SPAD Values and Nitrogen Nutrition Index for the Evaluation of Rice Nitrogen Status

Plant-based diagnosis is one of the most important methods to determine nitrogen (N) content of crops. Our objective was to establish the relationship between soil-plant analysis development (SPAD) values and N nutrition index (NNI) during the three developmental stages of rice and apply the SPAD meter as diagnostic tools for predicting grain yield response to N fertilization. We determined the SPAD values of four uppermost fully expanded leaves of two rice cultivars at six N fertiligation levels at three growth stages and examined the relationship between SPAD values and NNI. The critical N concentration (N-c) was 5.31 W-0.5 in Xiushui63, and 5.38 W-0.49 in Hang43, where W is the total shoot biomass. The correlation between SPAD value and NNI varied with the leaf position, developmental stage, and variety. The lower leaf appeared to be more sensitive to the N level than the upper leaf in the response of biomass, and could be more suitable as a test sample for N status diagnosis, especially in the booting and heading stage. The dependence of grain yield on SPAD values of the fourth fully expanded leaf (L4) was significant at booting stage (R-L4(2) = 0.82** in 2011, R-L4(2) = 0.72** in 2012). Ratio of SPAD values of L4 to that in the N-saturated plot (RSPAD) (R-L4(2) = 0.92** in 2011, R-L4(2) = 0.77** in 2012) and NNI (R-2 = 0.96** in 2011, R-2 = 0.86** in 2012) at booting stage demonstrated a closer relationship with grain yield.