Formulating warning signals for canopy growth status at different stages are an effective way to dynamically regulate the water and fertilizer management based on the relationship between growth information and orchard productivity. In the study, the response of leaf nitrogen content (LNC), leaf area index (LAI), fruit yield, quality and water and nitrogen use efficiency to irrigation levels and nitrogen rates was investigated in a field experiment from 2017 to 2022. Principal component analysis was used to construct an orchard production comprehensive evaluation index (OPCEI) to assess orchard productivity, and the relationship between LAI, LNC and OPCEI was analyzed by the structural equation modeling. The results showed that nitrogen rates had significant effect on OPCEI in the different years, but irrigation levels had significant effect in 2019 and 2021. Compared to no fertilization, the OPCEI of 120, 240, and 360 kg ha under low irrigation amount showed average increased 28.90%, 48.54%, and 25.85%, while it increased by an average of 33.51%, 55.65%, and 32.44% for 120, 240, and 360 kg ha under high irrigation amount. Also, low irrigation amount did not significantly reduce OPCEI at 240 kg ha compared to high irrigation amount. Structural equation modeling revealed that LNC had significant positive effect on fruit yield, while had significant negative effect on total soluble solids. Furthermore, LAI had a significant positive effect on fruit yield and sugar-acid ratio. On the basis of the relationship of LAI, LNC and OPCEI, the reference value of LNC was proposed. The suitable range of LNC in the shoot growth and young fruit stage is 2.96%-3.79% and 2.62%-3.30%, and that is 2.43%-3.17%, 2.21%-2.97%, and 2.13%-2.63% in the early, middle and late fruit expansion stage, respectively. The study results provide a basis for canopy nitrogen diagnosis, which is guide for achieving precise nitrogen application in apple orchards.

中文翻译：

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叶片氮素状况和叶面积指数对水氮施用的响应及其与苹果园生产力的关系

根据生长信息与果园生产力的关系，制定不同阶段冠层生长状况的预警信号是动态调控水肥管理的有效途径。本研究通过田间试验，研究了2017年至2022年叶片氮含量（LNC）、叶面积指数（LAI）、果实产量、品质以及水氮利用效率对灌溉水平和施氮量的响应。采用结构方程模型分析构建果园生产综合评价指数（OPCEI）来评价果园生产力，并分析LAI、LNC和OPCEI之间的关系。结果表明，不同年份施氮量对OPCEI影响显着，但2019年和2021年灌溉水平影响显着。与不施肥相比，低灌水量120、240和360 kg·ha的OPCEI平均增加。 28.90%、48.54%和25.85%，而120、240和360 kg ha在高灌水量下平均增加33.51%、55.65%和32.44%。此外，与高灌水量相比，低灌水量并没有显着降低 240 kg ha 的 OPCEI。结构方程模型表明，LNC对果实产量有显着的正向影响，而对可溶性固形物总量有显着的负向影响。此外，LAI对果实产量和糖酸比具有显着的正向影响。根据LAI、LNC和OPCEI的关系，提出LNC的参考值。地上部和幼果期LNC的适宜范围为2.96%-3.79%和2.62%-3.30%，前期为2.43%-3.17%、2.21%-2.97%和2.13%-2.63%。分别为果实膨大期、中后期。研究结果为冠层氮肥诊断提供依据，为苹果园实现精准施氮提供指导。