## 4 Conclusions  
Our results for the selected _Brassica_ crops determined kale to be the most tolerant, Chinese cabbage as moderately tolerant, and white cabbage as the most sensitive to drought stress. Drought stress induced no visible signs of damage on kale seedlings, the primary photochemistry was not disturbed, and the electron flow was not blocked at the PSII level, as well as intersystem electron carriers. Moreover, the better stability of the system led to the enhanced conservation of energy through electron transport compared to Chinese and white cabbage. However, the slight drought-induced disturbances observed at PSI recovered completely after re-watering, corroborating its high drought tolerance. As for Chinese and white cabbage, drought induced significant disturbances in PSII photochemistry. In addition to the lower connectivity of PSII units and the decoupling of OEC, the inactivation of reaction centers and the reduced electron flow rate between \(\text{Q}_{\text{A}}\) and \(\text{Q}_{\text{B}}\) decreased the ability to efficiently utilize absorbed and trapped light energy. Nevertheless, the higher dissipation of excess light reduced the capacity for photochemical \(\text{Q}_{\text{A}}\) reduction by increasing the PQ pool in both Chinese and white cabbage. In white cabbage, less pressure on PSI consequently caused a slower transfer of electrons to the PSI acceptor side, which could be a compensatory mechanism for protecting PSI from overreduction and, thus, coping with drought stress. The fully recovered PSII photochemistry in Chinese cabbage suggested the reversible downregulation of PSII reactions. However, the re-watering of white cabbage did not induce a full recovery of most of the parameters, suggesting more severe damage to the photosynthetic units and corroborating its higher drought sensitivity. The biophysical interpretation of Chl \(a\) fluorescence parameters offers a convenient framework for distinguishing drought-tolerant and -sensitive _Brassica_ crops. To detect the downregulation of specific events in primary photochemistry before harmful and irreversible consequences occur, it is necessary to identify specific parameters that would indicate the possible sequence of events. In the present study, positive L and K bands, an increase in the \(\text{PI}_{\text{total}}\) and the SFI factor, as well as positive \(\Delta\text{DF}\) and DRI could suggest drought tolerance. On the other side, negative L and K bands, in addition to a lower \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **F1** & **F2** & **F3** \\ \hline RWC & \(0.066\) & \(0.502\) & \(0.782\) \\ \(\Phi_{\text{PQ}}\) & \(0.776\) & \(-0.428\) & \(0.434\) \\ \(\Phi_{\text{EQ}}\) & \(0.988\) & \(0.038\) & \(0.108\) \\ \(\Psi_{\text{EQ}}\) & \(0.982\) & \(0.150\) & \(0.012\) \\ ABS/RC & \(-0.916\) & \(0.388\) & \(-0.037\) \\ \(\text{TR}_{0}/\text{RC}\) & \(-0.935\) & \(0.326\) & \(0.116\) \\ \(\text{ET}_{0}/\text{RC}\) & \(-0.249\) & \(0.820\) & \(0.349\) \\ \(\text{DI}_{0}/\text{RC}\) & \(-0.832\) & \(0.456\) & \(-0.259\) \\ \(\text{PI}_{\text{ABS}}\) & \(0.974\) & \(0.035\) & \(0.025\) \\ \(\Phi_{\text{PQ}}\) & \(0.894\) & \(0.382\) & \(-0.208\) \\ \(\delta^{\text{RD}}\) & \(0.753\) & \(0.541\) & \(-0.354\) \\ \(\text{RE}_{0}/\text{RC}\) & \(0.549\) & \(0.809\) & \(-0.158\) \\ \(\text{PI}_{\text{total}}\) & \(0.960\) & \(0.138\) & \(-0.048\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Variable contributions (loadings) for the principal component analysis model in Figure 6b. PItotal, SFI, and RC/CS\({}_{0}\), could distinguish a variety with a lower tolerance or even a higher sensitivity to drought-induced stress. Detecting the drought-resistant varieties by noninvasive methods such as Chl \(a\) fluorescence is a useful tool for the fast screening of crops exposed to drought in the field to anticipate the response and to develop efficient strategies for protection, thus keeping the yield stable. Conceptualization, B.S.-S.; methodology, J.A.D., S.M., I.P. and H.L., validation, S.M. and I.P.