Plant Density and N Rate Impacts On Maize Plant N Uptake Dynamics During Vegetative and Reproductive Stages.

The improvement of the N use efficiency (NUE, defined as grain or biomass production per unit of N applied or available N), which is widely acknowledged to be a complex plant process, may need a new approach. From our standpoint, one of the most promising avenues to enhance maize NUE is related to the study of N uptake partitioning during vegetative and reproductive stages. With this objective, two field experiments were established in Indiana to explore the interactions of plant density (PD) (low-54,000, medium-76,000 and high-104,000 plants ha^-1) and fertilizer N rate (low-0, medium-112 and high-224 kg N ha^-1) on N uptake dynamics of two maize hybrids. Total above-ground biomass and N uptake were measured at six developmental stages (V5, V10, V15, R1, R3 and R6 stages). Maize plants were separated into leaves and stems at all vegetative stages (including at R1 stage, when the ear component was also evaluated), and into ears versus the combined leaf and stem biomass at R3 and R6 stages. Maize plant %N declined with growth stage progression from a maximum value of 4.6% (low density and high N- at V5) to a minimum value of 0.5% (high density and low N- at R6). At silking time, if we ignore the “ear fraction”, the leaf to the stem N uptake ratio ranged from ~1.0 (low density and high N rate) to ~1.4 (high density and low N rate) presenting an important N rate and plant density effect. Ear biomass progression was strongly associated with the ear N uptake (effective ratio ~1.2%) from R1 to R6 at both experimental sites. This high slope value confirms once again the high N requirement for optimal ear development. The slope for the relationship between ear biomass and ear N uptake was very similar to the slope for the whole plant biomass and N uptake; the latter confirmed the importance of the ear fraction in the proportion of the final biomass and N uptake plants achieved at maturity. Nitrogen remobilization from the vegetative structures to ears was greater at the low N rate and low density treatment combination; the implication is that a higher efficiency in the N remobilization process correlated with a lower efficiency in overall plant N uptake. Thus achieving improved maize NUE requires the selection of genotypes with high N uptake during vegetative growth and high remobilization capacity.