Responses of leaf nitrogen status and leaf area index to water and nitrogen application and their relationship with apple orchard productivity

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−1 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−1 under high irrigation amount. Also, low irrigation amount did not significantly reduce OPCEI at 240 kg ha−1 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.