## 3 Results and Discussion
### Spectra Feature  
The mean reflectance spectra curves of sugar beet and celery leaf samples with four levels of N treatments growing under different water conditions are shown in Figure 4. All studied variants (N and water treatments) are characterized by a similar profile of the  
Figure 3: Leaf chlorophyll content of sugar beet (**a**) and celery (**b**) plants in accordance with four nitrogen supplies and three water treatments. Each bar represents the mean \(\pm\) SD of 3 plants in each group. According to two-way ANOVA and [PERSON]’s test, distinct uppercase letters indicate statistical differences among the different N supplies and the same water treatment. Distinct lowercase letters indicate significant differences between the different water treatments and the same N supply. For each graph values followed by the same letter are not significantly different (\(p>0.05\)). LW: low water supply, OW: optimum water supply, HW: high water supply. Figure 2: Leaf nitrogen content of sugar beet (**a**) and celery (**b**) plants in accordance with four nitrogen supplies and three water treatments. Each bar represents the mean \(\pm\) SD of 3 plants in each group. According to two-way ANOVA and [PERSON]’s test, distinct uppercase letters indicate statistical differences among the different N supplies and the same water treatment. Distinct lowercase letters indicate significant differences between the different water treatments and the same N supply. For each graph values followed by the same letter are not significantly different (\(p>0.05\)). LW: low water supply, OW: optimum water supply, HW: high water supply. spectral curves. It can be observed that water and N treatment influence leaf reflectance spectra in the whole spectral region for both studied plants. The most important parts of the spectra for discriminating between N stress among water treatment are located in the red and green regions from 525 to 640 nm. It can be observed that the increasing N doses reduced leaf spectral reflectance in this region. This variation was more pronounced for celery than for sugar beet, which may be related to the different internal structures of the leaf structure. The reflectance peak at around 550 nm is characteristic of green plants and can be correlated with chlorophyll concentration. [PERSON] et al. observed that about 40 days after plant stress, chlorophyll content decreased by about 60%, resulting in an increase in reflectance in the range of approximately 550 to 710 nm. In the SWIR region, two values (minima): 1410 nm and 1940 nm referred to water content were observed. The differences in leaf reflectance at the SWIR region among all N treatments became more apparent under the OW treatment than the underwater stress (HW and LW). In the region, 1580 to 1850 nm plants growth under N deficiency (N33 and N67) had a lower reflectance value than those with optimal and higher N doses (N100 and N133). However, the spectral separability between the N treatment and HW/LW treatment is lower in SWIR than VNIR region. The spectral signatures obtained by our experimental setup concurred with these previous studies. Figure 5 presents the relationship between the leaf chlorophyll concentration and features from the second derivative of spectral data based on [PERSON]'s correlation coefficient. It is featured with a correlation coefficient higher than 0.4 indicating a strong correlation with the leaf nitrogen content. Overall, most of the features showing a strong correlation are in the VNIR range. The highest negative correlations were observed in the regions from 410-500 nm and 750-800 nm. In the region between 800 and 1000 nm, the correlation curve becomes noisy. Some features with strong positive or negative correlations with the leaf N content could be found in the SWIR region. The correlation coefficient decreased  
Figure 4: Raw spectral data for celery and sugar beet plants treated with four nitrogen fertilization doses and three water supply levels. LW: low water supply, OW: optimum water supply, HW: high water supply.