Abstract: For diagnosing nitrogen (N) nutrient status in winter wheat plant based on fluorescence parameters of leaves, two cultivars with Aikang 58 (mid-protein content) and Zhengmai 366 (high-protein content) were used in experiments with N application rates of 0, 90, 180, and 270 kg ha^{-1 in 2008–2009 and 2009–2010 growing seasons. The fluorescence parameters were measured in the first, second, third, and fourth leaves from top of plant from jointing to maturity stage. The N contents and biomasses of leaf and culm were determined through destructive sampling and chemica1 assay. The leaf N contents at different 1eaf positions were significantlycorrelated with fluorescence parameters from jointing to booting stage (P < 0.01). The correlation coefficients of N content with F}_m, F_v, F_v/F_m, and F_v/F_o were 0.557, 0.601, 0.619, and 0.633, respectively. The fluorescence parameters of the top three leaves had small differences, which were much higher than those of the fourth leaf from the top. However, the differences reduced gradually with the increase of N application rate. This indicated a sensitive response of leaf position to N application. The differences of F_v/F_o and F_v/F_m between the fourth and the first leaves (LPD_4-1) were significantly correlated with N content in plant (PNC) at jointing stage (R² = 0.644 for F_v/F_o and R² = 0.651 for F_v/F_m); whereas, the differences of F_v/F_o and F_v/F_m between the fourth and the second leaves (LPD_4-2) had less accurate predictions with smaller R² values (R² = 0.626 for F_v/F_o and R² = 0.592 for F_v/F_m). At booting stage, linear equations were obtained for F_o, F_v, and F_m differences (LPD_4-n) between the fourth and the top three leaves (R² = 0.726 for F_o, R² = 0.791 for F_v, and R² = 0.784 for F_m). Using independent datasets to predict PNC at jointing stage with the LPD_4-1 and LPD_4-2 equations based on F_v/F_o and F_v/F_m, the R² values for LPD_4-1 were 0.844 and 0.828 with relative errors of 13.0% and 16.7%, respectively; and the R² values for LPD_4-2 were 0.793 and 0.813 with relative errors of 16.9% and 18.4%, respectively. The result showed that the estimations were reliable. At booting stage, the best prediction of PNC was from the LPD_4-n equation based on F_v and F_m, and R² values were 0.831 and 0.815 with relative errors 13.2% and 16.4%, respectively. In conclusion, the LPD_4-1 equation based on F_v/F_o at jointing stage and the LPD_4-n model based on F_v at booting stage are applicable to estimate PNC of wheat.

Key words: Winter wheat, Nitrogen content in plant, Leaf position, F1uorescence parameters