Estimating the proportion of nitrogen remobilization and of postsilking nitrogen uptake allocated to maize kernels by nitrogen-15 labeling

Estimating the proportion of N remobilization and postsilking N uptake allocated to kernels may help to improve N-use efficiency in maize (Zea mays L.). In this study, we show theoretically and experimentally, that N-15 labeling at the beginning of stem elongation can be used in the field to estimate the proportion of N remobilized from the vegetative parts to the kernels of maize by measuring N-15 distribution only at maturity. In the same way, N-15 labeling at silking allows a determination of the proportion of postsilking N uptake allocated to the kernels by measuring N-15 distribution only at maturity. Two 1-yr experiments with three and four genotypes and one 2-yr experiment with testcross progenies from 66 recombinant inbred lines were developed. Nitrogen-15 labeling during vegetative growth provided an estimate of the proportion of N remobilized with a greater accuracy compared with the "balance" method, which computes the difference between the amount of N present in the stover at silking and the amount of N present in the stover at maturity. The validity of our N-15 method is mainly based on the assumption that less than 15 to 20% of N-15 is taken up after silking. The determination of the proportion of postsilking N uptake allocated to kernels requires assumptions that are more difficult to fulfill. Nitrogen-15 labeling in the field at the beginning of rapid stem growth appears to be a useful tool for studying the genetic variability of N remobilization using a large number of genotypes.