ROOT MORPHOLOGY OF MAIZE UNDER HOMOGENEOUS OR SPATIALLY SEPARATED SUPPLY OF AMMONIUM AND NITRATE AT 3 CONCENTRATION RATIOS

Maize (Zea mays L. cv. Anjou 256) seedlings were grown hydroponically for 10 d in a split-root system (3 mM N; pH 5.5) under either a homoge neous supply (HS) or a simultaneous, but spatially separated supply (S S) of NH4+ and NO3-. Treatments comprised three NH4+:NO3- ratios (1:4, 1:1, 4:1). Shoot dry matter and various root traits (dry matter, numb er of laterals, length of main axes, total root length and total root surface area) were determined. For all NH4+:NO3- ratios, shoot dry mat ter, root dry matter, total root length, and root surface area, were g reater under HS than under SS. Under both SS and HS, increasing NH4+:N O3- ratios resulted in decreased shoot and root dry matter production, but did not alter the shoot:root dry matter ratio. Under SS, root dry matter, root length, and root surface area was greater on the NO3--fe rtilized side than on the NH4+-fertilized side. The allocation of root dry matter, root length, and root surface area to the NH4+ or NO3- co mpartments was unaffected by changes in the NH4+:NO3- ratio. Enhanced NH4+ nutrition has detrimental effects on top growth, but roots are ap parently unable to avoid excessive NH4+ uptake by proliferating in zon es where NO3- is the only form of N.