Vertical Variations of Leaf Photosynthetic and Biochemical Parameters Within Winter Wheat and Paddy Rice Canopies at Different Growth Stages

During crop growth, leaf photosynthetic capacity changes continuously, and the vertical distribution of leaf nitrogen (Na, in g m−2) and chlorophyll (Chla, in μg cm−2) affects photosynthesis in different canopy layers. Understanding stratified photosynthesis is vital for the accurate prediction of crop photosynthetic capacity. We conducted a two-year field study on winter wheat and paddy rice in Eastern China, measuring the leaf maximum carboxylation rate (Vcmax25), maximum electron transport rate (Jmax25), Na, and Chla every 7–10 days from greening to maturity. We analyzed vertical variations in these parameters in the upper (T-1), middle (T-2), and lower (T-3) canopy layers and explored relationships between Na/Chla and Vcmax25. The results showed significant vertical variations: Vcmax25 and Jmax25 in T-1 were higher than in T-2, and T-2 was higher than T-3. The vertical distribution of Na and Vcmax25 was more pronounced than that of Chla. The correlation between Na and Vcmax25 increased from T-1 to the lower layers, while the Vcmax25-Chla correlation decreased. A single Vcmax25 estimation model based on Na performed well across layers (R2 = 0.619, RMSE = 15.751 µmol m−2 s−1). Differentiating T-1 from T-2/T-3 improved the Chla-based models. Na was better than Chla for characterizing the Vcmax25 vertical variation, with the Chla-based models requiring separation of T-1 from T-2/T-3. This study provides key insights for remote sensing of photosynthetic parameters and improves the understanding of crop canopy photosynthesis.