Kinetics of ammonium and nitrate uptake among wild and cultivated tomatoes

Concentration dependence of net ammonium and nitrate uptake was monitored for a cultivar of tomato, Lycopersicon esculentum, and two accessions of a neotropical wild relative, L. hirsutum. The kinetics of net NH ₄⁺ uptake differed among these taxa and were not dependent on the ionic composition of the nutrient solution. The kinetics of net NO ₃^- uptake were dependent on the composition of the nutrient solution; the presence of NH ₄⁺ or Cl^- enhanced net NO ₃^- uptake for the cultivated species and for a highland accession of the wild species. The capacity for net NO ₃^- uptake was greater than the capacity for net NH ₄⁺ uptake in all three taxa; the proportion of NO ₃^- to NH ₄⁺ absorbed was much greater for the wild taxa. Our data suggest that NO ₃^- may be a more important source of mineral nitrogen than NH ₄⁺ for these tropical taxa.