Indicators for diagnosing nitrogen status of rice based on chlorophyll meter readings

Abstract

Various indices established on a portable SPAD-502 meter can serve as indicators of the nitrogen status of a plant. We compared di-positional SPAD readings and indices with several reliable nitrogen indicators during vegetative growth stage of rice (Oryza sativa L.) and developed a prediction model for diagnosing nitrogen status. Three field experiments were conducted in Jiangsu province of east China during 2013 and 2014. Different nitrogen application rates were used to generate contrasting conditions of nitrogen availability in three Japonica, Wuyunjing 19, Yongyou 8, and Wuyunjing 24, and one Indica rice hybrid, Yliangyou 1. The SPAD values of the uppermost four fully expanded leaves were measured from tillering to heading stages, and these values were further used to calculate the normalized SPAD index (NSI), relative SPAD index (RSI), difference SPAD index (DSI), relative difference SPAD index (RDSI), and normalized difference SPAD index (NDSI). Five hills from each plot were simultaneously sampled, and four nitrogen indicators, including leaf nitrogen concentration (LNC), plant nitrogen concentration (PNC), plant nitrogen accumulation (NA), and nitrogen nutrition index (NNI), were measured. The results of linear correlations among the SPAD and nitrogen indicators indicated that NSI of the fourth fully expanded leaf from top (NSI4) was the most reliable and generally applicable SPAD indicator. In total, 24 potential single-stage and duration models were established, taking into consideration the fact that duration models covering the period from stem elongation to booting stages are more robust, two duration diagnostic models (Model 1: NA=0.0279e8.6957NSI4, R2=0.730**, n=45; Model 2: NNI=0.0163e4.13NSI4, R2=0.767**, n=45, NSI4=0.80–1.00) were developed and calibrated. Both models can provide accurate and appropriate N diagnosis from the stem elongation to booting growth stages for rice production.