Rapid non-destructive estimation of the plant nitrogen (N) status is needed for the precise management of N in small-scale farms. Our objectives were to describe the responses of the soil–plant analyses development (SPAD) measurements to the plant N status of winter wheat, and to evaluate whether the SPAD index is more precise than SPAD readings in estimating the plant N status. During 2009–2011, a field experiment with winter wheat was conducted in the North China Plain. There were eight N supply levels ranging from 0 to 420 kg N ha −1 to create gradients of leaf greenness. The SPAD readings were measured on the topmost fully expanded leaf; the SPAD index was expressed relative to the SPAD readings of sufficiently fertilized plants. Combing over years, significant quadratic responses of SPAD reading to the plant N concentration (PNC) occurred at each/similar Zadoks growth stage (ZGS) but differed largely from ZGS 39/43 to ZGS 58/59, the r 2 varied between 0.88 and 0.97; the SPAD readings exhibited a high logarithmic correlation to plant N accumulation (PNA, r 2 = 0.94) irrespective of growth stages. Compared with SPAD readings, the SPAD index showed the same precision in response to PNC and PNA at each growth stage in each year, but presented less precision for the combined datasets across years, indicating that the SPAD index is not a more preferable tool in estimating plant N status. In addition, both the SPAD readings and SPAD index demonstrated poor to moderate responses to basal stem nitrate content (BSNC) with large differences among ZGS 39–59 and between the 2 years, indicating that the SPAD measurements could not be effectively used to estimate the BSNC.

中文翻译：

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SPAD读数和SPAD指数对估计冬小麦植株氮素状况的评价

需要对植物氮 (N) 状态进行快速无损评估，以精确管理小型农场中的氮。我们的目标是描述土壤-植物分析发展 (SPAD) 测量对冬小麦植物 N 状态的响应，并评估 SPAD 指数在估计植物 N 状态时是否比 SPAD 读数更精确。2009-2011 年，在华北平原进行了冬小麦田间试验。有八个 N 供应水平，范围从 0 到 420 kg N ha -1，以产生叶绿度梯度。SPAD 读数是在最上面完全展开的叶子上测量的；SPAD 指数表示相对于充分受精植物的 SPAD 读数。多年的梳理，SPAD读数对植物氮浓度（PNC）的显着二次响应发生在每个/相似的Zadoks生长阶段（ZGS），但与ZGS 39/43至ZGS 58/59差异很大，r 2 在0.88和0.97之间变化；无论生长阶段如何，SPAD 读数都与植物 N 积累（PNA，r 2 = 0.94）呈高对数相关性。与 SPAD 读数相比，SPAD 指数在每年各生长阶段对 PNC 和 PNA 的响应精度相同，但跨年合并数据集的精度较低，表明 SPAD 指数不是更可取的估计工具植物 N 状态。此外，SPAD 读数和 SPAD 指数均显示对基部茎硝酸盐含量 (BSNC) 的响应较差至中等，ZGS 39-59 之间和 2 年之间差异很大，