## 3 Results and Discussion
### Responses of ELUE and GPP to Bio-Physical and Environmental Factors
#### 3.2.2 Responses of ELUE and GPP to Climatic Factors  
Based on the linear regression analysis, ELUE was positively correlated to T\({}_{\rm a}\) across the four crop fields, but only significantly in paddy rice and summer maize, with the r of 0.48 and 0.50 (\(p<0.01\)), respectively. ELUE was only significantly correlated to R\({}_{\rm n}\) in summer maize croplands (r = 0.36, \(p<0.01\)), while no consistent relationship was found between PAR and ELUE in the four agroecosystems. However, GPP was significantly positively correlated to R\({}_{\rm n}\) in the fields (Table 3), while significantly related to T\({}_{\rm a}\) and PAR in the fields, except for the soybean fields. The results were consistent with previous reports that GPP generally increased linearly with rising T\({}_{\rm a}\) and light intensity (reflected by R\({}_{\rm n}\), PAR) [65; 66]. Lower light intensity and temperature could inhibit photosynthesis by influencing stomatal behavior and intrinsic biochemical reaction (e.g., thylakoid rection, Rubisco catalytic activity, electron transport capacity) [23]. Hence, the photosynthesis rate was lower during cold weather as was the ELUE [7; 67]. However, the low affinity of the enzyme for CO\({}_{2}\) and its dual nature as an oxygenase limit the possible increase in net photosynthesis as temperature rises; although the catalytic activity of Rubisco increases, there can be a rapid fall-off of the photosynthetic rate at high temperatures [68; 69]. For example, a quadratic regression result between ELUE and PAR was found in a semiarid savanna ecosystem [4]. However, for farmlands distributed at middle and high latitudes, the temperature usually did not reach the level that incurred a strong negative effect on photosynthesis. Research also demonstrated that T\({}_{\rm a}\) tended to exert a more significant  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Crop Types** & **NDVI** & **g\({}_{\rm c}\)** & **T\({}_{\rm a}\)** & **R\({}_{\rm n}\)** & **PAR** & **VPD** & **EF** \\ \hline Paddy rice & 0.85 ** & 0.24 & 0.47 ** & 0.39 ** & 0.28 ** & \(-\)0.06 & \(-\)0.07 \\ Soybean & 0.91 ** & 0.68 ** & 0.27 & 0.37 * & 0.17 & \(-\)0.20 & 0.82 ** \\ Maize & 0.81 ** & 0.54 ** & 0.56 ** & 0.62 ** & 0.40 ** & \(-\)0.14 & 0.67 ** \\ Winter wheat & 0.76 ** & 0.40 ** & 0.44 ** & 0.59 ** & 0.62 ** & \(-\)0.28 * & 0.42 ** \\ \hline \end{tabular} Note: T\({}_{\rm a}\) represents daily mean at temperature; VPD is vapor pressure deficit; R\({}_{\rm n}\) refers to net solar radiation; GPP means gross primarily productivity; NDVI is normalized difference vegetation index; PAR represents photosynthetically active radiation; EF means evaporative fraction; G\({}_{\rm c}\) represents total plant canopy resistance. ** represents significant level \(p<0.01\),* represents significant level \(p<0.05\).  
\end{table}
Table 3: [PERSON]’s correlation analysis between daily gross primary production (GPP) and the selected variables in different crop fields.  
positive influence on photosynthesis on colder sites [7]; for instance, T\({}_{\text{a}}\) was not significantly correlated to the actual ELUE in hot humid ecosystems [18], while T\({}_{\text{a}}\) determined the intra-annual variability of the ELUE in an energy-limited forest [7]. Therefore, the impact of climate control on carbon fixation will differ depending on the ecosystem types and vegetation growth conditions. More attention should be paid to studying differences in the photosynthetic limiting process according to different species and growth conditions.