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 I-max and K-m, 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). I-max varied among root class, plant age, and nitrate deprivation combinations, but was most affected by plant age, which increased I-max, and nitrate deprivation time, which decreased I-max. K-m was greatest for crown roots. The functional-structural simulation SimRoot was used for sensitivity analysis of plant growth to root segment I-max and K-m, as well as to test interactions of I-max with root system architectural phenes. Simulated plant growth was more sensitive to I-max than K-m, and reached an asymptote near the maximum I-max observed in the empirical studies. Increasing the I-max of lateral roots had the largest effect on shoot growth. Additive effects of I-max and architectural phenes on nitrate uptake were observed. Empirically, only lateral root tips aged 20 d operated at the maximum I-max, 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 I-max for all maize root classes merits attention as a promising breeding goal.