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Soybean nitrogen (N) demands can be supplied to a large extent via biological nitrogen fixation, but the mechanisms of source/sink regulating photosynthesis/nitrogen fixation in high yielding cultivars and current crop management arrangements need to be investigated.We investigated the modulation of symbiotic nitrogen fixation in soybean [Glycine max (L.) Merrill] at different plant densities. A field trial was performed in southern Brazil with six treatments, including non-inoculated controls without and with N-fertilizer, both at a density of 320,000 plants ha −1 , and plants inoculated with Bradyrhizobium elkanii at four densities, ranging from 40,000 to 320,000 plants ha −1 . Differences in nodulation, biomass production, N accumulation and partition were observed at stage R5, but not at stage V4, indicating that quantitative and qualitative factors (such as sunlight infrared/red ratio) assume increasing importance during the later stages of plant growth. Decreases in density in the inoculated treatments stimulated photosynthesis and nitrogen fixation per plant. Similar yields were obtained at the different plant densities, with decreases only at the very low density level of 40,000 plants ha −1 , which was also the only treatment to show differences in seed protein and oil contents. Results confirm a fine tuning of the mechanisms of source/sink, photosynthesis/nitrogen fixation under lower plant densities. Higher photosynthesis and nitrogen fixation rates are capable of sustaining increased plant growth.

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Alfalfa, the most important forage crop in Argentina, shows considerable variability in forage production caused by variations in inter-annual rainfall and intra-annual radiation and temperature regimes. Such variation may affect radiation use efficiency and water use efficiency. This paper seeks to study the effects of temperature and water availability on radiation use efficiency and water use efficiency. We conducted the experiment in Córdoba, Argentina, under irrigated and droughted conditions. Drought was imposed by mobile rainout shelters during 3 consecutive periods. We measured forage, intercepted photosynthetically active radiation and water use to calculate radiation use efficiency and water use efficiency between cuttings. Under irrigation, radiation use efficiency and water use efficiency normalised by daytime vapour pressure deficit, were not limited by mean temperature above 21.3 and 21.9°C, respectively. Below those critical values, both variables decreased consistently with temperature decrements. Under drought, radiation use efficiency tended to decrease and water use efficiency tended to increase. In addition, the relationship between relative dry matter and relative water use was not linear, as reported in previous studies for annual crops.

Adjusting soybean [Glycine max (L.) Merrill] density can be critical to reduce inter-plant competition for water, nutrients, and sunlight, and to increase intercepted radiation, photosynthesis, and biomass production. The objective of this study was to evaluate the effects of soybean-population density on soybean nodulation, plant nutrient status, yield, and grain quality. Three field experiments were performed in southern Brazil with soybean cultivar BRS 284, of indeterminate growth type and maturity group 6.6, at 80,000 and 320,000 plants ha -1 . For N supply plants were dependent either largely on biological fixation of atmospheric N-with a naturalized population of Bradyrhizobium or inoculated with Bradyrhizobium japonicum strain CNPSo 2050-or largely on N fertilizer-200 kg N ha -1 , split-applied at sowing and R1 growth stage. The lower plant density resulted in increased nodulation parameters (number and mass), but plant nutritional status-evaluated by the diagnosis and recommendation integrated system (DRIS) method-in general was not affected. Seed oil content increased by 3.4%, but protein decreased by 4.5% at the lower plant density. The N source affected nodulation, but not nutritional status or grain yield. Although plant density was reduced by 75%, yield decreased by 16% during only one of the three cropping seasons. These results indicate a high plasticity of soybean to adapt photosynthesis and N fixation to different plant densities. Furthermore, planting at the lower density has the advantages of lower input costs and less susceptibility to environmental and plant nutritional stresses.Supplemental material available online. M.J. de Luca, Instituto Nacional de Tecnología Agropecuária (INTA), Rivera Indarte 72 2° 216 (5000), Córdoba, Argentina; M.J. de Luca, M.A. Nogueira, and M. Hungria,