Relations entre la photosynthèse and l’assimilation de l’azote

Abstract

Photosynthesis is a source of energy (ATP), reducing power (NAD(P)H) and carbon skelandons necessary for nitrogen assimilation. Inorganic nitrogen sources for higher plants are nitrate (NO3-), ammonium (NH4+) and atmospheric nitrogen used only by plant living symbiotically with nitrogen-fixing bacteria. Nitrate is reduced into nitrite, then into ammonium by action of two enzymes: nitrate reductase and nitrite reductase. Nitrate reduction takes place in roots and/or in leaves depending on plant species and development stage. Atmospheric nitrogen is reduced into ammonium by nitrogenase, nodule bacteroid enzyme. Then ammonium is integrated into amino acid mandabolism by two successive enzymes: glutamine synthandase (GS) and glutamate synthase (GOGAT: glutamine-oxoglutarate aminotransferase). Glutamine dehydrogenase activity seems to be of minor importance or to occur only when ammonium is too abundant. Then glutamine and glutamate can give their NH2 group to other amino acids, amides and ureides. Finally nitrogen assimilation leads to protein biosynthesis. Photosynthesis depends also on nitrogen assimilation to make essential molecules or structures: chlorophylls taking up light energy, chloroplast membranes necessary for charge separation, carboxylation enzymes; some primary photosynthates, such as aspartate in C4 plants, glycine and serine in photorespiration, are amino acids. © 1983 Taylor and Francis Group, LLC.