Effect of Ammonium and Nitrate Nutrition on Protein Level and Exudate Composition

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Abstract
After removal of the cotyledons from 10-day-old sunflower plants, N was provided for 3 days in the form of ammonium chloride, ammonium chloride plus potassium nitrate, or potassium nitrate. Dry weight, total protein, protein concentration, and protein as a percentage of total N were all greater in leaves of NH4 + NO3 plants. Neither the total N, inorganic N, nor amide N of the exudates, collected after the final addition of N, appeared to be of determinative significance in the establishment of protein level. For most of the period between N applications total amino-acid N was greatest in the exudate of NH4 + NO3 plants, suggesting that this fraction may contribute to the high protein level. Also indicative of a role played by translocated amino acids in the control of leaf protein level was the low concentration of arginine in the exudate of NO3 plants and the absence of [gamma]-aminobutyric acid in NH4 plant exudate.