The practice of inoculating forage legumes with rhizobia strains is widespread. It is assumed that the inoculated strain determines the performance of the symbiosis and nitrogen fixation rates. However, native-naturalized strains can be competitive, and actual nodule occupancy is often scarcely investigated. In consequence, failures in establishment, and low productivity attributed to poor performance of the inoculant may merely reflect the absence of the inoculated strain in the nodules. This study lays out a strategy followed for selecting a Rhizobium leguminosarum sv. trifolii strain for white clover (Trifolium repens) with competitive nodule occupancy. First, the competitiveness of native-naturalized rhizobia strains selected for their efficiency to fix N₂ in clover and tagged with gusA was evaluated in controlled conditions with different soils. Second, three of these experimental strains with superior nodule occupancy plus the currently recommended commercial inoculant, an introduced strain, were tested in the field in 2 years and at two sites. Plant establishment, herbage productivity, fixation of atmospheric N₂ (¹⁵N natural abundance), and nodule occupancy (ERIC-PCR genomic fingerprinting) were measured. In both years and sites, nodule occupancy of the native-naturalized experimental strains was either higher or similar to that of the commercial inoculant in both primary and secondary roots. The difference was even greater in stolon roots nodules, where nodule occupancy of the native-naturalized experimental strains was at least five times greater. The amount of N fixed per unit plant mass was consistently higher with native-naturalized experimental strains, although the proportion of N derived from atmospheric fixation was similar for all strains. Plant establishment and herbage production, as well as clover contribution in oversown native grasslands, were either similar or higher in white clover inoculated with the native-naturalized experimental strains. These results support the use of our implemented strategy for developing a competitive inoculant from native-naturalized strains.

Drought stress is one of the most important factors in the limitation of plant productivity worldwide. In order to cope with water deprivation, plants have adopted several strategies that produce major changes in gene expression. In this paper, the response to drought stress in the model legume Lotus japonicus was studied using a transcriptomic approach. Drought induced an extensive reprogramming of the transcriptome as related to various aspects of cellular metabolism, including genes involved in photosynthesis, amino acid metabolism and cell wall metabolism, among others. A particular focus was made on the genes involved in the cellular stress response. Key genes involved in the control of the cell cycle, antioxidant defense and stress signaling, were modulated as a consequence of water deprivation. Genes belonging to different families of transcription factors were also highly responsive to stress. Several of them were homologies to known stress-responsive genes from the model plant Arabidopsis thaliana, while some novel transcription factors were peculiar to the L. japonicus drought stress response.

Crotalaria spectabilis and Crotalaria juncea are cover crops (CC) that are used in many different regions. Among the main attributes of these species are their high potential for biomass production and biological fixation of nitrogen (BNF). Attempting to maximize these attributes, while minimizing water consumption through high transpiration efficiency (TE), is a challenge in the design of sustainable agricultural rotations. In this study, the relationship between biomass productivity, BNF, and TE in C. spectabilis and C. juncea was evaluated. For this purpose, an experiment was carried out under controlled conditions without water limitations and using non-inoculated soil. BNF was determined by the natural abundance of ¹⁵N, while TE was estimated by several different methods, such as gravimetric or isotopic method (¹³C). C. juncea produced 42% less dry matter, fixed 28% less nitrogen from the air, and had 20% less TE than C. spectabilis. TE results in both species were consistent across methodologies. Under simulated environmental conditions of high temperature and non-limiting soil water content, C. spectabilis was a relatively more promising species than C. juncea to be used as CC.

Summer legume cover crops (CC) such as Crotalaria juncea, Crotalaria spectabilis, Crotalaria ochroleuca, and Cajanus cajan could offer diverse advantages for the environment and productive cropping systems. A low transpiration efficiency (TE) of CC can induce soil water content to levels that present a challenge for the subsequent crop. In a 75-day growth chamber experiment, using the natural abundance of ¹³C, ¹⁸O, and ¹⁵N we evaluated the TE and BNF under two soil water conditions. Our results showed that the four species tested are good candidates for their use as CC because they showed good results in terms of productivity parameters, TE, and BFN. Cajanus cajan had the highest TE, a high shoot dry matter production, and accumulated more N from BFN in the shoot than C. spectabilis, C. juncea, and C. ochroleuca. ∆¹⁸O increased under moderate water deficit and showed an inversely proportional relationship with the amount of transpired water, supporting the use of this isotopic indicator as a proxy for transpiration and stomatal conductance. For the isotopic parameters no interaction between the factors water regimen and species were found. We propose the mass ratio of nitrogen fixed by the volume of transpired water and the isotopic discrimination of ¹³C as useful indicators of drought fixing legumes tolerance.