Abstract:

Non-destructive monitoring and diagnosis of plant nitrogen (N) status is necessary for precision N management. The present study was conducted to determine if canopy reflectance could be used to evaluate leaf N status in wheat (Triticum aestivium) of two cultivars, `Huaimai 18' and `Xuzhou 26'. Ground-based canopy spectral reflectance, leaf N concentration and leaf N accumulation were measured at seven growth stages (jointing, booting, heading, anthesis, initial filling, mid-filling and late filling) under four different treatments of N fertilization (0, 12, 21 and 30 g N·m-2). Analyses were made on the relationships of seasonal canopy spectral reflectance, ratio indices (R(λ1,λ2)=ρλ1/ρλ2) and normalized difference indices (ND(λ1,λ2)=|ρλ1－ρλ2|[]ρλ1+ρλ2) to leaf N concentration and N accumulation in wheat under different N treatments at different growth stages and during the whole growing season. The results showed that canopy spectral reflectance in the near infrared increased with increasing N content, whereas reflectance in the visible band decreased. Leaf N status was significantly correlated with the ratio or normalized difference indices in both cultivars. Regression models of leaf N status to spectral indices based on single growth stages did not improve the prediction over whole growth cycle; hence, a single regression model based on the whole growth cycle should be adequate for diagnosing N status in wheat plants. The relationships between leaf N concentration and spectral indices were better than those of leaf N accumulation. The best indices were the combination of a MIR (mid-infrared) band (1 220 nm) and red band (660 nm) for leaf N concentration (LNC=3.894 5×R(660, 460)-1.028 7, R2=0.834　2; LNC=3.918 7×e-2.179 6ND(660,460), R2=0.835 2) and the combination of the red band (660 nm) and blue band (460 nm) for leaf N accumulation LNA=1.55×e0.115 3R(1 220, 660), R2=0.724　2; LNA=0.219 4×e4.091 4ND(1 220, 660), R2=0.629 9). These results indicate that canopy spectral reflectance can be used to non-destructively monitor leaf N status in wheat plants.