Nitrogen availability determines the vertical patterns of accumulation, partitioning, and reallocation of dry matter and nitrogen in maize

Context or problem: The reallocation of dry matter (DM) and nitrogen (N) from vegetative tissues to the grains are critical for both yield quantity and protein content of cereal crops. However, it is unclear to which extent the dynamics in DM and N reallocation depend on N availability, and how individual leaves within the maize canopy respond to different N availabilities in terms of these processes. Objective or research question: This study aimed to quantify DM and N accumulation, partitioning, and reallo-cation from vegetative to reproductive parts in maize in relation to soil N availability. Methods: A long-term N fertilizer trial was conducted in Jilin province, Northeast China, growing maize at three N fertilizer levels (low N availability, N0; intermediate N availability, N1; and high N availability, N2). The accumulation, partitioning, and reallocation of DM and N were quantified at the whole-plant, organs, and single -leaf scales in 2015 and 2016.Results: Although both post-silking DM accumulation and post-silking N uptake increased in response to higher N availability, 8.3-38.8% of grain N still needed to be reallocated from vegetative organs with a larger fraction coming from leaves (10.5 - 36.5%) than from stems (4.4 - 11.6%). This dependency of grain N on N reallocation from vegetative parts increased at lower soil N availability. Furthermore, the vertical patterns of reallocated leaf DM and reallocated leaf N changed with N availabilities. While in general leaves in the middle part of the canopy tended to reallocate more DM and N to the grains than leaves from the upper or lower canopy parts, these most -contributing leaves were higher in the canopy at low than at high N availability.Conclusions and implications: Grain N relies on reallocation more than does grain DM. At lower N availability, more grain DM and grain N need to be reallocated from vegetative organs. Furthermore, the leaf ranks from which most DM and N were reallocated, gradually increased from the bottom to upper ranks as N availability decreased. Together, our results on the dynamics of N and DM uptake and reallocation during the reproductive phase are important for plant and crop models that require these processes for accurate predictions of maize performance.