Effect of nitrogen on vertical distribution of canopy nitrogen and chlorophyll relative value (SPAD value) of summer maize in sub-humid areas

A field experiment was carried out in sub-humid area using Eum Orthic Authrosols as sample soil type. The experiment analyzed the vertical distribution pattern of canopy leaf nitrogen and leaf chlorophyll relative value (SPAD value), differences in different layer-leaves for different nitrogen levels in summer-maize (Zeamays L.) at three growth stages (jointing, grain filling and maturity) and correlation for layer-leaf nitrogen content, leaf SPAD value and applied N. The results show that different layer-leaves have different nitrogen content which obviously diminishes from the upper to middle, then the lower leaf canopy. Based on the average of different leaf layers for the entire growth stage, upper leaf layer is 6.64% higher than middle leaf layer, which in turn is 5.18% higher than the lower leaf layer. With increasing application of nitrogen, the difference between upper and middle leaf layers increases. However, the difference between middle and lower leaf layers decreases with increasing nitrogen application. The vertically changing trend of canopy leaf chlorophyll relative value is similar to that of nitrogen content of leaves. Correlation analysis indicates a significant (R = 0.503^**) linear correlation between chlorophyll relative value and leaf nitrogen content throughout the growth stage. Further analysis indicates that the upper leaf layer has the most closed correlation of leaf nitrogen content, chlorophyll relative value with applied nitrogen. So the best diagnosis of nitrogen nutrition for summer-maize is in the upper leaf layer.