## 2 Materials and Methods
### Method  
#### 2.3.1 Plotting of Light-Response Curve  
The light-response curves were plotted by using an LI-6400 XT portable photosynthesis system (Licor Inc., Lincoln, NE, USA). Measurements were taken during the 2021 cotton seedling period (18 d after sows), budding period (55 d after sows), flowering and bold development period (86 d after sows), and maturation period (145 d after sows) on windless sunny days between 11:00 and 13:00. Three plants with consistent growth status were selected. The fully unfolded third leaves were measured, and three biological replicates were measured for each configuration treatment. To reduce environmental interference, the open-air path was used, and the parameters were set as follows: air flow rate 500 \(\mu\)mol\(\cdot\)mol\({}^{-1}\), CO\({}_{2}\) concentration (400 \(\pm\) 20) \(\mu\)mol\(\cdot\)mol\({}^{-1}\), leaf temperature (25 \(\pm\) 2) \({}^{\circ}\)C, and relative humidity (50 \(\pm\) 5)%. The gradient of PAR simulated by red and blue light sources from weak to strong was 0, 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, and 2000 \(\mu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), with a total of 11 gradients. Before each measurement, the leaves were induced for approximately 60-120 s under 1200 \(\mu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) light intensity, and the minimum stable time after light intensity change was set to 180 s. When the variation rate of the measurement results was less than 0.05, measurements were automatically recorded by the instrument [28; 29].  
The measurement items included the net photosynthetic rate (Pn, \(\mu\)mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)), transpiration rate (Tr, mmol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)), stomatal conductance (Gs, mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)), intercellular CO\({}_{2}\) concentration (Ci, \(\mu\)mol\(\cdot\)mol\({}^{-1}\)), and leaf instantaneous water-use efficiency  
Figure 3: Diagram of field configuration modes.  
(WUE, \(\mu\)mol-mmol\({}^{-1}\)) = Pn/Tr, and the light-response curve was fitted using the photosynthetic calculation software (4.1.1) (CHN Jiangxi, jinggangshan university, college of life sciences, Ye, Z.) [30]. The best-fitting effect of the rectangular hyperbolic correction model was selected [24]. The fitting effect of the rectangular hyperbolic correction model is shown in Figure 4, and the R\({}^{2}\) value is 0.9996. The main physiological parameters, including maximum Pn (Pmax), apparent quantum efficiency (AQE), light saturation point (LSP), light compensation point (LCP), dark respiration rate (Rd), and determination coefficient (R\({}^{2}\)), were also obtained. The fitting model is given by  
\[\mathrm{Pn}=\alpha\frac{1-\beta\mathrm{I}}{1+\gamma\mathrm{I}}\mathrm{I}- \mathrm{Rd} \tag{1}\]  
where \(\alpha\) is the initial slope of the light-response curve, \(\beta\) and \(\gamma\) are coefficients, I is the PAR, and Rd is dark respiration.  
LSP is given by  
\[\mathrm{LSP}=\frac{\sqrt{(\beta+\gamma)/\beta-1}}{\gamma} \tag{2}\]  
Pmax is given by  
\[\mathrm{Pmax}=\alpha\bigg{(}\frac{\sqrt{\beta+\gamma}-\sqrt{\beta}}{\gamma} \bigg{)}^{2}-\mathrm{Rd} \tag{3}\]