Comparison of Plant Measurements for Estimating Nitrogen Accumulation and Grain Yield by Flooded Rice

Knowledge about N accumulation during the vegetative growth stage of flooded rice ( Oryza sativa L.) may be useful in determining the need for topdressing fertilizer N at panicle differentiation (PD). In a 3‐year field study, plant area, N concentration of the Y‐leaf (most recently matured leaf blade) and the whole plant, and chlorophyll meter (SPAD) readings measured during vegetative and early reproductive growth stages were used to estimate total N accumulation. The techniques were then used to determine the growth stages that maximized correlation with grain yield. Five preflood (PF) N rates (0, 33.6, 67.2, 100.8, and 134.4 kg ha −1 ), two PD N rates (0 and 67.2 kg ha −1 ), and two cultivars (LaGrue and Lacassine) were used. The treatments were chosen to represent an array of dry matter and total N accumulations. No interactions of PF N rate × cultivar on grain yield and total N accumulation were observed. Plant area was linearly correlated ( r = 0.84 to 0.93, P 60% of the variation in total N accumulation every year. However, Y‐leaf N concentration and SPAD readings accounted for 60% only in 1995. A combination of plant area and Y‐leaf N concentration or SPAD readings accounted for more variation in total N accumulation than did individual plant measurements. Plant area and whole‐plant N concentration was the best combination, accounting for 80 to 90% of the variation in total N accumulation. The maximum variation in grain yield accounted for by the measured traits was 50% at PD for plant area, and 37% at 2 wk after the PF N application for Y‐leaf N concentration and SPAD reading. These low correlations of grain yield with plant measurements during the vegetative stage confirm that environmental and other conditions prevailing during later growth stages profoundly influenced grain yield of rice.