GROWTH AND AMMONIUM-NITRATE UPTAKE RATIO OF SPRING WHEAT CULTIVARS UNDER A HOMOGENEOUS AND A SPATIALLY SEPARATED SUPPLY OF AMMONIUM AND NITRATE

In a split-root experiment, seedlings of two German spring wheat cultivars (Remus, Star) were grown hydroponically to investigate the effect of a homogeneous and a spatially separated supply of ammonium (NH4) and nitrate (NO3) (HS and SS, respectively) on the production and distribution of dry matter, and on the uptake of NH4 and NO3, Under HS, both halves of the seminal root system were exposed to a 1:1 mixture of NH4-N and NO3-N. At SS, one half of the seminal root system was fed with NH4, while the other half received NO3. The pH of the nutrient solutions was 6.5, and the nitrogen (N) concentration was 3 mM in all compartments. Plants grown under HS produced more shoot dry matter, and showed a higher concentration of N in the shoots and a higher NH4-N: NO3-N uptake ratio, while root dry matter and the root:shoot dry matter ratio were greater at SS. Irrespective of N form localization, cv. Star clearly favoured NH4 as source of N, while cv. Remus exhibited a more balanced NH4-N: NO3-N uptake ratio. Under SS, the contribution of the compartment supplied with NH4 to seminal root dry matter production and total water consumption was significantly greater for cv. Star than for cv, Remus. A supplementary experiment showed that cv. Star had lower levels of nitrate reductase activity and lower concentrations of NO3 in the shoots than did cv. Remus.