ADJUSTMENT FOR SPECIFIC LEAF WEIGHT IMPROVES CHLOROPHYLL METERS ESTIMATE OF RICE LEAF NITROGEN CONCENTRATION

The chlorophyll meter provides a simple, quick, and nondestructive met hod to estimate leaf N status of rice (Oryza sativa L.), but the linea r relationship between leaf N concentration on a dry-weight basis (N(d w)) and the meter reading differs depending on developmental stage and genotype. The objective was to determine whether prediction of N(dw) with the chlorophyll meter can be improved by a simple correction for specific leaf weight (SLW). Leaf N status was estimated by a chlorophy ll meter (SPAD-502) and measured directly by micro-Kjeldahl procedure. Specific leaf weight was calculated as the ratio of dry weight to lea f area. In one field study with 'IR72', measurements were taken at mid tillering, panicle initiation, and flowering stages on the uppermost f ully expanded leaves of both N-deficient and N-sufficient plants. Ther e was a linear relationship between N(dw) and SPAD values at each stag e, but regression lines differed significantly between growth stages. Based on pooled data from all stages, the degree of linear fit was poo r (r2 = 0.49). Adjusting SPAD values for SLW (SPAD/SLW improved the pr ediction of N(dw) (r2 = 0.93). For another set of measurements made on the flag leaves of five genotypes grown in the field and greenhouse, prediction of N(dw) was also improved, from r2 = 0.51 based on SPAD va lues alone to r2 = 0.87 based on the SPAD/SLW ratio. These results dem onstrate that SLW influences the prediction of N(dw) by the chlorophyl l meter, and that the adjustment of SPAD values for SLW greatly increa ses the accuracy of the prediction. However, when SPAD values are adju sted for SLW, the chlorophyll meter's estimate of N(dw) is no longer a s quick, simple, or nondestructive as the nonadjusted SPAD values.