## 2 Materials and Methods
### Measurements of Photosynthetic Parameters
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Recorded Data and Technical Parameters**} \\ \hline \(\mathbf{F_{0}}\) & Fluorescence intensity at 20 μs, when all PSII RCs are assumed to be open \\ \(\mathbf{F_{m}}\) & Maximal fluorescence intensity when all PSII RCs are closed \\ \(\mathbf{S_{m}}\) & Normalized total area above the OJIP curve, reflecting \\  & multiple-turnover events \\ \(\mathbf{V_{t}}\) & Relative variable fluorescence at time \(t\) \\ \(\mathbf{V_{k}/V_{j}}\) & Indicator of PSII donor-side limitation, a relative measure of \\  & OEC inactivation \\ \(p\) & Overall connectivity parameter; \\ RC/\(\mathbf{C_{0}}\) & Measure for Q\({}_{\text{A}}\) ‘reducing RCs per excited leaf cross-section (CS) \\ \hline \multicolumn{2}{c}{**Quantum Efficiencies and Flux Ratios**} \\ \hline \(\Phi_{\text{PQ}}\)= \(\mathbf{TR_{0}/ABS}\) & Maximum quantum yield of primary photochemistry; the probability \\  & that an absorbed photon \\  & will be trapped by the PSII RC and will reduce one Q\({}_{\text{A}}\) \\  & Probability that an absorbed photon will enter the electron \\ \(\Psi_{\text{EO}}\)= \(\mathbf{ET_{0}/TR_{0}}\) & transport chain; \\  & electron transport efficiency \\ \(\Phi_{\text{EO}}\)= \(\mathbf{ET_{0}/ABS}\) & Quantum yield for electron transport \\ \(\delta_{\text{RO}}\)= \(\mathbf{RE_{0}/ET_{0}}\) & Probability that an electron is transported from the reduced PQ to the \\  & electron acceptor side of PSI \\ \(\Phi_{\text{RO}}\)= \(\mathbf{RE_{0}/ABS}\) & Quantum yield of electron transport from Q\({}_{\text{A}}\) ‘to the PSI end \\  & electron acceptors \\  & Effective antenna size of an active reaction center (RC). Expresses the \\ ABS/RC & total number of photons absorbed by Chl molecules of all RCs divided by the total number of active RCs \\ ET\({}_{0}\)/RC & Electron transport per active RC \\ TR\({}_{0}\)/RC & Maximal trapping rate of PSII. Describes the maximal rate by which an excitation is trapped by the RC \\ DI\({}_{0}\)/RC & Effective dissipation per active RC \\ RE\({}_{0}\)/RC & Electron flux reducing end electron acceptors at the PSI acceptor side \\  & per RC \\ \hline \multicolumn{2}{c}{**Performance Index and Driving Forces**} \\ \hline \hline \multicolumn{2}{c}{**SFI**} & Structure–function index on an absorption basis; (RC/ABS) \(\times\)\(\Phi_{\text{PQ}}\)\(\times\)\(\psi_{\text{EO}}\) \\  & Performance index (potential) for energy conservation from photons \\  & absorbed by PSII to the reduction of intersystem electron acceptors; \\  & \([\gamma_{\text{RC}}/(1-\gamma_{\text{RC}})][\phi_{\text{PO}}/(1-\phi_{\text{PO}})][ \psi_{\text{EO}}/(1-\psi_{\text{EO}})]\) \\  & Performance index (potential) for energy conservation from photons \\  & Performance index (potential) for energy conservation from photons \\  & absorbed by PSII to the reduction of PSI end acceptors; \\  & \([\gamma_{\text{RC}}/(1-\gamma_{\text{RC}})][\phi_{\text{PO}}/(1-\phi_{\text{PO}})] [\psi_{\text{EO}}/(1-\psi_{\text{EO}})]\) \\  & Total driving forces for the photosynthesis of the observed system, \\ \(\mathbf{DF_{total}}\) = & Total driving forces for the photosynthesis of the observed system, \\ \(\mathbf{log PI_{total}}\) & created by summing up the partial driving forces for each of the \\  & several bifurcations \\
**DRI** & Drought resistance index; log\(([\text{PL}_{\text{d}}/\text{PL}_{\text{c}})(\text{PL}_{\text{t}}{}^{2}/\text{PL}_{ \text{c}}{}^{2})]\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Definitions of measured and calculated JIP-test parameters [47; 60; 61; 62; 63]. The drought resistance index (DRI) was calculated for each _Brassica_ variety, as described for heat stress and recovery [63], with a modification for drought stress, based on the principle defined in [50], by using the driving force (DF) of PI\({}_{\text{ABS}}\) (log PI). The recovery outcome was doubled compared to the one measured after the drought (Table 1).