Root Growth Patterns in Relation to Nitrogen Availability

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Nitrogen is the mineral element that plants require in greatest quantity and its availability determines to a large extent the productivity of both natural and cultivated ecosystems. The two mineral forms of nitrogen accessible to plants are ammonium and nitrate. Little is known abut the relative importance of ammonium and nitrate as nitrogen sources under field conditions, in part, because of the methodological limitations. Nitrogen has no long-lived radioisotope. Use of the stable isotope 15N-the most common approach for measuring plant absorption from soils-has major disadvantages. The proposed research shall develop new field methods for studying plant nitrogen absorption utilizing methylamine, a chemical analog for ammonium, and chlorate, a chemical analog for nitrate. These analogs have several advantages. They are not readily metabolized in the soil. They can be used at low concentrations because they have long-lived, radioactive forms, 14C-methylamine and 36Cl-chlorate. Moreover, use of the radioactive forms can separate ammonium from nitrate absorption. The objective is to design a procedure which uses only small amounts of radioactive compounds and ensures that virtually no radioactive material is left in the field. Field experiments using these techniques will access seasonal and spatial changes in the availability of ammonium and nitrate. The distribution of tomato roots through the soil should indicate whether roots develop special branching patterns to optimize the absorption of ammonium or nitrate. Tomato mutants that have diminished abilities to absorb ammonium or nitrate or unusual root branching patterns have been isolated. Differences among the nitrogen accumulation in these mutants and that in normal tomatoes should reflect the relative importance of plant physiological mechanisms and root morphology on ammonium or nitrate absorption from soils. This research brings together innovative techniques and unique genetic material to examine nitrogen acquisition under realistic conditions. This information should stimulate the development of cultivars and agricultural practices that increase the efficiency of nitrogen fertilizer use. Because the production of nitrogen fertilizer expends from 1/3 to 1/2 of the total energy inputs in modern agriculture, increased nitrogen fertilizer efficiency would dramatically reduce the energy requirements of agriculture.