Estimating Canopy Nitrogen Status From Hyperspectral Parameters of Single Leaves in Wheat.

Leaf nitrogen status is the key indicator for evaluating growth and forecasting yield in agronomic crops. Hyperspectral sensing can provide an effective means for fast estimation of crop leaf nitrogen content. The present study was undertaken to determine the sensitivity of hyperspectral parameters of top single leaves to canopy leaf nitrogen status, and then quantify the relationships between canopy leaf nitrogen content and single leaf hyperspectral parameters in wheat. The field experiments under varied nitrogen rates and cultivar types were carried out in four wheat-growing seasons, and time-course measurements were taken on hyperspectral reflectance and nitrogen concentrations of top four leaves. The results showed that the spectral indices of the 2nd and 3rd leaves had the stronger capacity to estimate the canopy leaf nitrogen content (LNC), and could be considered as the indicators of canopy LNC in wheat. Of the normalized differential spectral index (NDSI) and ratio spectral index (RSI), the parameters NDSI (R₆₁₀, R₄₈₀) and RSI (R₆₁₀, R₄₈₀) of the 2^nd leaf, NDSI (R₁₈₂₁, R₅₇₁) and RSI (R₁₈₂₁, R₅₇₁) of the 3^rd leaf could be used for reliably predicting canopy LNC. Of several red edge parameters, REP-IGAUS, symmetry of REP were the best indices for evaluating canopy LNC based on the 2^nd or 3^rd leaf. With the previously reported spectral indices, RSI (R₅₆₀, R₄₅₀) and FD₇₂₃ were seen as the proper parameters for canopy LNC on the 2^nd or 3^rd leaf. In addition, for the spectral indices of combined leaves, the monitoring equations based on DSI[NDSI(R₁₈₂₁, R₅₇₁)₃, NDSI(R₆₁₀, R₄₈₀)₂], NDSI[RSI(R₁₈₂₁, R₅₇₁)₃, RSI(R₆₁₀, R₄₈₀)₂] gave R² values greater than 0.8, with better performance than the spectral indices of single leaves. From testing of the derived equations with independent experiment data, the models based on DSI[NDSI(R₁₈₂₁, R₅₇₁)₃, NDSI(R₆₁₀, R₄₈₀)₂], NDSI[RSI(R₁₈₂₁, R₅₇₁)₃, RSI(R₆₁₀, R₄₈₀)₂] exhibited greater stability and accuracy. These results provide a technical approach to estimating canopy LNC from hyperspectral parameters of single leaves in wheat.
Key words: Leaf position; Leaf hyperspectra; Canopy nitrogen content; Quantitative relationship; Monitoring