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Nondestructive Estimation of Shoot Nitrogen in Different Rice Genotypes

Soil and Water Sciences Div., Int. Rice Res. Inst. (IRRI), P.O. Box 933, Manila, Philippines

^* Corresponding author (j.k.ladha{at}cgnet.com).

Plant N uptake and grain yield are important components of N use efficiency. Grain yield is easily measured, but plant N analysis is time consuming and requires hazardous chemicals or expensive equipment. A nondestructive method involving the least equipment and skill, to determine N uptake, is needed in agronomic and plant breeding experiments. A nondestructive method to determine shoot (aboveground biomass) N of transplanted rice (Oryza sativa L.) was developed based on SPAD-502 chlorophyll meter readings, leaf area, and tiller number. In two dry-season and one wet-season field experiment, shoot N of various genotypes at flowering were highly correlated (P < 0.05) with CLAT, the product of SPAD reading from a selected leaf (C), area of that leaf (LA), and number of tillers (T) (r² = 0.46, 0.90, and 0.85 in Exp. 1, 2, and 3, respectively); and with LAT, the product of LA and T (r² = 0.56, 0.88, and 0.76). Shoot N may be estimated using LAT for larger differences in leaf area compared with SPAD readings. Lower correlations in Exp. 1 were due to the lower range in shoot N contents. The highest correlation between shoot N and CLAT was observed in the third uppermost leaf. Regressions of shoot IV on LAT and CLAT varied across growth stages and seasons. Thus, LAT or CLAT can be used to evaluate N uptake among N fertilizer treatments and different rice genotypes at a given stage within a season. Further work is needed to assess the reliability of this method under different seasons and cultural practices.

Received for publication June 9, 1996.

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