Abstract:Canopy leaf pigment status is a key index for evaluating crop potential photosynthetic efficiency and nutritional stress. Leaf pigment density per unit ground area provides a rapid and non-destructive method to evaluate yield predictions and management of nitrogen applications. This study investigated the quantitative relationships of leaf pigment density to canopy hyperspectral reflectance in wheat (Tritium aestivum L.) with two field experiments consisting of different cultivars and nitrogen levels in two growing seasons. On the basis of measured protein content, the cultivars, Yumai 34, Yangmai 12 and Ningmai 9 were considered as high, medium and low protein types, respectively. Four nitrogen rates were applied as 0, 75, 150, 225 kg N hm-2 in the form of urea in 2004-2005, and 0, 90, 180, 270 kg N hm-2 in the form of urea in 2005-2006. Canopy hyperspectral reflectance over 350-2500 nm was measured, and the density of chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll a+b (Chl a+b), and carotenoid (Car) were determined for leaves in the wheat canopies. The densities of the different pigments (Chl a, Chl b, Chl a+b, Car) in wheat leaves increased with increasing nitrogen application rates. With respect to plant development, the densities of the different pigments (Chl a, Chl b, Chl a+b, Car) initially increased and then decreased after jointing with differences between the cultivars, while gradually decreasing in the non-nitrogen treatment. The sensitive wavebands for pigment density occurred mostly within the visible light range, and a close correlation existed between the first derivatives of reflectance in the red-edge region (680-760 nm) and leaf pigment density. The analyses between 21 vegetation indices and leaf pigment density indicated that the pigment densities were highly correlated to all these 21 vegetation indices. The correlations of leaf chlorophyll density to the following five vegetation indices, were higher than for the other spectral parameters, with the coefficients of determination (R2) for linear correlations above 0.858. These five vegetation indices were Vogelmann indices 2 (Vog2), Vogelmann indices 3 (Vog3), the ratio vegetation index of 810 to 560 nm (RVI(810,560)), the ratio of the red-edge (680-760 nm) slope to the blue-edge (490-530 nm) slope (SRE/SBE), and the ratio of the integral of the first derivative on the red-edge region to that on the blue-edge region (SDr/SDb). All the values of R2 between Car density and the different spectral indices were below 0.780. Testing of the derived regression equations with independent datasets indicated that Vog2, Vog3, SRE/SBE and SDr/SDb were the best to predict leaf pigment density with a relative error (RE, (simulated value - observed value)/observed value) below 17.6%. The R2 associated with the Chl b density was below 0.804, lower than that of other pigment densities. The overall results suggested that the Vog2, Vog3, SRE/SBE and SDr/SDb indices have stable relationships with the pigment density in wheat leaves, especially the densities of leaf Chl a and Chl a+b.