helped the host legumes to adapt to many different habitats (2), while the great diversity and vast geographic distribution of the legumes also shaped their distinct rhizobial populations and drove their diversification (3)(4)(5). Therefore, the diversity of rhizobia present in a certain ecosystem is the result of interactions among the rhizobia, their host legumes, and the biotic and abiotic factors of the ecosystem, as revealed by the previous biogeographic studies on the rhizobia associated with faba beans (6), Caragana spp.…”

Delayed inoculation of alfalfa with Sinorhizobium meliloti and Penicillium bilaiae

“…Although inoculation at seeding is a widely recommended practice, discussions with producers reveal that some do not inoculate seed and instead rely on existing soil populations of rhizobia to nodulate plants. These indigenous rhizobia are often ineffective strains, which are less efficient in fixing N 2 than introduced strains (MarquesPinto et al 1974;Thies et al 1991;Drew and Ballard 2010;Drew et al 2011). Little scientific information is available addressing situations where producers fail to inoculate the forage legume at seeding, or where inoculation fails and biological N fixation (BNF) is limited.…”