Physiological Perspectives of Changes Over Time in Maize Yield Dependency On N Uptake and Associated Efficiencies: II. Physiological Basis of Changes.

Over preceding decades, the study of various mechanisms involved in maize grain yield (GY) formation and its relationship with nitrogen (N) uptake dynamics has been increasingly acknowledged in the scientific literature. Still changes in the physiological processes associated with the mechanisms of the plant N uptake and associated efficiencies are not clearly known. We reviewed 100 published and unpublished reports) related to the plant N uptake process and GY formation. Our analyses were limited nearly exclusively to experiments involving hybrid (as distinct from inbred) response to M input levels where plant density data was available. Dissection of the complex interactions among years, plant densities and N rates began with division of treatment mean data (close to ~3000 individual points) into two time periods defined by year(s) of the original research: i) studies from 1940 to 1990 - “Old Era” and, ii) studies from 1991 to 2011 - “New Era”. The key physiological processes affected in the New Era versus Old Era hybrids  were i) a consistently strong dependency between dry matter and N allocation to the ear organ in both Eras; ii) higher total plant biomass (BM) accumulation and N uptake, on an absolute basis, during the post-silking period with New Era genotypes accompanied by relatively smaller changes in grain harvest index (HI) and NHI; iii) a strong correlation between plant N uptake at silking time and per-plant GY and its components (kernel number and kernel weight) in both Eras; iv) NIE improvement from Old to New Era was primarily associated with reduced grain %N, and to a minor degree with NHI gains, and v) due to the equivalent levels documented for the pre-to post-silking N uptake ratio (averaging for both Eras, 64.5%) and the NHI parameter (~for both Eras, 63.6%), a straight positive association (close to the 1:1 line) was consequently documented between the N stored in the grain organ at physiological maturity versus the N taken up by the whole plant at silking time. This improved understanding of the physiological factors underlying progress in maize yield response to N over time, within the context of changing G x E x M factors, serves to help guide maize programs focused on achieving further improvements in N use efficiency.