Crown roots dominate nitrate uptake in maize (Zea mays L.) under varying soil moisture conditions

Nitrate (NO3-) uptake is primarily driven by soil water flow and varies among maize root types. This study quantified NO3- uptake by embryonic (primary and seminal) and crown roots under contrasting soil water conditions to determine their relative contributions to total plant N acquisition during early vegetative growth.

Maize was grown in a split-root pot that segregated the embryonic and crown roots. The sandy-loam soil was moistened to water potentials of either -5 kPa (wet) or -30 kPa (relatively dry). A partial N mass balance was made by destructively sampling shoots, roots, and soils after 0, 24, and 48 h following 15N-KNO3 injection at the V3 (three-leaf) and V6 (six-leaf) stages. Root hydraulic conductance was quantified using an osmotic-driven root exudation method. Gross nitrification was assessed using a 15N isotope dilution technique.

At the V3 stage, crown roots had 202% more N uptake than embryonic roots in wet soil (-5 kPa). However, in relatively dry soil (-30 kPa), N uptake was similar for embryonic and crown roots, and crown root hydraulic conductance was 80% lower than in wet soil. By the V6 stage, crown roots dominated N uptake, with embryonic roots supplying < 20% of N uptake. Gross nitrification rates did not differ significantly between root types.

Maize NO3- uptake depends primarily on the crown roots, due to their capacity to extract water and NO3- from soil, even under dry conditions.