Nitrogen-utilization efficiency in rice: an analysis at leaf, shoot, and whole-plant level

Aims If rice has a higher nitrogen use efficiency for dry matter production (NUE) and grain yield (NUEGY) than wild annuals, we may question whether the higher NUE is due to a higher productivity per plant N (NP) or a longer retention time of plant N (MRT) or both, and whether the higher NUEGY results also from a higher harvest index (HI). Methods Stands of rice were established at three N levels. Censuses were done for birth and death of every shoot and leaf from germination to full maturity. Nitrogen uptake, dry matter production, grain yield, HI, NUE, and NUEGY were determined at shoot and whole-plant levels. Results Rice had a higher NUE, NUEGY and NP, but hardly higher plant-N MRT and HI than wild annuals. Leaf-N MRT was higher than leaf longevity in fertile shoots, while the opposite was true in sterile tillers. Shoot NUEGY was higher in late tillers due to higher HI. Conclusions High NUE of rice results from its high NP, not from high MRT at both whole-plant and leaf levels. Revenues gained from enhancing MRT would have been lower than the opportunity costs of reducing NP. N recycles between shoots. Sterile tillers function as an N storage for grain yield in fertile shoots.