THE INFLUENCE OF NITROGEN CONCENTRATION AND AMMONIUM-NITRATE RATIO ON N-UPTAKE, MINERAL-COMPOSITION AND YIELD OF CITRUS

In short-term water culture experiments with different N-15 labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH4+ at a higher rate than NO3-. Maximum NO3- uptake by the whole plant occurred at 120 mg L-1 NO3--N, whereas NH4+ absorption was saturated at 240 mg L-1 NH4+-N. (NH4+)-N-15 accumulated in roots and to a lesser degree in both leaves and stems. However, (NO3-)-N-15 was mostly partitioned between leaves and roots. Adding increasing amounts of unlabeled NH4+ (15-60 mg L-1 N) to nutrient solutions containing 120 mg L-1 N as N-15 labeled nitrate reduced (NO3-)-N-15 uptake. Maximum inhibition of (NO3-)-N-15 uptake was about 55% at 2.14 mM NH4+ (30 mg L-1 NH4+-N) and it did not increase any further at higher NH4+ proportions. In a long-term experiment, the effects of concentration and source of added N (NO3- or NH4+) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH4+ versus NO3- nutrition whereas the reverse was true for Ca, K, Zn and Mn. The effects of different NO3--N:NH4+-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L-1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO3- or NH4+ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO3--N/NH4+-N ratio. With increasing proportions of NH4+ in the N supply, leaf nutrients such as P. Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH4+ although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO3--N:NH4+-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity , and colour index of fruits decreased with increasing NH4+ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO3-:NH4+ ratio.