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Doctor of Philosophy




A series of field, greenhouse, and growth chamber experiments were conducted to study the physiological and practical implications of different forms of nitrogen nutrition;Nutrient uptake by corn plants is strongly regulated toward a balanced cation-anion uptake. The form of nitrogen absorbed clearly affects the uptake of other ions, and similarly, the availability of other nutrients influences the NO(,3)-N/NH(,4)-N ratio absorbed. Moreover, when nitrogen is supplied as an unbalanced NO(,3)-N/NH(,4)-N ratio, corn plants favor absorption of the less abundant nitrogen form;Cytoplasmic pH is not affected by the form of nitrogen and it does not seem to be involved in the cation-anion balance effect. Organic acid synthesis or breakdown may directly respond to H('+) or OH('-) efflux during unbalanced cation-anion uptake, without changes in cytoplasmic pH. Moreover, excess anion uptake, for example, would generate electropotential differences that would reduce anion uptake and stimulate cation uptake while electrochemical gradients would favor leaking of those anions absorbed in excess. Nitrogen assimilation into organic compounds in the roots would produce H('+) or OH('-) (for NH(,4)('+) and NO(,3)('-), respectively) that would be exchanged for further nitrogen uptake. The combination of these mechanisms together with the effect of the external pH on nutrient uptake would explain most of the results obtained in this work;Nitrate retarded the acidification of the cytoplasm during development of anaerobic conditions. Part of the NO(,2)('-) generated under these conditions is extruded to the rhizosphere. This could have important implications in denitrification due to the higher reactivity of NO(,2)('-) in the soil;Under field conditions, the form of nitrogen has different implications. By using NH(,4)('+) plus a nitrification inhibitor or by splitting the applications, the efficiency of nitrogen fertilizer can be significantly improved given that losses of nitrogen by leaching are important.



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Fernando Hector Andrade



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131 pages