Differential effects of coumarin on root growth and nitrate uptake kinetics in nitrogen-contrasting wheat germplasm lines

Abstract

Enhancing Nitrogen Use Efficiency (NUE) is critical for sustainable wheat production, yet how root exposure to allelopathic coumarins influences NUE remains poorly understood. This study tested the hypothesis that coumarin exerts dose-dependent and variety-specific effects on root growth and nitrate uptake kinetics in wheat. Two wheat lines, pre-characterized as nitrogen-efficient (90003) and nitrogen-inefficient (90002), were pre-cultured for five days and then exposed to a factorial combination of coumarin concentrations (0, 25 μM, 0.1 mM, and 1 mM) under nitrate-induced (50 μM KNO₃) conditions. After a 24-hour exposure, the relative growth rate for root length (RGRrl), net nitrate uptake kinetics (via depletion assay), and tissue nitrate concentrations were quantified. Coumarin significantly inhibited RGRrl in a dose-dependent manner, with the strongest suppression at 0.1 mM, where RGRrl decreased by 45% and 55% in the efficient and inefficient lines, respectively, relative to controls. The N-efficient line consistently maintained higher RGRrl across all concentrations. Strikingly, at 0.1 mM coumarin, the net nitrate uptake rate was enhanced approximately 3.7-fold in the efficient line compared to controls, while the inefficient line showed a diminished response. Coumarin accelerated the induction rate constant (K1) and reduced the half-induction time (t1/2 K1) while delaying system inhibition (increased t1/2 K2), effects that were more pronounced in the N-efficient line. Furthermore, coumarin shifted nitrate partitioning from roots to shoots in a concentration-dependent manner. We conclude that coumarin acts not merely as an inhibitor but as a dose-dependent modulator of nitrogen acquisition, with 0.1 mM representing an optimal stimulatory concentration for uptake kinetics. Our physiological evidence suggests that coumarin may modulate the inducible high-affinity nitrate transport system (iHATS), although direct confirmation of transporter gene expression is required. We propose that the nitrogen-efficient line demonstrates superior physiological resilience to coumarin, potentially linking NUE with tolerance to allelochemical stress.