Deconfounding Phenology in SPAD-Based Rice Nitrogen Diagnosis Using Physiological Time and Canopy-Stratified...

Area of Science:

• Agronomy
• Plant Physiology
• Soil Science

Background:

• Phenology complicates rice nitrogen diagnosis using SPAD readings due to nonlinear changes in leaf greenness and nitrogen concentration during development.
• Existing methods lack robustness across different growth stages and environments.

Purpose of the Study:

• To evaluate if physiological time (growing degree days, GDD) can mitigate developmental bias in SPAD-based rice nitrogen diagnosis.
• To enhance the portability and accuracy of nitrogen diagnosis across diverse field experiments.

Main Methods:

• Analysis of 1141 observations from 20 independent field experiments across five sites.
• Inclusion of japonica, indica, and hybrid rice cultivars with varying nitrogen fertilizer treatments (0-300 kg N ha⁻¹).
• Measurement of SPAD on multiple leaf positions (LFT1-LFT5) and prediction of leaf nitrogen concentration (LNC), plant nitrogen concentration (PNC), and nitrogen nutrition index (NNI) using SPAD and GDD.

Main Results:

• Incorporating GDD alongside SPAD readings consistently improved cross-environment accuracy for LNC (R² up to 0.75) and PNC (R² up to 0.79).
• GDD addition significantly reduced residual trends along physiological time, indicating reduced developmental bias.
• Diagnostic information was found to be concentrated in mid-canopy leaves and varied with physiological time.
• A two-leaf SPAD protocol combined with GDD maintained high accuracy for nitrogen concentration targets.

Conclusions:

• Physiological time (GDD) effectively reduces developmental bias in SPAD-based nitrogen diagnosis for rice.
• Combining GDD with a simplified SPAD protocol offers a time-aligned, field-practical, and robust approach for nitrogen management.
• This method enhances the reliability and cross-environmental applicability of nitrogen status assessment in rice production.