High responsiveness of maize grain yield to nitrogen supply is explained by high ear growth rate and efficient ear nitrogen allocation

Increasing the response of maize yield to nitrogen (N) application is essential for food security and economic benefits of the farmers. However, the physiological mechanism underlining N responsiveness is unclear. Three maize cultivars with different N-responsiveness, BMY (the low), ZD2 (the medium) and ZD958 (the high) were grown in the field under a wide range of N supply (from 0 to 240 kg N per ha) in two years. Plant growth rate, plant N uptake rate, ear growth rate and ear N allocation rate, and their relationship to grain yield formation were investigated during the early critical period (14 days before silking). The direct path coefficient of grain number to grain yield was 0.82, which explained 96% of the variation in grain yield (R² = 0.91). There was not a trade-off between grain number and other yield components. ZD958 got more grains per ear than the other cultivars in response to N supplies. The variation in grain number was mostly explained by the number of floret primordia (57% for BMY, 93% for ZD2 and ZD958). When N supply increased from 0 to 240 kg N ha⁻¹, ZD2 and ZD958 did not differ in nitrogen nutrition index, and were 7.2–32.8% higher than BMY. The number of grains and floret primordia was independent of plant growth rate and plant N uptake rate, although these rates increased with N supply. Ear growth rate and ear N allocation rate of ZD958 were higher compared with the other cultivars, especially under high N supply. Except BMY, the number of floret primordium and grain per ear depended on ear growth rate and ear N allocation rate. There was a positive correlation between ear growth rate and ear N allocation rate in ZD2 and ZD958. However, ear N allocation rate of BMY did not increase when its ear growth rate was greater than 19.2 mg ᵒC⁻¹. It is concluded that, with increasing N supply, N-responsive maize cultivars had high ear growth rate and allocated more N into the ear during the early critical period, which enhanced the differentiation of floret primordia, hence increasing grain number and final grain yield.