Physiological Processes Affected by Plant Density and Nitrogen Stress Tolerance In Maize Hybrids: Association Between Maize Grain Yield and N Uptake.

A better understanding of the relationship between maize grain yield and N uptake should be pursued with the purpose of advancing the knowledge in the N use efficiency (NUE) at the system-level. Two field experiments involving different hybrids were conducted to investigate possible individual and interacting effects of plant density (PD) (low-54,000, medium-76,000 and high-104,000 pl ha^-1) and fertilizer N rate (low-0, medium-112 and high-224 kg N ha^-1) on maize yields and NUE. The objectives of this work were to: (i) evaluate the correlation between plant N uptake rate (NUR) and plant growth rate (PGR), (ii) assess physiological correlation between maize grain yield (and its components) with the plant N uptake, and (iii) evaluate the association between the grain and N partitioning components with the plant NUR and PGR processes. A high association was found between NUR and PGR around the critical maize period bracketing silking (from V15 till R3 stage) regardless of location, hybrid and plant density factors. The highest values were associated with the low density and high N rate treatment combination. Maize grain yields and its components (kernel number and weight) presented a strong correlation with the amount of N stored prior to silking stage, increasing both grain components as N uptake increases; the latter showed the importance of the N status on achieving higher kernel number and weights in the per plant (per ear) basis. Both parameters, PGR and NUR, were highly correlated with the plant biomass and plant N uptake accumulated at silking time, respectively. This result suggests that more vigorous plants in accumulation of biomass, in concomitance with richer N status (higher plant N stored) tend to present higher PGR and NUR around the period bracketing silking. In addition, the grain harvest index (HI) was correlated with the PGR regardless the location, hybrid, plant density and N rate factors. Furthermore, the grain HI was strongly associated with the proportion of biomass accumulated during the post-silking period, being the highest grain HI associated with the highest proportion of biomass accumulated after silking time. However, the N conversion efficiency or partitioning parameter (N harvest index) was neither correlated with the NUR nor with the post-silking N uptake. The latter confirmed the complexity of the N process and yet the lack of knowledge in the understanding of this physiological mechanism required to progress the NUE (through the increase in both N uptake and conversion efficiency) at the system-level.