Abstract
Nitrogen (N) fertilization is critical for spike and floret development, which affects the number of fertile florets per spike (NFFs). However, the physiological regulation of the floret development process by N fertilization is largely unknown. A high temporal-resolution investigation of floret primordia number and morphology, dry matter, and N availability was conducted under three N fertilization levels: 0 (N0), 120 (N1) and 240 (N2) kg ha−1. Interestingly, fertile florets at anthesis stage were determined by those floret primordia with meiotic ability at booting stage: meiotic ability was a threshold that predicted whether a floret primordium became fertile or abortive florets. Because the developmental rate of the 4th floret primordium in the central spikelet was accelerated and then they acquired meiotic ability, the NFFs increased gradually as N application increased, but the increase range decreased under N2. There were no differences in spike N concentration among treatments, but leaf N concentration was increased in the N1 and N2 treatments. Correspondingly, dry matter accumulation and N content of the leaf and spike in the N1 and N2 treatments was increased as compared to N0. Clearly, optimal N fertilization increased leaf N availability and transport of assimilates to spikes, and allowed more floret primordia to acquire meiotic ability and become fertile florets, finally increasing NFFs. There was no difference in leaf N concentration between N1 and N2 treatment, whereas soil N concentration at 0–60 cm soil layers was higher in N2 than in N1 treatment, implying that there was still some N fertilization that remained unused. Therefore, improving the leaf’s ability to further use N fertilizer is vital for greater NFFs.
