Testing Whether Pre-Pod-Fill Symbiotic Nitrogen Fixation in Soybean Is Subject to Drift or Selection Over 100 Years of Soybean Breeding

Abstract

Soybean [Glycine max (L.) Merr.] is the world's leading legume crop and the largest oilseed crop. It forms a symbiotic relationship with rhizobia bacteria residing in root nodules that provide fixed nitrogen to host plants through symbiotic nitrogen fixation (SNF). In soybean, it has been widely reported that the highest SNF occurs at the pod-filling stage, associated with the peak demand for nitrogen. However, the majority of seed nitrogen is derived from remobilizing root/shoot nitrogen, representing cumulative SNF from the seedling stage to the pre-pod-fill stage. Therefore, the question arises as to whether there has also been selection for improved SNF at these earlier stages, or whether pre-pod-fill SNF traits have drifted. To test this hypothesis, in this study, pre-pod SNF-related traits were evaluated in soybean cultivars that span 100 years of breeding selection in the Canadian Province of Ontario. Specifically, we evaluated SNF traits in 19 pedigree-related historical cultivars and 25 modern cultivars derived from the University of Guelph soybean breeding program. Field trials were conducted at Woodstock, Ontario, Canada in 2016 and 2017, and various SNF-related traits were measured at pre-pod-fill stages (R1-R3), including nitrogen fixation capacity. Considerable variation was observed among Canadian soybean cultivars released over the past 100 years for pre-pod-fill nitrogen fixation. The modern soybean cultivars had similar or moderately higher pre-pod-fill SNF compared to the historical lines in terms of the percentage of nitrogen derived from the atmosphere (%Ndfa) and total shoot fixed nitrogen. These findings suggest that, despite no direct selection by breeders, pre-pod-fill nitrogen fixation, and associated SNF traits have been maintained and possibly improved in modern soybean breeding. However, the low level of pre-pod-fill SNF in some modern cultivars, and generally wide variation observed in SNF between them, suggest some level of genetic drift for this trait in some pedigrees. Specific historical and modern soybean cultivars were identified as potential parents to enable targeted breeding for improved pre-pod-fill SNF. This retrospective study sheds light on our understanding of the impact of decades of recent selective breeding on pre-pod-fill nitrogen fixation traits in soybean in a temperate environment.

The effect of drought stress on nodulation, plant growth, and nitrogen fixation in soybean during early plant growth

“…Another possibility is that the increased average nodule size of the drought treated soybeans increased the N fixation capacity sufficiently enought to eliminate the difference. The %Ndfa rates reported in this study under well‐watered conditions are comparable with our previous study under field conditions, where SNF was measured at the pre‐pod‐fill stages (R1‐R3) (Thilakarathna et al, 2021).…”

“…However, we did not find a significant difference between the drought and well‐watered treatments for %Ndfa ( p = .0559, Figure 2e), and this can be due to a few reasons. Soybean generally has low N 2 ‐fixation during vegetative growth compared to the pod‐filling stage (Ortez et al, 2018; Thilakarathna et al, 2021), where our data also show that only 61% and 54% of the total shoot nitrogen was derived from N 2 ‐fixation under well‐watered and drought conditions, respectively. The potting media we used in this experiment with 2.5 L pots also contained some nitrogen, possibly supporting the plants' early growth nitrogen requirements.…”

“…Plants take up the ammonium which is used for amino acid and protein synthesis (Thilakarathna & Raizada, 2018). On average, the percent nitrogen derived from the atmosphere (%Ndfa) of soybean is around 56% (Ciampitti & Salvagiotti, 2018), but this value highly depends on the cultivated region, soybean varieties, and other biotic and abiotic factors (Peoples et al, 2021; Thilakarathna et al, 2021).…”

“…Although SNF in soybean peaks at the pod filling stage (Pitumpe Arachchige et al, 2020), nitrogen fixation during the vegetative stage can also significantly contribute to the seed nitrogen production through remobilization of nitrogen from vegetative tissues to seeds (Ortez et al, 2018; Thilakarathna et al, 2021). Therefore, any interruptions to nodulation and SNF during early growth stages can affect yield and grain protein production in soybean.…”

Inoculation of soybean seeds by rhizobia with nanometal carboxylates reduces the negative effect of drought on N₂ and CO₂ assimilation

“…All points related to plants previously exposed to drought were located in the region of the initial part of the curve describing the dependence of A n on N fx in plants grown all the time at optimal substrate moisture (control group) but below it. Some decrease, compared to the flowering stage, in the A n rate of plants which all the time grew at the optimum substrate moisture probably occurred due to the starting of nitrogen-containing compounds remobilisation from the leaves to pods (Thilakarathna et al 2021). As in the flowering period, at the pods' formation stage, the best physiological indices were in plants inoculated by rhizobia with nano-Ge and nano-Fe carboxylates.…”