## 3 Results and Discussion
### Drought Resistance Index and Principal Component Analysis  
The drought resistance index (DRI) was calculated for each _Brassica_ variety separately in order to quantify the reactions of seedlings to drought and subsequent recovery. Our results reveal positive DRI values for kale, while Chinese and white cabbage showed negative ones (Figure 6a). DRI could be used as a measure for the energy conservation potential of a variety in stressful conditions [63]. The calculation of the stress factor index was introduced as a practical tool for the classification of stress tolerance [108]. It is usually calculated as a relative performance index at crucial points of plant exposure to stress by taking into account and emphasizing the importance of the duration of exposure and the ability of plant to recover [80; 109]; however, it could be modified depending on different setups of experiments, including the extent of stress exposure or the recovery from it [63]. The performance index, drought factor index or drought stress/resistance indexes are extensively used indicators used to classify drought tolerance in various plants as well as phenotypic plasticity [110; 111; 112]. They have often been used to screen varieties, cultivars, or genotypes that have been subjected to various stresses, mostly drought [48; 50; 80; 109; 113; 114] and temperature stress [63; 108]. It has been suggested recently that this could be a useful parameter in identifying genotypes differing in their response to drought [80; 115]. The ability of a specific variety to recover after stressful conditions--in this case, after the drought--was shown to be genotype-specific [34; 107]. Therefore, drought-sensitive genotypes show a higher decrease in PI\({}_{\text{ABS}}\) and, consequently, lower DRI values [50]. Similar results were shown for soybean genotypes sensitive to dark chilling [108], barley cultivars [50], sesame lines [48], and sunflower hybrids [101] sensitive to drought, as well as for wild barely [36] sensitive to heat stress. Therefore, our results corroborate the fact that kale is the most resistant _Brassica_ variety, while white cabbage is the most sensitive one. The PCA (Figure 6b, Table 2) was performed based on a matrix of Pearson's correlation coefficients (\(p\leq 0.05\)) to compare correlations among _Brassica_ varieties in the control, drought, and after-recovery conditions in relation to selected photosynthetic parameters and RWC (data not shown; see [PERSON] et al. [9]). Before the analysis, the autoscaling of the average values was performed to standardize the parameters. The first two components explained 86.63% of the variability. Chinese cabbage and kale in the control and drought-stressed kale positioned in the fourth quadrant showed a negative correlation with RWC and parameters describing quantum efficiencies and probabilities, as well as  
Figure 6: Drought resistance index (DRI) (**a**) of three _Brassica_ seedlings subjected to drought relative to corresponding controls. Bars represent the means \(\pm\) SD of seven measurements (\(n=7\)); different letters represent significant differences at \(p\leq 0.05\) (ANOVA, HSD). Principal component analysis (PCA) (**b**) shows variation within and among three _Brassica_ seedlings (blue dots) in the control (C) and after drought (D) and recovery (R) in relation to the PSII functioning parameters, performance index, quantum efficiencies, and flux ratios shown as red dots. with \(\text{PI}_{\text{total}}\) and \(\text{RE}_{0}/\text{RC}\). However, the same parameters contributed the most to the reactions of white cabbage in the control and drought stress conditions. Energy flux ratios, especially those describing absorption, trapping, and dissipation, contributed negatively to reactions in drought-stressed white cabbage and in kale after recovery. However, the same parameters contributed the most to reactions in Chinese cabbage exposed to drought. Kale showed the lowest variability in performance, suggesting a higher efficiency due to a better tolerance to drought.