## 4 Discussion
### Effect of NR, PAR, SM, RH and VPD on GPP  
After sunrise, the NR gradually increased, reaching its peak value at around noon and before starting to decline gradually. The same trend was also observed in PAR. During the daytime, the GPP increased as there was greater CO\({}_{2}\) uptake with increasing PAR. A significant positive correlation was observed between PAR and GPP in both rice and wheat (0.225, 0.348; \(p<\) 0.01), respectively (Table 2). The GPP is mainly dependent on the amount of PAR absorbed (APAR) by green leaf area of the crop canopy ([PERSON] et al., 2019). When the PAR value was greater than 2500 \(\upmu\) mol m\({}^{-2}\) s\({}^{-1}\), the maximum carbon assimilation rate was observed at the booting stage in rice, whereas a PAR of more than 2000 \(\upmu\) mol m\({}^{-2}\) s\({}^{-1}\) in wheat led to a greater CO\({}_{2}\) uptake in wheat. These differences in the rates of CO\({}_{2}\) uptake were due to the different light response parameters of rice and wheat. Apparent quantum yield (\(\alpha\)) and Pmax (maximum photosynthetic capacity at light saturation) varied at different growth stages according to the crop phenology (Table 3). The highest P max was observed at the booting stage in rice, whereas it was highest at the heading stage in wheat. The Pmax values ranged from 0.347 to 2.412 mg CO\({}_{2}\) m\({}^{-2}\)s\({}^{-1}\) for rice and from 0.461 to 2.841 mg CO\({}_{2}\) m\({}^{-2}\)s\({}^{-1}\) in wheat. The initial value of \(\alpha\) was in the range of 0.0007 to 0.0008 mg CO\({}_{2}\)\(\upmu\) mole photon\({}^{-1}\) and started to increase at the vegetative stage as the plants started to grow rapidly. It was the highest at the heading stage in both rice and wheat. The average annual \(\alpha\) and Pmax values in our study were a little lower than those reported by [PERSON] et al. (2019) in a maize-winter wheat rotation system and by [PERSON] et al. (2019) in a rice-winter wheat system. The ecosystem \(\alpha\) and Pmax values, could be affected by the differences in air temperature, vapor pressure deficit (VPD), and other biotic factors, such as the green leaf area. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Environmental Variables**} & \multicolumn{2}{c}{**Ecosystem Respiration (RE)**} & \multirow{2}{*}{**Environmental Variables**} & \multicolumn{2}{c}{**Gross Primary Productivity (GPP)**} \\ \cline{2-2} \cline{4-6}  & **Rice (N = 4712)** & & **Wheat (N = 5472)** & & **Rice (N = 4712)** & **Wheat (N = 5472)** \\ \hline
**AT** & 0.060  & AT & 0.129  \\ \hline
**ST** & 0.543  & 0.010  & 0.348  \\ \hline
**SM** & \(-\)0.088  & \(-\)0.147 [FOOTNOTE:]Footnote : For each pair of correlation all the other variables in the column are the covariates; * Significant at (\(p<\) 0.05); ** * Significant at (\(p<\) 0.01); ** * = non-significant; N is number of observations.[ENDFOOTNOTE] \\ \hline
**SHF** & 0.084  & NR & 0.248  & RH & 0.140  & 0.471 [FOOTNOTE:]Footnote : For each pair of correlation all the other variables in the column are the covariates; * Significant at (\(p<\) 0.05); ** * Significant at (\(p<\) 0.01); ** * = non-significant; N is number of observations.[ENDFOOTNOTE] \\ \hline
**GPP** & 0.917  & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Partial correlation coefficient between ecosystem respiration (RE), gross primary productivity (GPP) and related environmental variables. In the current investigation, the VPD was between 1.5 to 2.5 kPa and influenced the GPP in both the crops. The VPD increases with air temperature, increasing the plant respiration rate and leading to a reduction in CO\({}_{2}\) uptake [34]. However, it was difficult to evaluate the effects of high VPD on overall photosynthesis and GPP because high VPD conditions mostly persist for a short duration around midday and the stress impact may not persist throughout the rest of the day [49]. The GPP was significantly negatively correlated with VPD in rice (\(-\)0.315, \(p\) < 0.01) (Table 2).