Nodulation and growth of pasture legumes with naturalised soil rhizobia. 1. Annual Medicago spp.

Abstract

The ability of 11 species of annual medics (Medicago doliata, M. laciniata, M. littoralis, M. minima, M. orbicularis, M. polymorpha, M. praecox, M. rigidula, M. rigiduloides, M. tornata and M. truncatula) to nodulate and fix nitrogen with naturalised rhizobia from 28 South Australian soils was assessed. The number of rhizobia in the soils was estimated. Medic shoot dry matter production and nodulation were measured, after inoculation of medic seedlings with a soil suspension, in 2 glasshouse experiments. The number of medic rhizobia ranged from 0.4 10 2 to 1.5 10 6 per gram soil. Medicago laciniata was the only medic species tested which was not consistently nodulated by the soil rhizobia. While all the other species formed nodules, they varied widely in their ability to form an effective symbiosis. Symbiotic performance (which indicates how much growth the medic line achieved, when compared to an effective inoculation treatment) of the medic species ranged from 3% (M. rigiduloides) to 67% (M. praecox). Herald (M. littoralis) achieved a symbiotic performance of 49% and it was estimated that this would be insufficient to meet the nitrogen requirements of a Herald-based pasture during early growth. The symbiotic performance of Santiago (M. polymorpha) was low (17%) and erratic (from –6 to 72%). The ability of the rhizobia to form an effective symbiosis varied widely also between soil regions. For example, the rhizobia in Riverland soils resulted in only 31% of the shoot dry matter of those in Eyre Peninsula soils, in association with M. polymorpha. There are significant opportunities to improve the symbiotic performance of a number of the species of annual medics examined in this study. Options to improve the effectiveness of the symbiosis of medics with naturalised soil rhizobia are discussed.

Genetic diversity of Sinorhizobium meliloti associated with alfalfa in Chilean volcanic soils and their symbiotic effectiveness under acidic conditions

“…Similar to our observations, several studies have demonstrated the wide variation in the symbiotic effectiveness of naturalized S. meliloti strains including strains capable to form effective symbiosis with alfalfa plants host (Ballard and Charman 2000;Evans et al 2005). Bradic et al (2003) not only observed wide variations in shoot dry matter yields in alfalfa plants inoculated with several indigenous S. meliloti strains, but also detected differences among alfalfa cultivars on several symbiotic parameters.…”

“…The higher numbers of rhizobia were obtained in neutral soils (pH~6.5; Las Encinas and Carillanca), while lower numbers were observed in acidic soils (pH~5.0; Rio Bueno) ( Table 1). Diverse studies have reported the negative effect of soil acidity on the persistence and the colonisation of S. meliloti in pasture systems (Brockwell et al 1991;Ballard and Charman 2000). Ballard et al (2003) demonstrated that soil acidity reduced the survival of rhizobia associated with alfalfa in Australian soils.…”

“…This species forms a symbiotic association with Sinorhizobium meliloti, which might supply most of the N required by plant growth. However, this symbiosis is highly sensitive to soil acidity, which decreases the survival of rhizobia and plant nodulation when soil acidity increases (Cheng et al 2005), with a concomitant reduction in productivity in pasture systems (Brockwell et al 1991;Ballard and Charman 2000).…”

Genetic diversity of resident soil rhizobia isolated from nodules of distinct hairy vetch (Vicia villosa Roth) genotypes

“…Resident rhizobia can be defined as those existing in a given soil, including native rhizobia and those previously introduced and naturalized over time. Ballard and Charman (2000) reported symbiotic efficiency, in which N is fixed and subsequently translocated to the host for improved growth and production, between resident clover rhizobia (R. leguminosarum biovar trifolii) and balansa clover (Trifolium michelianum Savi) to be as high as 128% of that of the recommended inoculant strain. Other studies have shown reduced symbiotic efficiency between resident rhizobia and pasture medics, with N-fixed ranging between −6 and 72% of that resulting from the inoculant strain (Ballard and Charman, 2000).…”

Genome sequence of the Medicago-nodulating Ensifer meliloti commercial inoculant strain RRI128

“…It also forms effective symbiosis with several species of annual medic ( M. truncatula , M. littoralis and M. tornata ) that happen to be closely related to each other based on their ability to be hybridized [5] and morphological and nucleotide sequence analyses of their relatedness [32]. RRI128 forms ineffective (white) nodules with Medicago polymorpha, a species that is generally recognized to have a more specific rhizobial requirement for effective symbiosis than Medicago sativa and Medicago littoralis [4,33] (Table 2). …”