为探明水稻叶片SPAD值的最佳测定叶位和最佳的SPAD值次级指标，并构建基于水稻叶片SPAD值的氮素营养诊断模型，开展以品种为主区，氮肥施用量为副区的双因素裂区设计试验。参试品种为Q优6号和宜香优2115，氮肥施用量设5个水平（纯N 0、75、150、225、300 kg/hm²），分析叶片的敏感性、代表性和稳定性，并探讨SPAD值次级指标与施氮量和叶片含氮量之间，及叶片氮积累量与叶片含氮量和产量之间的关系。结果表明，叶片敏感性、代表性和稳定性大小顺序分别为L4（顶4叶）>L3（顶3叶）>L1（顶1叶）>L2（顶2叶）和L3>L4>L2>L1、L2>L3>L4>L1，可见， L3、L4可作为氮素营养诊断的共同理想指示叶。选择L3、L4的SPAD值几何平均数（GMSI34）作为最佳的SPAD值次级指标。由叶片氮积累量与产量和叶片含氮量的抛物线方程、一元线性回归方程， GMSI34与叶片含氮量的指数方程，求得拔节期、孕穗期、抽穗期的SPAD值次级指标的临界值（GMSI34_临）分别为48.5、44.3和42.9，与实际获取的GMSI34_实相比较，若GMSI34_实-GMSI34_临<0时，则表明水稻缺氮，需要追氮，若GMSI34_实-GMSI34_临≥0，则表明水稻氮营养充足，无需追氮。综上，基于杂交水稻叶片SPAD值的氮素营养诊断模型，可为杂交水稻高效氮肥管理提供技术支持。

A split experiment of two-factor was conducted to explore the best measured leaf position of the SPAD value of rice leaves, the best secondary index of SPAD value, and build a nitrogen (N) nutrition diagnosis model based on the SPAD value of rice leaves. The main plots consisted of variety treatment (two variety: Qyou 6 and Yixiangyou 2115) and the subplots consisted of the N application rate (five N levels: 0, 75, 150, 225, 300 kg/hm²). The sensitivity, representativeness, and stability of rice leaves were analyzed. The relationship between SPAD value index and N application rate and leaf N content (LNC), and the relationship between leaf N accumulation (LNA) and LNC and yield were studied. The results showed that the sensitivity, representativeness and stability of rice leaf in order of magnitude were L4 > L3 > L1 > L2, L3 > L4 > L2 > L1, L2 > L3 > L4 > L1, respectively. Thus, L3 and L4 can be used as the common ideal indicator leaves for N nutrition diagnosis. The geometric mean of SPAD value of the top 3 and 4 leaves of rice plant (GMSI34) was selected as the best secondary index of SPAD value. According to the parabola equation of LNA and yield, univariate linear regression equation of LNA and LNC, and the exponential equation of GMSI34 and LNC, the critical values of SPAD value secondary index (GMSI34_cri) at jointing stage, booting stage, and heading stage are 48.5, 44.3, and 42.9, respectively. Compared with the actual GMSI34 (GMSI34_act), if GMSI34_act < GMSI34_cri, it indicates that rice lacks nitrogen and needs nitrogen topdressing, if GMSI34_act ≥ GMSI34_cri, it shows that the rice has sufficient nitrogen nutrition and does not need nitrogen topdressing. In conclusion, the N nutrition diagnosis model based on the SPAD value of hybrid rice leaves can provide technical support for efficient N fertilizer management of hybrid rice.