Assessment of plant nitrogen status using chlorophyll fluorescence parameters of the upper leaves in winter wheat

• We investigated using chlorophyll fluorescence (ChlF) to assess N status in wheat.
• ChlF parameters of the upper leaves were well correlated with plant N concentration.
• We developed the NDF index (normalized difference in ChlF between the upper leaves).
• The relationship between NDF and plant nitrogen sufficiency index (NSI) was stable.
• NDFs are potential predictors of NSI in wheat, and were easily applied in the field.

Non-destructive, rapid diagnosis of plant nitrogen status is important for the evaluation of wheat growth and the dynamic management of nitrogen nutrition. Two wheat cultivars, Zhengmai 366 (high protein content) and Aikang 58 (medium protein content) were grown in field trials at five different nitrogen levels (0, 90, 180, 270 and 360 kg ha−1) in two consecutive growing seasons at Zhengzhou, China. Leaf chlorophyll fluorescence (ChlF) parameters, leaf and stem biomass, and nitrogen content were measured simultaneously at different growth stages, establishing an evaluation model of plant nitrogen nutrition in wheat using ChlF parameters. The results showed that the differences in ChlF parameters between the three top leaves (1–3LFT) was small from the reviving to the flowering stages. With increasing nitrogen levels, the difference in ChlF parameters between the fourth leaf (4LFT) and the first three leaves (1–3LFT) decreased gradually, indicating that 4LFT is sensitive to N fertilizer application and has a disadvantage in competition for redistributed N. The correlation coefficients between ChlF parameters for the upper, fully expanded leaves and N concentration of the corresponding leaves were 0.628 for Fv, 0.607 for Fm, 0.579 for Fv/Fo, and 0.600 for Fv/Fm at P < 0.01, but only 0.248 for Fo at P < 0.05. At the reviving and jointing stages, the relationships between the normalized differences between 1–2LFT and 4LFT (NDF12/4) for Fv/Fo and Fv/Fm to plant nitrogen concentration (PNC) were the most significant (r < − 0.79, P < 0.001), the determination coefficient (R2) for Fv/Fm was much higher than for Fv/Fo, and the two regression equations were grouped at reviving and jointing with similar R2 values between the stages. At booting and flowering, the normalized differences between 1–2LFT and 4LFT for Fo, Fm, and Fv better reflected the changes in PNC; the R2 values were 0.654–0.797 (P < 0.001) at booting and 0.515–0.584 (P < 0.001) at anthesis, and the two regression equations were grouped at booting and anthesis with greater differences in R2 between the stages. The unified regression equation could be used to express the relationship between plant nitrogen sufficiency index (NSI) and ChlF parameters with R2 values of 0.623 (P < 0.001) for NDF12/4 for Fv/Fm, and 0.567 (P < 0.001) for NDF12/4 for Fv/Fo during the reviving and jointing stages, while R2 = 0.666 (P < 0.001) for NDF12/4 for Fm and 0.615 (P < 0.001) for NDF12/4 for Fv during booting and anthesis. These results show that the relationship between NDF and NSI was stable and reliable over the different years, varieties, and N supply levels. We conclude that the spatial differences in ChlF parameters between 1–2LFT and 4LFT should be ideal indicators of plant nitrogen status in wheat, and will provide a decision-making method for N diagnosis and regulation in field production.