## 4 Materials and Methods
### Experiment Design  
The experiment was conducted between 15 October 2020 and 30 April 2021 in the Northwest A&F University, Yangling, China. Winter wheat (_Tritcium aestivum_ L. cultivar Xi [PERSON] 979) was sown in plastic pots (27 cm height \(\times\) 21 cm diameter) filled with 9 kg of sieved, air-dried, loess toposoil and 1.95 g of urea. The field capacity of the soil was 24% and the wilting point was 9%. The plants were grown under a rainproof shed until the turning-green stage, and then moved to a climate chamber where CO\({}_{2}\) concentration, air temperature and relative humidity were controlled at 400 umol mol\({}^{-1}\), 12 \({}^{\circ}\)C and 60%. The light intensity above the canopy during the experiment was kept at 600 umol m\({}^{-2}\) s\({}^{-1}\) for 12 h per day, from 8 a.m. to 8 p.m. Beginning in late March, eight representative wheat plants in the heading stage were chosen for the experiment. Throughout the 28-day experiment, the gravimetric soil water content (\(\theta_{\text{SWC}}\), %) was continuously monitored using the weighing method [58; 59]. The wheat plants were subjected to one of two treatments: non-water stress (NS) and water stress (WS). Four wheat plants in each treatment were randomly selected as biological replicates. In the NS treatment, the \(\theta_{\text{SWC}}\) was maintained at 19.1% (well-watered plants, assuming 80% of field capacity) [60; 61] throughout the experiment, but, in the WS treatment, to simulate intensifying drought, \(\theta_{\text{SWC}}\) was gradually decreased to a final value of 6.3%. This steady decline in \(\theta_{\text{SWC}}\) ensured that plants under the WS treatment were subjected to progressive drought stress [62]. Measurements were made on attached leaves of the wheat plants under the two water treatments. The wheat leaf had to be placed in the 3 cm\(\times\) 3 cm clear chamber along the diagonal of the square and positioned in the chamber with its adaxial surface facing the LED light source. Since the leaves did not fill the leaf chamber, the area of each leaf had to be accurately measured. After 1 h of dark adaptation, a saturation flash from the PAM fluorometer was used to determine minimal fluorescence (F\({}_{\text{o}}\)) and maximal fluorescence in the dark (F\({}_{\text{m}}\)) of dark-adapted leaves. Next, the light-response curves and CO\({}_{2}\) response curves of gas exchange and fluorescence were measured. The CO\({}_{2}\) flow rate, air relative humidity and leaf temperature were kept constant at 500 umol s\({}^{-1}\), 50% and 12 \({}^{\circ}\)C, respectively. To obtain light-response curves, measurements were conducted at a CO\({}_{2}\) concentration of 400 umol mol\({}^{-1}\). Light-response measurements were made with photochemically active radiation (PAR) light intensities of 0, 40, 90, 180, 350, 700, 1300, 1700, and 2100 umol m\({}^{-2}\) s\({}^{-1}\). Next, we measured the CO\({}_{2}\) response curve under saturated light intensity (1500 umol m\({}^{-2}\) s\({}^{-1}\)) using values of the CO\({}_{2}\) concentration gradient of 30, 50, 100, 200, 300, 400, 600, 900, 1200, 1500 umol mol\({}^{-1}\). Throughout the course of each light-response curve and CO\({}_{2}\) response curve determination, spectral measurements (reflected radiance, transmitted radiance, forward and backward fluorescence spectral radiant energy flux) were continually recorded at 1 s intervals. Steady-state fluorescence emission (F\({}_{\text{t}}\)), induced by the measuring beam of the PAM fluorometer, was also included. Maximal fluorescence emission in the light-adapted state (F\({}_{\text{m}}\)) from the PAM was recorded under each light intensity and CO\({}_{2}\) concentration. From these measurements (F\({}_{\text{o}}\), F\({}_{\text{t}}\), F\({}_{\text{m}}\), F\({}_{\text{m}}\)), we also obtained NPQ, a fraction of open PSII reaction centers (\(q_{\text{L}}\)) [63], and the actual rate of electron transport (\(J_{\text{a,PAM}}\), umol m\({}^{-2}\) s\({}^{-1}\)). The details for estimating them are provided in Note S1.