Abstract
Soybean inoculation with elite strains of Bradyrhizobium to improve nodulation, N2 fixation, and grain yield is well established worldwide. However, when grown in soils where N is deficient, soy- bean undergoes an initial phase of N starvation that may last up to 20 days after seedling germina- tion due to the lack of synchronism between the phase when seed N reserves are exhausted and the moment when plants begin to benefit from the nitrogen fixed by the bacteria. Practices that promote early nodulation may play a key role in reducing the N starvation period. Azospirillum is a plant growth promoting rhizobacteria (PGPR) that can stimulate root hair formation and root growth, creating more sites for early root infection and nodule formation by N2-fixing Bradyrhizo- bium spp. In this study, the effects of co-inoculating soybeans with Bradyrhizobium spp. and Azos- pirillum brasilense on nodulation precocity and N2 fixation were evaluated under greenhouse and field conditions. Nodule number and dry weight, as well as plant and root dry weight and N accu- mulated in shoots at 15, 18, 21, 24 and 30 days after emergence (DAE) were evaluated in response to inoculation with Bradyrhizobium spp. alone or when co-inoculated with Azospirillum sp. In the greenhouse, co-inoculated plants nodulated precociously as indicated by a significant increase (p < 0.05) in nodule biomass observed at (include) 21 DAE. More pronounced effects of co-inocula- tion were observed in the field as early as 18 DAE, suggesting that the presence of Azospirillum helps plants to overcome environmental stresses. * Corresponding author.
Highlights
The practice of seed inoculation with nitrogen-fixing bacteria with the objective of increasing soybean [Glycine max (L.) Merr.] yield has long been successfully established worldwide
A significant increase (p < 0.05) in nodule dry weight (NDW) and precocity of nodulation was observed in response to co-inoculation at 21 days after emergence (DAE), as well as total N in shoots (TNS) at 24 DAE (Table 2)
Correlation coefficients between NDW and TNS obtained in this study are similar to those reported from soybean experiments grown in sterile substrate under greenhouse controlled conditions and in pots containing non-sterile soil [means of 152 cultivars, r = 0.697, p < 0.001 [38]]
Summary
The practice of seed inoculation with nitrogen-fixing bacteria with the objective of increasing soybean [Glycine max (L.) Merr.] yield has long been successfully established worldwide. When grown in soils where N is deficient, even when effective rhizobia are present, legumes may undergo an initial phase of N starvation that may last up to 15 to 20 days after seedling emergence [3]-[5]. This is due to the absence of synchronism between the phase when seed N reserves are exhausted and the moment when plants begin to benefit from the nitrogen fixed by the bacteria in the recently formed root nodules [3] [4] [6]. Any practice that promotes earlier nodulation may help to reduce the N starvation period
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
