Abstract In maize (Zea mays L.) crops, nitrogen (N) status at silking (R1) has been used to predict grain yield (GY) response to N fertilization and to develop strategies to manage crop nutrition in order to match N supply with crop demand during growing season, such as late N fertilizations. Crop N status can be estimated by N nutrition index (NNI), which is based on actual and critical N concentration in crop biomass. Optical measurements of N concentration (e.g. SPAD readings) of the leaf blade subtending primary ear have also been used as a proxy of crop N status. Sowing date (SD) and N rates could affect soil N availability and hence crop N uptake at R1. Additionally, the effects of SD and its interaction with plant density (PD), N rates and hybrids (H) on N uptake (NuptP) and particularly on N partitioning in leaf-blades, stem + sheaths and ears could affect SPADs readings. We hypothesized that variations of GY by crop N status at R1 promoted by SD, PD and H, would be better predicted by NNI than by SPAD readings. In this study, two Hs (DK 70−10 VT3P and DK 73−10 VT3P) were cropped in two contrasting SD (early and late) in Parana, Argentina (31°44′ S 60°32′ W) at three PD (5, 7 and 9 pl m−2) with three N rates (0, 90 and 270 kg N ha-1) in order to evaluate the effect of treatments on: i) N availability, N uptake at the plant and crop level, N partitioning in leaf-blades, stem + sheaths and ears, SPAD readings and NNI at R1, and ii) the relationships among N availability and N uptake at the plant and crop level, NNI, SPAD, and GY. N concentration of leaf-blades was negatively affected by PD, but this reduction was attenuated by N rates, especially in late SD (N x PD x SD interaction). Hence, in early SD, some data of both Hs corresponding to 270 N yielded low SPAD values for NNI greater than 0.86. Consequently, crop N status was better reflected by NNI than by SPAD readings, because NNI considers N stored in the whole plant. NNI at R1 adequately described relative GY variations promoted by SD, PD, N rates and, Hs, i.e. NNI was a more meaningful crop status index than SPAD readings. Overall, our study contributes to understanding mechanisms that regulate crop N status affected by agronomical practices and adds insights to explore in late N fertilization of maize crops.

中文翻译：

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不同植株密度下早播和晚播玉米作物氮素状况

摘要 在玉米 (Zea mays L.) 作物中，吐丝 (R1) 时的氮 (N) 状态已被用于预测谷物产量 (GY) 对施氮的响应，并制定管理作物营养的策略，以匹配 N 供应与生长季节的作物需求，例如晚施氮肥。作物 N 状况可以通过 N 营养指数 (NNI) 进行评估，NNI 以作物生物量中实际和临界 N 浓度为基础。对着初级穗的叶片的 N 浓度的光学测量（例如 SPAD 读数）也被用作作物 N 状态的代理。播种日期 (SD) 和氮肥率可能会影响土壤氮的有效性，从而影响 R1 时的作物氮吸收。此外，SD 及其与植物密度 (PD)、N 率和杂交种 (H) 的相互作用对 N 吸收 (NuptP) 的影响，特别是对叶片中 N 分配的影响，茎 + 鞘和耳朵可能会影响 SPAD 读数。我们假设由 SD、PD 和 H 促进的 R1 作物 N 状态对 GY 的变化，由 NNI 比由 SPAD 读数更好地预测。在这项研究中，两个 Hs（DK 70-10 VT3P 和 DK 73-10 VT3P）在阿根廷巴拉那（31°44′ S 60°32′ W）的三个 PD（ 5、7 和 9 pl m-2) 具有三个 N 比率（0、90 和 270 kg N ha-1），以评估处理对以下方面的影响：i) 植物和作物水平的 N 可用性、N 吸收，叶片、茎 + 鞘和穗中的 N 分配，R1 处的 SPAD 读数和 NNI，以及 ii) 植物和作物水平的 N 可用性和 N 吸收之间的关系，NNI、SPAD 和 GY。叶片的 N 浓度受到 PD 的负面影响，但这种减少会因 N 率而减弱，特别是在 SD 后期（N x PD x SD 交互）。因此，在早期的 SD 中，对应于 270 N 的两个 H 的一些数据产生了大于 0.86 的 NNI 的低 SPAD 值。因此，NNI 比 SPAD 读数更好地反映了作物 N 状态，因为 NNI 认为 N 储存在整个植物中。R1 处的 NNI 充分描述了 SD、PD、N 率和 Hs 促进的相对 GY 变化，即 NNI 是比 SPAD 读数更有意义的作物状况指数。总体而言，我们的研究有助于理解受农艺实践影响的作物 N 状态的调节机制，并为探索玉米作物后期施氮提供了见解。因为 NNI 认为 N 存储在整个植物中。R1 处的 NNI 充分描述了 SD、PD、N 率和 Hs 促进的相对 GY 变化，即 NNI 是比 SPAD 读数更有意义的作物状况指数。总体而言，我们的研究有助于理解受农艺实践影响的作物 N 状态的调节机制，并为探索玉米作物后期施氮提供了见解。因为 NNI 认为 N 存储在整个植物中。R1 处的 NNI 充分描述了 SD、PD、N 率和 Hs 促进的相对 GY 变化，即 NNI 是比 SPAD 读数更有意义的作物状况指数。总体而言，我们的研究有助于理解受农艺实践影响的作物 N 状态的调节机制，并为探索玉米作物后期施氮提供了见解。