Responses of Nitrogen Status Indicators to Nitrogen Rates and Soil Moisture in Corn (Zea mays L.)

The present study compared the sensitivity of tissue nitrogen (N) concentration, SPAD readings, Dualex readings, and SPAD/Dualex ratios for assessing the N status of corn (Zea mays L. cv. Pioneer 38B84) under different water supply conditions. A greenhouse trial was conducted with 4 rates of N fertilizer (0, 50, 50+75, and 200 kg ha⁻¹) and 3 watering levels (drought, drought followed by rewatering, and fully-watered). Tissue N concentration, SPAD, Dualex, and SPAD/Dualex values were influenced significantly by N rates and soil water content (SWC). Tissue N concentration, SPAD value, and SPAD/Dualex ratio increased with N rates, whereas Dualex value decreased. In the first phase of reaction to drought, tissue N concentration, SPAD value, and SPAD/Dualex ratio decreased rapidly, but Dualex value increased; however, the opposite pattern of response was observed in the long term. Under rewatering, tissue N concentration, Dualex value, and SPAD/Dualex ratio gradually recovered, whereas SPAD values did not change significantly as they did in the drought treatment. There were highly significant relationships between tissue N concentration and SPAD reading (r = 0.92, P ≤ 0.001), Dualex value (r = (0.86, P ≤ 0.001), or SPAD/Dualex ratio (r = 0.63, P ≤ 0.001). However, SPAD and Dualex values were better predictors of tissue N concentration under drought condition (r= 0.90, P ≤ 0.001 for SPAD; r = −0.83, P ≤ 0.001 for Dualex) than under fully-watered condition (r = 0.39, P ≤ 0.05) for SPAD; r = −;0.44, P ≤ 0.05 for Dualex) at the end of the trial. Among the indicators, Dualex is better to discriminate N treatments with consistent results across SWC levels.