Mechanisms For The Coordination Of High Yield And High Nitrogen Remobilization Efficiency In Maize

During the past several decades, maize (Zea mays L.) grain yield (GY) has increased dramatically, while grain nitrogen (N) concentration (GNC) has declined in modern hybrids. Genetic improvement to increase both high GY and GNC is necessary to improve maize nutritional quality. Using maize hybrids with different N remobilization efficiency (NRE) as plant materials, multi-years field studies were conducted to analyze (1) the post-silking dry matter (DM) accumulation and remobilization, N uptake and remobilization and their responses to N supply and (2) the temporal and spatial trait of post-silking photosynthate production and N remobilization, so as to elucidate the mechanism to achieve high GY together with high GNC. The main results were as follows:(1) In comparison to YD13, ZD958had higher GY but lower GNC, whereas XY335had higher GY and similar GNC. Both ZD958and XY335had higher total and post-silking DM accumulation and N uptake than YD13, and were also characterized by delayed leaf senescence and a sustained net photosynthetic rate (PN) after silking. In addition, XY335also had higher NRE and higher photosynthetic N use efficiency (PNUE) than ZD958.(2) Throughout the grain filling period, the PN, stomatal conductance (gs), and PNUE in XY335were similar with that in ZD958in lower leaves, but higher in upper and middle leaves. XY335had higher stomatal density, maximum Rubisco saturated rate of carboxylation and maximum rate of electron transport than ZD958in the upper and middle leaves, respectively.(3) In accordance with the light distribution withing maize canopy, the upper leaves had higher SLN, PN and PNUE. Low-N stress facilitated leaf N remobilization but increased PNUE in all the leaves, and maintained largely the light interception by middle leaves, which might contribute to the increased whole-plant physiological N utilisation efficiency. Low-N did not further optimizethe vertical distribution of SLN, PN, and PNUE.(4) There was a significant year x N x genotype interaction in the amount of vegetative N remobilization and NRE, and residual stalk N concentration at maturity. However, the response of GNC to increasing N levels was the same between XY335and ZD958and was not affected by year conditions. The N level required to obtain the highest GY was the same in the two hybrids (156±13kg ha-land159±19kg ha-1), but that required to obtain the highest GNC was greater in XY335than in ZD958. Due to the higher vegetative N remobilization amount and NRE, XY335had lower residual stalk N concentration.In conclusion, it is supposed that higher NRE together with higher PNUE are the target traits to improve the modern stay-green cultivars to increase GNC without penaly in GY. Precise N fertilizer management as well as the selection of high-yielding hybrids with high NRE can increase GNC without negatively affecting GY or leading to surplus N storage in vegetative organs. The physiological mechanism controlling PNUE in maize leaves needs further investigation.