Reduced soybean photosynthetic nitrogen use efficiency associated with evolutionary genetic bottlenecks

Soybean has a narrow genetic base thought to limit future yield genetic gains. However, there is no evidence whether this reduction in genetic diversity correlates with diversity loss for any yield trait. We tested how photosynthetic nitrogen use efficiency (leaf photosynthesis per unit nitrogen, NUEp) evolved from the wild relative Glycine soja Siebold & Zucc. to the current Glycine max (L.) Merr. Five populations resulting from different evolutionary bottlenecks were evaluated under field conditions. Populations were wild ancestors, domesticated Asian landraces, North American ancestors, and modern cultivars. Genotypic differences in photosynthesis and leaf nitrogen were evident, creating a significant 3-fold variation in phenotypic NUEp. There was a parallel reduction in molecular marker and phenotypic NUEp diversity after each evolutionary bottleneck. G. soja had three times more NUEp diversity and 25% more average NUEp compared with the elite modern cultivars. Two strategies for increasing NUEp were identified: (i) increases in light saturated photosynthesis (Pmax), and, alternatively, (ii) reductions in leaf nitrogen. A modelling approach showed that NUEp will increase yield only if based on increased Pmax. Our study quantified the genetic potential of exotic germplasm available for trait-directed breeding. Results antagonise the concept that elite germplasm is always superior for any relevant yield trait when compared with undomesticated germplasm.