Exploration of Genetic and Management Consequences for Organ-Specific Partitioning of Dry Matter and Nitrogen During Maize (Zea mays L.) Development

<p dir="ltr">The pathways to genetic improvement in maize can be condensed to two main approaches: 1) better resource capture, or 2) better resource partitioning to the ear. While better resource capture (i.e., stress tolerance, disease resistance, greater LAI, etc.) is mostly well understood, better resource partitioning (i.e., initial partitioning, progressive remobilization) has been less frequently described and understood. While it is clear that modern hybrids have partitioned significantly greater dry matter and nitrogen to the ear than older hybrids at maturity, the kinetics of the partitioning process during the reproductive period is less clear. Current field-grown maize physiology investigations involving N response are often conducted with limitations involving single genotypes, too few management treatments, insufficient sampling stages, and (or) limited organ-component partitioning. The net result is an incomplete understanding of the storage and remobilization kinetics of plant dry matter and N, especially as plant densities increase. Our new studies aimed to elucidate the vertical-profile specific partitioning and remobilization patterns of dry matter and N from leaves and stems across a wide range of densities, N treatments, and hybrids. The 2-year trial conducted in Indiana involved over 9,000 individual tissue samples of three unique maize hybrid platforms grown under moderate N stress (45 kg N ha^-1) versus supra-optimal N (270 kg N ha^-1) conditions at 5 unique plant densities from 4.9 to 10.9 plants m^-2. Above-ground plants were sampled at four stages: V8-V9, R1, R3, and R6. Leaves and stems were separately partitioned into the 3 vertical canopy zones: below ear zone (BEZ), ear zone (EZ), and above ear zone (AEZ). The EZ was centered on the primary ear and consisted of the ear leaf and internode plus one leaf or internode above and below. From our analysis, it was evident that the partitioning and remobilization of dry matter and N across different vertical partitions of leaves and stems is a complex process driven by sink demand in balance with the supply of current assimilates and N uptake. We found that intra-plant dry matter partitioning and remobilization follow specific programmed patterns affected by N supply, plant density, and hybrid. Increases in stem and leaf dry matter with higher plant density was mostly a result of increased dry matter production before R1 and in lower intra-plant partitions. From R1 to R3, dry matter from below ear zone (BEZ) leaves decreased, while leaves above the ear (AEZ) simultaneously increased. Additionally, we found that stems served as primary and significant storage organs for excess assimilates that continued to accumulate up to R3. The additional assimilate accumulation in high densities in stems and leaves enabled nearly double the dry matter remobilization of the low density from R3 to R6. From R1 to R3, stems concurrently stored significant additional dry matter while also initiating the remobilization of N reserves for use in ear growth. This confirmed that N remobilization and dry matter partitioning are independent in post-anthesis stems. Stored assimilates were remobilized to supply the ear with carbohydrates when the current plant growth rate (PGR) was lower than the current ear growth rate (EGR). Like dry matter, N remobilization followed a specific but different pattern consistently across hybrid and density treatment factors. Stems served as the first source of remobilized N from R1 to R3. After stem N reserves were depleted at R3, maize plants began to remobilize N from leaves, starting at the BEZ and progressing upward in the canopy through the ear zone (EZ) and AEZ. Relative N remobilization from leaves compared to the peak N content, attained at either R1 (BEZ and EZ) or R3 (AEZ) across years and N treatments, to R6 decreased from the bottom up with BEZ at ~70%, EZ at ~52% and AEZ at 47%. Of the stem N remobilized during the reproductive period, 69% to 87% was remobilized from R1 to R3, while leaves remobilized 81% to 100% during the R3 to R6 period. The transitions of N remobilization through vertical partitions of stems and leaves thus depended upon the current N uptake and ear N demand. While stems serve as the initial source of supplemental N, leaves provide the bulk of N remobilization later in grain fill. Although overall N rates and density significantly impacted dry matter partitioning and remobilization, hybrid differences were smaller in magnitude. However, Hybrid 1 achieved higher N concentrations in both stems and leaves across the vertical profile, which generally drove greater N content across the season that was associated with grain yield gains of 10% versus Hybrid 2 and ~3% versus Hybrid 3. Our canopy partitioning results during the season demonstrated the intricate nature of programmed intra-plant N kinetics necessary to support grain formation. Additionally, we conducted separate experiments to elucidate the grain yield and potentially unique physiology of DP202216 hybrids in elite germplasm from Corteva Agriscience. DP202216 hybrids have enhanced and extended expression of <i>zmm28</i>, a native maize MADS-box transcription factor that previously demonstrated increased grain yield and yield stability in stress and favorable environments. This trial was conducted at 4 locations in Indiana and Nebraska across 2 years with 3 hybrids fertilized at 0N and 200N. In contrast to previous literature, we found no difference in the max CER at R2 or R5 in DP202216 hybrids compared to the wild-type (WT) hybrids. However, our results showed that N uptake from R2 to R5 was 80% greater in the DP202216 hybrids than in the WT hybrids. This was concurrent with a 7.5% increase of R5 green leaf dry matter, a 6.9% increase in total N content at R5, and a 2.6% greater final grain yield in the DP202216 hybrids. The N uptake gains in DP202216 hybrids occurred post-flowering, not pre-flowering as previously reported. This higher rate of N uptake from R2 to R5 in some environments contributed to higher whole-plant dry matter production at R5 and greater grain yield. This further demonstrates the potential of the unique genetic diversity of DP202216 hybrids and shows that transcription factors can achieve additive step changes in N uptake and genetic yield potential. The results of both series of experiments improve scientific understanding of the in-season vertical distribution of dry matter accumulation and N uptake, and particularly their unique canopy-position partitioning and remobilization at progressively higher densities, that helps maize plants prioritize later-reproductive leaf assimilate production in the upper and middle canopy to enable grain yield improvements in responsive hybrids.</p>