## 4 Discussion
### Change in Primary and Secondary Metabolism Caused by Glyphosate Treatment
The third parameter that was investigated in the plant material in the current experiment was the dry matter content because glyphosate application is obviously associated with a loss of water. The loss of water, expressed as an increase in dry matter, content became statistically significant 7 days after glyphosate application. This character became significant so late due to the fact that one of the glyphosate-treated containers reacted with a time lag to the application. Nevertheless, it is important to emphasize that the chlorophyll content showed a significant response despite the delayed dieback and therefore seem to be a more sensitive indicator. Chemical analysis revealed that photosynthetic pigments, and here especially chlorophyll \(a\), are very sensitive parameters to detect glyphosate application followed by the polyphenols and the dry matter content. The polyphenol content in the fresh material was closely correlated with the dry matter content. Differences caused by drought stress became significant only at the last two samplings. However, it is important to mention that on the last sampling dates no differences were observed between any of the measured parameters in the glyphosate-treated and drought-stressed plants. Therefore, when vegetation dieback is widely proceeded or completed it is not possible to draw conclusions on the cause of dying off. Spectral Vegetation Indices for Early Detection of Changes in Vegetation Due to Glyphosate Treatment  
Hyperspectral reflectance measurements were used to investigate the vegetation cover with respect to its composition (e.g., nitrogen, polyphenols) and important growth characteristics (e.g., ripening, senescence) [38; 39; 40; 41] but also to distinguish between herbicide-treated and non-treated plants [28]. [PERSON] et al. [42] could show that hyperspectral reflectance data are suitable to distinguish between glyphosate-resistant and non-resistant varieties of amaranth even without glyphosate application as the plants slightly differed in light reflectance. If hyperspectral reflectance data were also suitable to distinguish glyphosate-application from other factors that result in vegetation dieback, spectral data could be useful for monitoring glyphosate application. In literature, multitudinous spectral vegetation indices were described. They refer to different wavelengths and by this to different plant constituents. From the large number of available plant spectral vegetation indices, a set of eight indices was chosen which address crop parameters of interest in the context of glyphosate treatment, i.e., photosynthetic pigments, dry matter, lignin and canopy or leaf water content (Table 1). There are many indices described in literature that exhibit sensitivity to the chlorophyll content, for example the pigment specific simple ratio index (PSSRa) and the inverted red-edge hhl chlorophyll index (IRECI). The PSSRa is a simple ratio algorithm and was actually developed for estimating photosynthetic pigment concentrations in deciduous tree leaves showing strongest and most linear relationship with chlorophyll \(a\) and \(b\) concentrations [20]. [PERSON] et al. [21] presented the IRECI with regard to the retrieval of biophysical variables from Sentinel-2 MSI sensor. It showed significant relationship to canopy chlorophyll content but also to leaf area index (LAI). While there are relatively many approaches for the estimation of chlorophyll from reflectance measurements, only a few models support the retrieval of other pigments and biochemical components such as carotenoids, anthocyanins and lignin content. The photochemical reflectance index (PRI) was shown to correlate with the epoxidation state of xanthophyll cycle pigments and photosynthetic radiation use efficiency [23; 24]. The PRI measures the relative reflectance on either side of the green peak (550 nm) where reflectance at 531 nm is dominated by chlorophyll absorption only and reflectance at 570 nm is affected by chlorophyll and carotenoid absorption.