Tissue location of the enzymes involved in nitrate and ammonia nitrogen assimilation in lupin nodules

The nodules of flowering lupin (Lupinus luteus L.) plants transferred for 5 days to media containing nitrate at various concentrations (0, 11.2, and 2 2.5 mM) were found to possess a low nitrate accumulation capacity compared to that of the root, stem, and leaves of the same plants. Most of the nitra te entering the nodules appeared to be located in their cortex. It has been established that a low potential NR activity is inherent to this nodule zo ne while the bacteroid-containing zone of the nodules has the lowest nitrat e content. These data provided evidence that the nodules cannot utilize the overall capacity for nitrate assimilation. This conclusion was confirmed b y the results regarding the activities of glutamine synthetase, glutamate d ehydrogenase, and asparagine synthetase assayed in both tissue zones of the nodules. The evidence obtained suggests that some structural or metabolic barriers constrain the nitrate entrance into the nodules and its assimilati on.