Physiological Responses of Barely Leaves to Foliar Applied Urea‐Ammonium Nitrate1

Abstract

Foliar applied supplemental N could be a convenient and rapid method to enhance plant growth and increase seed yield and protein content. The magnitude and duration of physiological responses in greenhouse‐grown barley (Hordeum vulgare L., ‘Scio’) leaves to foliar applied urea‐ammonium nitrate (UAN) were examined by determining changes in leaf content of urea, ammonium, sugars, amino acids, soluble protein, and chlorophyll, and activities of nitrate reductase (NR), urease, glutamine synthetase (GS), and glutamate synthase (GOGAT). Within 4 h of spraying 30 g (N) kg−1 fresh weight, seedling leaves contained 44‐ and 8‐fold more urea and ammonia, respectively, than the control with a consistent increase of 17‐ and 28‐fold in the in vivo activity of urease and NR. The increases declined to control levels within 4 days. Leaf amino acid, soluble protein, and chlorophyll content were significantly increased, and the activity of GS and GOGAT was stimulated by the UAN spray. Three weekly UAN applications to 12‐week‐old plants resulted in greater total plant weight, leaf area, and leaf chlorophyll content. Leaves of mature greenhouse‐grown plants responded similarly. An additional UAN‐spray one week after anthesis significantly increased seed protein to 11% greater than that of controls. Experimental results indicated that applied UAN provided substrates for the synthesis of amino acids, chlorophyll, and soluble protein by increased enzyme activity. Induction of enzyme synthesis by substrates was indicated but not verified.