## 4 Discussion
### Change in Primary and Secondary Metabolism Caused by Glyphosate Treatment  
The mode of action of glyphosate is an inhibition of the 5-enolpyruvateylshikimate-3-phosphate synthetase in the shikimate pathway, which interferes in the production of proteins and other molecules that require the aromatic amino acids tryptophan, tyrosine or phenylalanine as precursors [3]. Aromatic amino acids are important precursors for many different compounds belonging to the primary and secondary plant metabolism, such as growth promotors like indoleacetic acid, tannins, anthocyanins, flavonoids and lignin. The disturbance in the metabolism of aromatic amino acids result in the very quick dieback of plants. It was assumed that the photosynthetic pigments and the polyphenols are good indicators to observe the changes caused by glyphosate application on a metabolic level. Figure 8: CRI-1 calculated from hyperspectral image data of the Specim IQ camera for control (Ctr), glyphosate (Gly) and drought (Dry) treatment of replication 1. Data were acquired every second day of sampling starting on 9 July until 17 July and on day 16 (25 July) and day 23 (1 August) of the experiment. Also, other studies revealed that glyphosate and its main degradation product aminomethylphosphonic acid (AMPA) caused a disturbance in the chlorophyll biosynthesis of non-resistant species, which led to decreased chlorophyll contents and a decreased photosynthetic activity in conjunction with oxidative stress [31; 32]. In the present study, a clear effect of glyphosate-treatment on chlorophyll _ab_ and carotenoids was found, too. Already 2 days after glyphosate application the contents of photosynthetic pigments decreased significantly (Tables 2 and 3). Therefore, spectral indices that refer to the chlorophyll content or the photosynthetic activity seem to be good candidates to recognize changes caused by glyphosate application at an early stage. Polyphenols are ubiquitous products of the secondary plant metabolism with very diverse functions including formation of pigments in flowers and fruits, a contribution to biotic and abiotic stress tolerance, pollen fertility, structural roles and signalling [33]. The main classes of polyphenols that can be commonly found in plants are flavonoids, tannins, phenolic acids, stillenes and lignans [34], and their biosynthesis is steeply increasing under stress conditions [35; 36]. According to [PERSON] et al. [36], polyphenols are \"the most versatile secondary metabolites, thus allowing plants to respond promptly to unpredictable stress\". Phenolic compounds are defence compounds, which are produced to protect plants from herbivores or photo-damage mainly by their high antioxidant activity [37]. Under stress conditions, reactive oxygen species (ROS) can be produced which need to be detoxified to prevent plants from being damaged. Polyphenols are very active metabolites [34; 35]. When chlorophyll biosynthesis is disturbed through glyphosate application, absorbance of light energy by photosynthetic pigments is disturbed, resulting in an excess of light energy, which can enhance the production of ROS. Therefore, as glyphosate application affects chlorophyll biosynthesis, limited photosynthesis will increase oxidative stress and an increase in polyphenols can be expected. [PERSON] et al. [31] observed a decrease in chlorophyll biosynthesis with glyphosate as well as AMPA application and both chemicals induced ROS accumulation. On the other hand, the shikimate pathway, which is disturbed by glyphosate application, is important in the biosynthesis of polyphenols. From the data (Tables 2 and 3), it is obvious that polyphenols increased with glyphosate application, but they react much slower than photosynthetic pigments. Differences were significant only 6 to 7 days after application. Nevertheless, spectral indices that capture changes in polyphenols are also interesting for the prediction of plant stress.