*3.1. Hyperspectral and Biophysical Parameters Comparison*

The water stress caused a reduction in practically all biophysical parameters. The exception was for the root development node treatment inoculated with the rhizobacteria. This indicates that the influence of the bacteria was more evident in the root system. The bacteria group had mean values similar or even higher to those found for the control group, both in the fresh mass and in the dry root (Table 1). Chlorophyll content also expressed high differences between the control group and the others.


**Table 1.** Results from the mean test between the biophysical parameters mean.

Mean values followed by equal letters in lines do not present differences at 5% in the Turkey test. C. V. = coefficient of variation.

At the end of the experiment, the lowest values of reflectance and the highest values of absorbance were observed in the control group (Figure 3). The spectral behavior of each treatment increased in difference as the stress progress continued. Stress and bacteria groups were both submitted to water stress, and their spectral response curves were distanced from the control curve. This condition can be explained by the reduction in fresh leaf mass and leaf diameter, which caused an increase in chlorophyll concentration in both groups (stress and bacteria). Furthermore, a continuous analysis over time showed that reflectance and absorbance values both increased and decreased, respectively. The stress group; however, was the one that presented a higher discrepancy over time.

**Figure 3.** Comparison between the mean spectral reflectance and absorptance at selected wavelengths for each of the three treatment groups.

To evaluate the correlation between each spectral behavior in the final day of the experiment, a matrix (Figure 4) was organized with the correlation values between all groups. In general, the control group presented better correlations with the bacteria group than the stress group. The correlation was higher in the near-infrared region for all curves. The green region (from 520 to 580 nm) was more evident on the reflectance curve in the bacteria group and was better isolated when evaluated on the absorbance curve (Figure 4C,D). This could prove difficult in differentiating both groups' spectral behavior. Still, both groups returned different amplitudes in the averaged values (Figure 3), which indicates a feasible separation between them.

The relationship between the biophysical parameters and all the spectral wavelengths of the three groups is shown in Figure 5. The parameters that presented best correlations with the spectral curves were chlorophyll, fresh masses and aerial dry matter, and dry weight of roots. With the exception of the dry root weight, the aforementioned parameters presented higher negative and positive correlations in the visible region (Figure 5A,C,D), specifically in the blue region (from 380 to 460 nm) and in a smaller range in the red region (between 640 and 680 nm), which coincides with the absorption regions of chlorophyll. The weight of the root dry mass was the parameter that presented the highest correlation (r = 0.80) with the near-infrared region (700 nm onwards).

The mean values of the correlation of each parameter with the reflectance and absorbance curves were also compared (Table 2): the chlorophyll, diameter, fresh aerial weight, and dry root weight had better correlations (r = −0.803; 0.540; 0.822; and 0.709, respectively) with absorbance than with reflectance. The difference between averages ranged from 0.797 (fresh root) to 1.602 (fresh aerial), which demonstrates how much the reflectance values differ from those of absorbance. This behavior is better observed in the absorbance wavelengths of chlorophyll, fresh weight of the aerial part and in the dry-root weight, which presented values higher than 0.7, and the lowest values in root mean squared error (RMSE).

**Figure 4.** The correlation matrix between the mean spectral wavelengths.

**Figure 5.** The correlation coefficient (r) between the spectral wavelengths (380–1020 nm) and the biophysical parameter; reflectance (blue curve) and absorbance (pink curve).


**Table 2.** Differences from mean values to correlations from absorbance and reflectance.
