## 3 Discussion  
Identification of the cascade of drought-induced changes in photosynthetic characteristics is highly relevant to fully understanding the relationships between photosynthesis, NPQ, and fluorescence under water stress conditions. Stomatal closure is the earliest response to drought, paralleling a decrease in photosynthetic CO\({}_{2}\) assimilation but with a lower magnitude (Figure 2). In the early stages of drought stress, exposure to the volatile plant hormone methyl jasmonate (MeJA) can mitigate drought stress by stomatal closure [37; 38; 39] During WS1-WS6, under mild-to-moderate water stress, energy partitioning in PSII (i.e., \(\phi_{\mathrm{P}}\), \(\phi_{\mathrm{N}}\), \(\phi_{\mathrm{D}}\), and \(\phi_{\mathrm{F}}\)) remained virtually unchanged under low or intermediate light levels (Figure 4). However, we noticed a decline in \(\phi_{\mathrm{P}}\), and a rise in the other three pathways, \(\phi_{\mathrm{D}}\), \(\phi_{\mathrm{N}}\) and \(\phi_{\mathrm{F}}\), in the presence of high light levels during that period (Figure 4). These observations may suggest that the proportion of energy dissipated in each pathway does not change much regardless of stomatal closure levels during the initial period of drought [40]. At moderate-to-severe drought (WS7-WS9), G\({}_{\mathrm{S}}\) almost vanished (Figure 2) and the observed drop in \(A_{\mathrm{net}}\) indicated the predominance of non-stomatal limitations to photosynthesis [41]. Given the lower G\({}_{\mathrm{S}}\), the excess light energy can ultimately lead to the production of reactive oxygen species and reflect the MeJA response [37; 38; 39]. As a consequence, \(\phi_{\mathrm{P}}\), the quantum yield of photochemistry in PSII, showed a decreasing trend, especially at high light levels (Figure 4). To compensate for the decline in \(\phi_{\mathrm{P}}\), regulated heat dissipation (\(\phi_{\mathrm{N}}\)) tended to increase as a main photoprotective mechanism, and \(\phi_{\mathrm{D}}\) remained fairly constant (Figure 4). The engagement of thermal energy dissipation also resulted in a small decrease in \(\phi_{\mathrm{F}}\). Under extreme drought conditions (WS10-WS12), \(\phi_{\mathrm{P}}\) continued to decrease, and the stressed plants kept increasing their use of thermal energy dissipation and fluorescence emission (\(\phi_{\mathrm{F}}\)) to cope with excess light energy (Figure 4). However, the process of thermal energy dissipation changed from a regulated form that can be rapidly activated by excess light to a sustained form that is rather insensitive to fluctuating light [42; 43], resulting in the simultaneous rise of \(\phi_{\mathrm{D}}\) and decline of \(\phi_{\mathrm{N}}\) (Figure 4). This result is consistent with the findings that this transformation of energy dissipation characteristics tends to occur during periods of harsh environmental stress [44]. The results confirm that water stress increases nonlinearity in the overall relationship between photosynthesis and fluorescence (Figure 3). Under typical high light levels, as what might occur for early afternoon spaceborne SIF retrievals, the 'NPQ phase', driving a positive SIF-GPP relationship, is representative of non-stressed plants. We showed that the transition from the 'PQ phase' to the 'NPQ phase' is driven by both irradiance and the severity of drought. During moderate or severe drought stress, the decrease in \(\phi_{\mathrm{P}}\), and increase in \(\phi_{\mathrm{F}}\), pushes the stressed plant from the 'NPQ phase' to the 'PQ phase', resulting in a more nonlinear SIF-GPP relationship when considering both the drought and non-drought periods together. When the stomata are closed, [PERSON] et al. (2020)[16] found that the photosynthetic rate decreases rapidly, and SIF is relatively less affected. SIF cannot track the changes of photosynthesis, resulting in the phenomenon of decoupling fluorescence and photosynthesis. During a short drought duration, [PERSON] et al. (2020)[29] showed that the degree of reduction in leaf photosynthesis was much greater than the reduction in SIF.