A model for monitoring leaf nitrogen content of Jinsha pomelo based on portable pomelo spectrometer

Abstract:【Objective】Jinsha pomelo is one of the high-quality honey pomelo varieties planted in Ji’an city, Jiangxi province, China. The leaf nitrogen content (LNC) of Jinsha pomelo is an important index for quantitative monitoring and diagnosis of the nitrogen nutrition status and growth characteristics. Traditional LNC measurement, which mainly relies on the laboratory chemical testing, can obtain accurate and reliable results, but the applications are time- consuming and restricted to relatively small spatial scales. The spectral monitoring technology is recognized as one of the most popular means for real-time,fast and non-destructive monitoring fruit tree growth status in orchard scale. It can provide an effective technical mean for accurately analyzing the quantitative relationship between the LNC and spectral vegetation indices. Consequently, the spectral monitoring models of the LNC were established to provide a basis for non- destructive, convenient and quantitative estimation of the LNC. The previous studies proved the application of spectral monitoring technology for estimating fruit tree growth indices based on the spectral vegetation indices. Nevertheless, which spectral vegetation indices is strongly related to the LNC of Jinsha pomelo remians unclear. The objective of this study was to test the precision of the portable pomelo spectrometer (PPS, wavelengths range 400-1100 nm, which was developed by our research group) in monitoring the LNC of Jinsha pomelo, and establish the spectral monitoring model of the LNC of Jinsha pomelo.【Methods】The orchard experiments were conducted in Jizhou district and Jishui county in Ji'an city, Jiangxi province in 2021, where eight year old Jinsha pomelo with four nitrogen fertilizer application amounts (0, 0.5, 1.0 and 1.5 kg·plant^-1 ) and three replications were conducted. Three spectrometers, i.e., PPS, ASD (wavelengths range 325-1075 nm, which is made in USA) and RapidSCAN (including 670, 730 and 780 nm spectral bands, which is made in USA) were set at a height of 1 m above the top of the canopy to measure canopy spectral reflectance at young fruit stage and fruit expanding stage under clear sky conditions at 10:00 to 14:00 of Beijing local time. The NDRE (normalized difference red edge) and NDVI (normalized difference vegetation index) were calculated by the canopy spectral reflectance. The canopy vegetation indices change trends were compared between PPS, ASD and RapidSCAN, and their correlations were analyzed. The LNC were measured by the traditional chemical testing in the laboratory based on the leaf samples, which were collected from the canopy middle periphery of the each sampled pomelo tree in the four directions (including south, north, east and west. Then, the correlations between the canopy vegetation indices and LNC were analyzed. The PPSbased monitoring models of the LNC were established from orchard experimental data set at Jizhou district, which was tested with orchard experimental data set at Jishui county.【Results】The LNC, NDVI and NDRE of Jinsha pomelo were increased the increase of nitrogen fertilizer application amount at different stages. The R2 (determination coefficient) of fitting for the NDVI and NDRE from PPS and ASD were 0.909 5 and 0.900 5, respectively. The R2 of fitting for the NDVI and NDRE from PPS and RapidSCAN were 0.954 3 and 0.900 2, respectively. This result proved that the measurement results of PPS were highly consistent with ASD and RapidSCAN. The model performance for the young fruit stage or fruit expanding stage was better than that for the middle growth stages. Comparison the NDVIPPS and NDREPPS were measured by PPS, the correlation between the NDVIPPS and LNC was higher than that of the NDREPPS. The power function equation NDVIPPS-based could exactly predict the LNC with the R2 between 0.821 0-0.847 2, and the r (correlation coefficient), RRMSE (relation root mean square error) and RMSE (root mean square error) of the model test were between 0.895 2-0.933 3, 4.4%-8.1% and 0.1%- 0.2%, respectively.【Conclusion】In this study, the LNC and canopy vegetation indices of Jinsha pomelo presented an increasing trend the increase of the nitrogen fertilizer application amount. The canopy vegetation indices collected by the three spectrometers were highly consistent, and the power function model based on NDVIPPS could monitor the LNC of Jinsha pomelo more accurately. Compared with the traditional chemical test methods, the use of PPS could conduct in real-time and accurately get the LNC information of Jinsha pomelo. It would have wide application prospects for the high-efficient cultivation and non-destructive monitoring and diagnosis of the nutrients in Jinsha pomelo production.