# Using Fluorescence Spectroscopy to Detect Rot in Fruit and Vegetable Crops
## 3 Results and Discussion
### Experiments with a 527 nm Pulsed Laser  
In experiments with a 527 nm pulsed laser, fluorescence spectra were obtained in the 560-720 nm range. In this spectral range, chlorophyll peaks at 680 nm were clearly visible, especially in green apples. The rotten apple is easily distinguishable by the fluorescence spectra from the healthy apple (Figure 8). The intensities of fluorescence are increased, and fluorescence peaks are shifted to shorter wavelengths for rotten apple. Interestingly, controversial influence of rot on laser-induced fluorescence spectra can be observed while comparing Figures 5b and 8c. In the case of 527 nm pumping, the fluorescence signal is also increased for rot as expected, but for 405 nm, the excitation situation is the opposite. This can be explained by the difference in absorption coefficient for healthy and rotten apple surfaces that resulted in different sampling depths. For example, a penetration depth of 405 nm is smaller compared to 527 nm radiation, so fluorescence spectra will originate from different layer thicknesses. Figure 6: (**a**) Continuous-wave fluorescence spectra of healthy (blue) and rotten (red) surface of potato; (**b**) continuous-wave fluorescence spectra of healthy (blue) and rotten (red) pulp of potato. On the top right, the fluorescence spectra of healthy potato pulp with a thickness of 2 mm (green) and 0.5 (crimson) mm. Figure 7: (**a**) Changes in the fluorescence spectra of potato pulp for 10 min; (**b**) fluorescence kinetics of potato pulp at a wavelength of 680 nm for samples with a thickness of 0.5 mm. Typically, any damage of plant material will increase the intensity of fluorescence spectra due to the decreased efficiency of quenching. In the case of 527 nm pumping, the penetration depth is significantly larger, so apple rotting will result in an increase of fluorescence signal. Alternatively, the 405 nm wavelength had a small penetration depth that was further increased for the rotten apple surface due to the increased absorption coefficient of the decomposed organic material. In the case of the green apples (Figure 9), the intensity of fluorescence is conversely dramatically decreased, although fluorescence peaks are shifted to shorter wavelengths in the same manner for rotten red apple. The maximal shift is observed for the rotten pulp. Intensity of fluorescence is decreased, and contribution of the 630 nm peak is increased in the following order: healthy surface, rotten surface, rotten pulp. Figure 8: Pulsed fluorescence spectra of healthy (black) and rotten (red) red apples: (**a**) initial spectra of surfaces; (**b**) normalized spectra of surfaces; (**c**) initial spectra of pulp; (**d**) normalized spectra of pulp. Significant differences between emission spectra of healthy and scalp-affected apple samples were also found in the region of 680-750 nm in [26], where a conventional serial fluorescence spectrometer with a xenon lamp as the radiation source was used. The fluorescence spectra of washed and unwashed potatoes were compared to assess the contribution of dust and soil to the overall spectrum (Figure 10a). The overall intensity is higher for washed potatoes, and the spectrum of the unwashed sample is noisier. However, the envelope of the spectrum is the same in both cases. The contamination does not distort the spectrum and it is possible to analyze directly unwashed potatoes in the future in field experiments. The spectra of whole potatoes of different potato samples are quite different from each other (Figure 10b). There is a significant shift of chlorophyll peak to the region of shorter wavelengths for the rotten sample. The 680 nm peak is more pronounced than the 630 nm peak for healthy potato samples of the \"Udacha\" cultivar. Figure 10: (**a**) Pulsed fluorescence spectra of surface of “Udacha” (healthy) dirty (green) and washed (violet) potato; (**b**) pulsed fluorescence spectra of surface of “Udacha” (conditionally healthy) (blue) and “Zhukovsky” (conditionally rotten) (red) dirty potato.