## 3 Results  
### Dynamics of Plant Spectral and Biological Properties  
Figure 2a and b show the temporal dynamics of reflectance properties for Spring Barley (a) and Sugar Beet (b) separately for all combinations of water (WC) and nitrogen (N) treatments. Spectra for each crop type/N-WC-treatment combination vary not only between single time steps but also in between different N- and WC-treatments due to the changes in a canopy structure and differences in  
pigment and water content, both caused by different availability of water and nutrients. It is obvious from Figure 2 that spectral response significantly changes over time, however, to a different degree for different treatments and for different crop types. As pointed out by [45,46], spectral domains of strong chlorophyll (blue and red) and water (water bands) absorption are more sensitive to low concentration of chlorophyll and water, while others (green and regions between water bands) are more sensitive to high water and N-concentration. From the reflectance spectra in Figure 2, where we see both, a clear temporal development that seems to be dominated by the plant growth and therefore LAI dynamics, and (to a much smaller extent) differences under different irrigation conditions, we would expect at least some potential for the extraction of LAI and WC from narrow band SI's.  
Differences in plant development for different water and nutrient availability, as indicated by measured LAI values and crop height, are illustrated in Figure 3. As expected, samples under low N treatment (Figure 3a left) indicate low LAI values, while those cultivated under high N conditions (Figure 3a right) show higher values. However, this behavior is much more distinct for spring barley (o) than for sugar beet (+), where this difference grows with increasing water level. The analysis of crop height (Figure 3b) indicates a clear difference between both species. While different in their phenology, it can be observed that under low nitrogen conditions (Figure 3b left), the height for spring barley (o) decreases during the entire experiment and for all water levels. Sugar beet (+) height decreases very rapidly only under low water treatment, while under medium or high water conditions such behavior is only observable towards the end of the experiment. Interesting is the height development of spring barley under high nitrogen conditions (Figure 3b, right), where a decrease in height is similar to those under low nitrogen treatment.  
In summary, plant spectral reflectance as well as the temporal development of LAI and crop height responds significantly to differences in water and nutrient availability. Even though reflectance sensitivity is not wavelength independent and varies within the entire spectral range, this behavior  
Figure 2: Temporal dynamics of spectral properties of Spring Barley **(a)** and Sugar Beet **(b)**, plotted with regards to applied WC and N treatments (indicated in an upper right corner of all plots), during the BL 2008 experiment.  
suggests the possibility of an inverse retrieval of crop characteristics and controlling water and nutrient conditions. This potential will be explored in the next sub-sections.