CULTIVAR AND NITROGEN RATE INFLUENCE ON NITROGEN UPTAKE AND PARTITIONING IN RICE

Numerous empirical studies have shown that semidwarf rice (Oryza sativa L.) cultivars require higher N fertilizer rates for optimum yields than standard-statured cultivars; however, these studies do not indicate the reason for this increased N requirement. Field studies were conducted with semidwarf 'Lemont' and standard-stature 'Lebonnet' rice to evaluate N uptake and partitioning as related to fertilizer N-15 rates and developmental stages. Four fertilizer N rates bracketing the recommended ranges for the two cultivars were applied as N-15-labeled urea in split applications at preflood (PF) and particle differentiation (PD). Aboveground plant portions were sampled at PD, heading, and heading + 21 d for dry matter, total N, and fertilizer N-15. Lebonnet produced more total dry matter at all N rates and had higher panicle weights at the lower two N rates, whereas Lemont had higher panicle weights at the highest N rate. Lemont continued to accumulate total N between heading and heading + 21 d, whereas Lebonnet showed either a decline or leveling. Fertilizer N-15 uptake and accumulation decreased after heading for both cultivars, thus Lemont appears to continue accumulating soil N through grain filling. Nitrogen harvest index for total and fertilizer N with Lemont was greater than with Lebonnet, therefore, N utilization differences between the two cultivars may be associated only with total N uptake and partitioning after heading.