Mycorrhizae enhance nitrogen fixation and photosynthesis in phosphorus-starved soybean (Glycine max L. Merrill)

Nitrogen (N) and phosphorus (P) are nutrients that frequently limit plant performance and crop yield. Currently, awareness of the importance of plant-microbe interactions in plant nutrient acquisition and growth promotion persist, particularly under stressful conditions, such as those resulting from P scarcity. The aim of the present study was to evaluate the effects of P starvation on the influence of arbuscular mycorrhiza (AM) symbiosis on N2-fixing capacities, plant growth, nutrient uptake and photosynthetic performance of soybean plants at flowering and grain filling stages. To this end, soybean (Glycine max) plants were nodulated with Bradyrhizobium elkanii and inoculated or not with the AM fungus Glomus macrocarpum. The +AM and −AM plants were grown under low P conditions (50 μM), and a control treatment with −AM plants under sufficient P concentration (500 μM) was also performed (−AM + P). Plants were grown until the beginning of the flowering and another set till the grain filling stage. The results showed that mycorrhization improved soybean plant performance under P scarcity compared with non-AM associated plants under the same conditions. AM positively influenced growth and plant nodulation at both flowering and grain-filling stages, without significant changes in plant P content. AM symbiosis increased nitrogenase activity, P content in nodules and leaf N content, the maximum carboxylation rate of Rubisco and reduced the metabolic limitation of photosynthesis under P starvation, which indicated the AM stimulus to symbiotic N2 fixation (SNF) and photosynthesis. Under these conditions, mycorrhizal symbiosis could not completely meet soybean P demand compared with well-nourished soybean plants, which produced higher plant and nodules biomass. In conclusion, AM association in nodulated soybean plants is likely an important strategy for maintaining SNF functioning, conferring increased plant growth and better N status under P scarcity conditions and significantly attenuating the negative effects of low Pi in nodulated-soybean plants.