Seasonal Changes of Leaf Photosynthetic Capacity in Relation to Biochemical Parameters of Winter Wheat and Rice in Eastern China

Accurate representation of photosynthetic capacity (PC) and its seasonal variations is critical for modeling carbon uptake of cropland ecosystems. Current studies indicated that PC can be mapped by biochemical parameters, e.g. leaf nitrogen content (N) and leaf chlorophyll content (Chl). However, relationships between PC and these biochemical parameters in cropland ecosystems over the whole growing seasons remain unclear and controversial. In this study, We conducted intensive field campaigns for monitoring seasonal variations of PC, N, and Chl of winter wheat and rice in Eastern China, including growing seasons for winter wheat in 2017 and 2018 and for rice in 2017. We took measurements of leaves from full expansion to senescence every 7 to 10 days over crops growing seasons (from the beginning of jointing stage to harvest).

Results show that leaf photosynthesis varied significantly during the growing seasons. Maximum Rubisco activity normalized to 25 °C (V_cmax25) ranged from 9.2 to 141.8 µmol m^-2 s^-1 for winter wheat and from 6.3 to 123.2 µmol m^-2 s^-1 for rice respectively. Leaves at different positions had obvious photosynthesis differences, due to leaf ages and environmental factors. From flowering, all measured parameters showed a turning trend. The photosynthetic parameters of flag leaves (i.e., canopy leaves) peaked at the flowering stage, increased slightly before flowering and decreased significantly after flowering. Correlations of V_cmax25 with Chl varied largely before and after flowering, with R² =0.30 (n=28, p<0.001) and 0.89 (n=39, p<0.001) over these two periods, respectively. The relationship between V_cmax25 and N also exhibited similar seasonal changes, with R²=0.41 (n=20, p<0.001) and R²=0.63 (n=38, p<0.001) before and after flowering. If all samples of winter wheat and rice were lumped together, the R² values of V_cmax25-Chl and V_cmax25-N were 0.70 and 0.72, respectively. Our findings indicate the potential of remotely sensed N and Chl used to map Vcmax of crops at regional and global scales.