Seasonal changes of leaf chlorophyll content as a proxy of photosynthetic capacity in winter wheat and paddy rice.

[Display omitted] • V cmax25 , N a and Chl a of winter wheat and paddy rice exhibited distinguishable seasonality. • The seasonality of leaf N a and Chl a were inconsistent with that of V cmax25. • Differentiating pre- and post-flowering stages significantly improved the estimation of V cmax25 from Chl a. Accurate representation of photosynthetic capacity and its seasonal variations is critical for modeling carbon sequestration of cropland ecosystems through photosynthesis. Previous studies indicated that the maximum carboxylation rate at 25 °C (V cmax25) is the key determinant of photosynthetic capacity and can be mapped according to leaf nitrogen (N) and chlorophyll (Chl) contents. However, whether relationships of V cmax25 with leaf nitrogen and chlorophyll contents in cropland ecosystems change over the whole growing season remain unclear. To address this question, we conducted intensive field campaigns in winter wheat and paddy rice farmlands of Eastern China, with leaf V cmax25 , the maximum electron transport rate at 25 °C (J max25), N, and Chl measured every 7 to 10 days from the tillering stage to maturity. Results showed that V cmax25 and J max25 varied significantly during the growing seasons and maximized at flowering stages. For both winter wheat and paddy rice, Chl a (Chl content at the area unit) showed significantly different relationships with V cmax25 pre- and post- flowering while N a (N content at the area unit) did not. When different empirical models were used for pre- flowering and post- flowering stages, V cmax25 estimated from Chl a was closer to observations (R2 = 0.875, RMSE = 9.479 µmol m−2 s−1) than that estimated from N a (R2 = 0.565, RMSE = 17.718 µmol m−2 s−1). The findings in this study imply that the combination of remotely sensed leaf Chl a content and phenology would improve the mapping of V cmax25 for crops at regional and even global scales. [ABSTRACT FROM AUTHOR]