Spatiotemporal variation of nitrate uptake kinetics within the maize (Zea mays L.) root system is associated with greater nitrate uptake and interactions with architectural phenes

Increasing maize nitrogen acquisition efficiency is a major goal for the 21st century. Nitrate uptake kinetics (NUK) are defined by Imax and Km, which denote the maximum uptake rate and the affinity of transporters, respectively. Because NUK have been studied predominantly at the molecular and whole-root system levels, little is known about the functional importance of NUK variation within root systems. A novel method was created to measure NUK of root segments that demonstrated variation in NUK among root classes (seminal, lateral, crown, and brace). Imax varied among root class, plant age, and nitrate deprivation combinations, but was most affected by plant age, which increased Imax, and nitrate deprivation time, which decreased Imax. Km was greatest for crown roots. The functional-structural simulation SimRoot was used for sensitivity analysis of plant growth to root segment Imax and Km, as well as to test interactions of Imax with root system architectural phenes. Simulated plant growth was more sensitive to Imax than Km, and reached an asymptote near the maximum Imax observed in the empirical studies. Increasing the Imax of lateral roots had the largest effect on shoot growth. Additive effects of Imax and architectural phenes on nitrate uptake were observed. Empirically, only lateral root tips aged 20 d operated at the maximum Imax, and simulations demonstrated that increasing all seminal and lateral classes to this maximum rate could increase plant growth by as much as 26%. Therefore, optimizing Imax for all maize root classes merits attention as a promising breeding goal.