Physiological Dynamics of Maize Nitrogen Uptake and Partitioning in Response to Plant Density and Nitrogen Stress Factors: II. Reproductive Phase

Improved plant N utilization and partitioning is critical for future improvements in maize (Zea mays L.) grain yield (GY). The overall research objective was to gain understanding of the physiological mechanisms underpinning biomass (BM), N uptake partitioning, and GY processes during the reproductive period for two maize hybrids grown at varying plant density (PD) (low is 54,000 plant ha (1), medium is 79,000 plants ha (1), and high is 104,000 plants ha(-1)) and N inputs (low is 0 kg N ha(-1), medium is 112 kg N ha(-1), and high is 224 kg N ha(-1)) over four site-years. At the community level, maize GY was maximized in both genotypes at the medium PD and highest N rate. At maturity, grain harvest index improved as the whole-plant N uptake increased following a linear-plateau model and, for N allocation, both grain and shoot N concentrations increased similarly as BM increased. Around flowering (+/- 15 d), dry mass and N partitioning rates were unmodified by treatment factors. Treatment factors only marginally influenced potential kernel number near flowering. Allometric analyses confirmed a lack of treatment impact on whole-plant N uptake and N remobilization coefficients. Greater reproductive-stage N uptake was associated with superior ear strength (kernel number and weight) and late shoot N remobilization, but GY was also positively related to vegetative-stage N uptake. Future research should identify genotypic variation for overcoming the documented N uptake trade-off mechanisms (vegetative-stage N uptake vs. shoot N remobilization) as related to the maize GY improvement process.