- Foliar applications of urea-ammonium nitrate (UAN) around anthesis increased seed protein content by about 40% in 1983 and 1984 in 'Scio' barley (Hordeum vulgare L.). In exploring the mode and the mechanism of this increase, agronomic and physiological studies were conducted. The late N applications did not affect yield or harvest index, but elevated the seed amino acid content and reduced the sugar content. The 40% increase of total seed protein was unevenly distributed to globulin-albumin, hordein, and glutelin fractions by 27, 67, and 40%, respectively. No new protein subunits were observed after the treatment as revealed by SDS-polyacrylamide gel electrophoresis of the protein fractions. The approximate molecular weights of the hordein subunits were 33, 35.5, 38, 40.5, 42, 44, 50, 52, 54, 57, 61, 72, and 79 kD. The physiological responses to repeated foliar-UAN sprays were monitored in young and mature leaves. The absorbed urea and ammonia were metabolized in 2 days and stimulated the endogenous activities of nitrate reductase (NR) and urease from 2- to 77-fold within hours with the higher activity of NR sustained for several days. The activities of glutamine synthetase and glutamate synthase were also increased but much less in extent. These enhanced activities and added N substrates resulted in an increase of leaf contents of amino acids, chlorophyll, soluble protein, and a small decrease of sugars. Eight hours after the UAN spray, when amino acid content was highest in sprayed-seedling leaves, the absolute weights of glutamine, asparagine, alanine, glutamate, serine, glycine, valine, and histidine were 2535, 511, 460, 373, 361, 240, 129, and 59 ng g⁻¹ fresh weight over the control, respectively. The highest increase in weight percent of the extract was found in glycine, which is different from the results of ammonia or nitrate treated leaves, indicating that the absorbed urea may enhance the photorespiratory synthesis of glycine or reduce the degradation of glycine due to the high concentration of ammonia produced by urease hydrolysis of the influxed urea.