## 3 Results
### Spectral Vegetation Indices  
From the SVC reflectance data, eight spectral vegetation indices (see Table 1) were calculated to investigate if these data are suitable to distinguish between crops treated with glyphosate and those suffering from drought stress in comparison to healthy control plants. The development of the spectral vegetation indices during the course of the experiment is displayed in Figure 7; PRI, IRECI, NDVI and NDWI showed very similar trends for the control and the drought treatments up to and including day 8. Differences became visible only on the last two days of sampling. The NDLI, mARI and CRI-1 exhibited the first differences between the control and drought on day 7, whereas PSSRa on day 8. In contrast, glyphosate treatment differed already a few days after application. For NDLI, differences became obvious on day 2. The CRI-1, PRI, PSSRa and mARI indicated changes on day 3, NDWI and NDVI on day 4. Statistical analysis using [PERSON]'s test (Table 4) confirmed the different capacity of the spectral indices to differentiate between the control plants and the glyphosate treatment. The statistics revealed that significant differences between control and glyphosate treatment existed earliest on day 3, and latest on day 7. The indices responded in the following order: NDLI (3) \(>\) CRI-1 = PRI (4) \(>\) PSSRa (5) \(>\) IRECI = mARI = NDVI (6) \(>\) NDWI (7) with NDLI differentiating first already 3 days after glyphosate  
Figure 7: Development of spectral vegetation indices during the experiment. application and NDWI differentiating last with 7 days after application. With respect to drought, no differences were observed. This is likely due to the fact that water shortage had an effect on the development of the sod only very late. At this time, no more daily measurements were analysed. Only one index, the mARI, was able to differentiate between glyphosate and drought treatment at the last sampling date, when all other indices showed no statistically different values. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**DAA** & **Treatment** & \multicolumn{8}{c}{**Spectral Vegetation Indices \({}^{*}\)**} \\  & & **PSSRa** & **IRECI** & **mARI** & **CRI-1** & **PRI** & **NDLI** & **NDVI** & **NDWI** \\ \hline
**0** & Ctr & 12.0 \({}^{\text{a}}\) & 1.31 \({}^{\text{a}}\) & 0.66 \({}^{\text{a}}\) & 3  
Linear regression models were set-up to analyse the interrelationship between chemical parameters and the spectral vegetation indices (Table 5). The results showed that all relationships were highly significant but the strength of correlation as indicated by the coefficient of determination varied considerably. Generally, \(R^{2}\) was higher for the linear models between the spectral indices and the photosynthetic pigments than for the polyphenols. With regard to the photosynthetic pigments (carotenoids, chlorophyll), the highest \(R^{2}\) was found for CRI-1 and PRI (\(R^{2}>0.85\)). Scatterplots between spectral indices and plant pigments disclosed a positive linear relationship for CRI-1 and chlorophyll _ab_ as well as CRI-1 and carotenoids, whereas PRI exhibited a sigmoid curve shape (not shown). For the polyphenols highest \(R^{2}\) was retrieved using mARI indicating a broad scatter and a weak correlation (\(R^{2}=0.32\)). Furthermore, strong linear relationships were found between the dry matter content and the NDVI, NDLI, NDWI and PRI (\(R^{2}>0.86\)). Hyperspectral image data were processed and spectral vegetation indices were calculated for each replication and date. This was done for all indices except NDWI and NDLI as these indices require data outside the wavelength range of the Specim IQ camera. The results confirmed findings obtained from the SVC reflectance measurements. The CRI-1 and PRI are the first to recognize changes after glyphosate application. Figure 8 displays the CRI-1 of the first replication for all three treatments. The index revealed first signs of deterioration already on day 2.