Sucrose and the organic acids, which provide carbon skeletons for amino acid synthesis, have triose phosphate as common precursor. Different biological systems fed with a nitrogen source showed an increase in amino acid synthesis in parallel with a decrease in sucrose formation. The main objective of the present research was to study the interaction between carbon and nitrogen metabolism in maize, a C-4 type plant, in order to compare with the results previously published on wheat leaves. Leaves detached from two weeks N-limited seedlings were fed high nitrate concentrations by the transpiration stream. Nitrate uptake and assimilation was proportional to time and NO3- concentration, showing that detached maize leaves represent a suitable system for the present study. Sucrose synthesis and phosphoenolpyruvate carboxylase (PEPCase) activity were the main parameters analyzed. Feeding 40 mM NO3- for more than four hours decreased sucrose synthesis in leaves of both species, although in maize at a lesser extent than in wheat. In C-3 as in C-4 plants, PEPCase activity is responsible for the anaplerotic reactions, which assure the equilibrium between carbon and nitrogen metabolism. In wheat, it has been previously shown that enzyme activity is activated by nitrate. Maize leaves showed an increase of mow than 50% after one-hour uptake on 40 mM NO3-. The aim of furnishing C-13(1)-glucose to maize and wheat leaves was to identify by C-13-NMR the C-13-metabolites, particularly sucrose, synthesized within short times, and thus to compare the response of C-3 and C-4 plants to high NO3-. In wheat leaves the decrease in resonances corresponding to the direct synthesis of sucrose was more evident than in maize. No variation was noticed in the C-13 resonances of other metabolites for both types of leaves. The results are discussed in terms of competition between carbon and nitrogen metabolism upon NO3- supply in wheat and maize leaves.
|Journal||Phyton-Annales Rei Botanicae|
|Publication status||Published - 1 Jan 2000|