Growth-stage dependent changes of leaf chlorophyll content as a proxy for photosynthetic capacity in maize

Accurately quantifying leaf photosynthetic capacity and its seasonal dynamics is critical for simulating gross primary productivity of farmland ecosystems. Previous studies found that leaf chlorophyll (Chl) content was a better proxy for leaf photosynthetic capacity than leaf nitrogen (N) content for C3 crops. However, the relationships of leaf photosynthetic capacity with leaf Chl and N contents for C4 crops (i.e. maize) remain unclear. To address this question, we measured leaf N, Chl, carotenoid (Car) contents, and leaf photosynthetic capacity quantified by the maximum PEP carboxylation rate (V pmax ), maximum carboxylation rate (V cmax ) and maximum electron transport rate (J max ) weekly for a summer maize cropland in North China from the tillering stage to maturity. The results revealed that leaf photosynthetic capacity and leaf biochemistry parameters exhibited distinct dynamic patterns, increasing during vegetative stages and declining during reproductive growth. The relationships between photosynthetic capacity and biochemical variables (i.e., leaf N, Chl, Car contents) differed pre- and post-flowering. The strongest correlation was observed between the maximum carboxylation rate at 25 °C (V cmax25 ) and leaf Chl content by area (Chl area ) (R 2 = 0.81) before flowering, followed by N content by area (N area ) and Car content by area (Car area ) (R 2 = 0.78 and 0.62, respectively). The explanatory power of Chl area , N area , and Car area for V cmax25 declined to varying degrees after flowering (R 2 = 0.74, 0.50, and 0.52, respectively). The Variable Influence on the Projection (VIP) scores and loading analyses also showed that the relationship between V cmax25 and biochemical variables exhibited growth-stage dependency. Multiple linear regression analysis revealed that the combination of leaf N area , Chl area , and Car area had higher accuracy in estimating V cmax25 (pre-flowering: R 2 = 0.94, RMSE = 1.50 μmol·m −2 ·s −1 ; post-flowering: R 2 = 0.78, RMSE = 4.44 μmol·m −2 ·s −1 ) than individual leaf traits. However, the estimation accuracy of V cmax25 based on leaf N area , Chl area , and Car area showed R 2 value only 0.13 (pre-flowering) and 0.04 (post-flowering) higher than that of leaf Chl area alone. These findings imply that integrating leaf Chl with growth stage information could enhance the estimation of leaf photosynthetic capacity in C4 crops.