Grain Nitrogen Source Changes over Time in Maize: A Review

Understanding the sources of grain N uptake (Grain N) in maize (Zea mays L.) and especially the trade-off between reproductive-stage shoot N remobilization (Remobilized N) and reproductive-stage whole-plant N uptake (Reproductive N) is needed to help guide future improvements in yield and N use efficiency (NUE). Therefore, a literature review was performed to investigate the knowledge gap concerning changes over time in Grain N sources and on N partitioning to the grain and stover plant fractions at maturity. The synthesis-analysis was based on 100 reports, which were divided into two time intervals: (i) research conducted from 1940 to 1990-"Old Era"-and (ii) research conducted from 1991 to 2011-"New Era." The most remarkable results were (i) Grain N concentration was the main parameter that has changed over time, (ii) Reproductive N contributed proportionally more to Grain N for the New Era while Reproductive N and Remobilized N contributed equally to Grain N for the Old Era, (iii) Remobilized N was primarily associated with vegetative-stage whole-plant N uptake (Vegetative N), which was constant across eras, although the proportion of the Remobilized N itself seems to be driven by the ear demand, (iv) complex plant regulation processes (source: sink) appeared to influence Reproductive N, and (v) stover N concentration gains mirrored the grain N concentration as the plant N uptake increased at maturity in both eras. This new appreciation for the changes over time may assist directed selection for yield and NUE improvements.