## 2 Results
### The Variations in the Mechanisms Linking Photosynthesis and Fluorescence under Drought  
The increasing nonlinearity in the relationship between \(A_{\rm net}\) and \(\rm{ChlF_{P\_F}}\) (Figure 3) suggests that water stress causes shifts in the allocation of absorbed light energy dissipation pathways. At the beginning of the experiment (WS0, non-stress), \(\phi_{\rm P}\) showed an inverse correlation with PAR and exhibited less sensitivity to PAR with increased light levels (Figure 4). The trade-offs between these yields appear to be governed by the complementarity between PQ and NPQ: \(\phi_{\rm N}\) increased with increased PAR, and \(\phi_{\rm P}+\phi_{\rm N}\approx 0.8\) (Figure 4). In contrast, \(\phi_{\rm D}\) and \(\phi_{\rm F}\) had a muted sensitivity to changes in PAR: both of them showed a slight increasing trend at low light levels, and a decreasing trend at intermediate or high light levels (Figure 4). Figure 4: Light–response curves of the quantum yields (\(\phi\)) for the four different pathways during progressive drought stress: fluorescence (\(\phi_{\rm F}\), red), photosynthesis (\(\phi_{\rm P}\), green), regulated heat dissipation (\(\phi_{\rm N}\), yellow), and basal heat dissipation (\(\phi_{\rm D}\), blue). The number in the upper right corner of each plot indicates the day of progressive drought stress. The soil water content (\(\theta_{\rm SWC}\), %) of progressive drought stress treatment is also given. The inset panel illustrates the pattern of \(\phi_{\rm F}\) under changing light intensity. The quantum yields (\(\phi\)) for the four different pathways are obtained from PAM measurements. Figure 3: Relationships of net photosynthetic carbon assimilation (\(A_{\rm net}\), \(\rm{umol~{}m^{-2}~{}s^{-1}}\)) with full-band chlorophyll fluorescence emission at the photosystem level (\(\rm{ChlF_{P\_F}}\), \(\rm{umol~{}m^{-2}~{}s^{-1}}\)). Individual light–response curves are indicated by the black lines connecting measurements obtained at increasing light levels (0–2100 \(\rm{umol~{}m^{-2}~{}s^{-1}}\)). Circles indicate no water stress (\(\theta_{\rm SWC}=19.1\%\)), diamonds represent moderate water stress (\(\theta_{\rm SWC}=12.0\%\)), and squares indicate plants under high water stress (\(\theta_{\rm SWC}=9.7\%\)). The number of light–response curves under high water stress was smaller than those under no water stress and moderate water stress due to human error, leading to unrealistic values in the measurements. \(A_{\rm net}\) is obtained from the gas-exchange system. During the first five days after imposing water treatment (WS1 to WS5), when \(\theta_{\rm SWC}\) decreased from 19.1% to 12.0%, the light-response curves of \(\phi_{\rm P}\), \(\phi_{\rm N}\), \(\phi_{\rm D}\) and \(\phi_{\rm F}\) showed a weak response to drought under low PAR conditions. For example, \(\phi_{\rm P}\) taken at PAR = 180 umol m\({}^{-2}\) s\({}^{-1}\) decreased slightly from 0.63 to 0.61. However, a further reduction in \(\phi_{\rm P}\) occurred at higher PAR levels; for instance, \(\phi_{\rm P}\) at PAR = 700 umol m\({}^{-2}\) s\({}^{-1}\) decreased from 0.42 to 0.36 (Figure 4). \(\phi_{\rm N}\) showed a small increasing trend during each light regime, with a larger magnitude under intermediate and high light conditions (Figure 4). Both \(\phi_{\rm D}\) and \(\phi_{\rm F}\) increased over the entire light-response curve (Figure 4). From the 6 th to the 8 th day of treatment (WS6 to WS8), the variations in these four yields were still relatively limited when PAR \(\leq\) 700 umol m\({}^{-2}\) s\({}^{-1}\). However, \(\phi_{\rm P}\) showed a significant decline at high PAR values: at WS8, \(\phi_{\rm P}\) decreased to 0.13, 0.10, and 0.07 at 1300, 1700, and 2100 umol m\({}^{-2}\) s\({}^{-1}\), respectively (Figure 4). NPQ still remained complementary to PQ: \(\phi_{\rm NPQ}\) increased to 0.51, 0.62, 0.65 for these three PAR regimes (Figure 4).