Glutamine synthetase and nitrate assimilation in sorghum (Sorghum vulgare) leaves

The patterns of nitrate reduction, nitrate accumulation, and glutamine synthetase activity as a function of leaf development were studied in glasshouse-grown sorghum (Sorghum vulgare L. var. Dourado) seedlings. Leaf nitrate reductase activity increased during early leaf development and reached a maximum at full leaf expansion which was followed by a decline with leaf maturation. Leaf nitrate content closely paralleled the pattern of nitrate reductase activity for each individual leaf along the canopy. Glutamine synthetase activity increased during early leaf development but then remained constant until senescence. Etiolated leaves from nitrate-grown plants had minimal levels of nitrate reductase activity which increased markedly upon illumination. Glutamine synthetase activity was already high in those leaves and increased only slightly during greening. Crude preparations of glutamine synthetase were insensitive to cyanide and both the biosynthetic and transferase assays were enhanced by cysteine. The relatively high activities of glutamine synthetase throughout leaf development and greening indicate that this enzyme does not limit the assimilation of nitrate into amino acids and plays a role in the assimilation of ammonia from sources other than NO3− reduction.