NITROGEN METABOLISM OF CORN (ZEA MAYS) AS INFLUENCED BY AMMONIUM NUTRITION

In a previous publication (45) data were presented to show that large amounts of glutamine, asparagine, amino acids, and other forms of soluble nitrogen, accumulated in the sap of the tops of corn seedlings in response to the absorption of large amounts of ammonium nitrogen from culture solutions. Ammonia did not accumulate in the sap and visible injury did not result until the soluble forms of organic nitrogen had reached relatively high levels. The pH of the sap, the total and reducing sugars, and the "true protein" nitrogen remained relatively stable during the period of ammonium absorption. It is the purpose of this paper to present data for the roots and tops in greater detail showing the probable site of the metabolism of the absorbed ammonium and further information on the nature of the soluble nitrogen compounds synthesized. No attempt will be made to review the extensive literature on the nitrogen metabolism of plants since several excellent reviews are available (2, 20, 46), but attention will be confined to that dealing with the nitrogen nutrition of corn insofar as it is related to this investigation. In spite of the great economic importance of corn surprisingly little has been reported concerning its nitrogen metabolism. As early as 1868 BOUSSINGAULT (cited by CHIBNALL (2)) reported that asparagine was formed in corn germinated in the dark. More modern reports are those of PRIANISCHNIKOW (26) who showed the necessity of carbohydrates for the formation of asparagine in corn and JODIDI (13) who crystallized asparagine from etiolated corn seedlings. KLEIN and TAUBOCK (16) demonstrated that corn in sterile culture solutions would absorb arginine and would decompose it completely within the plant without urea being one of the products that accumulated. Alkaloids are rarely found in the monocotyledons. Hydrocyanic acid and eyanogenic glycosides rarely occur in corn and when present occur only in traces according to a review of the literature by VAN DER WALT (36). Tests at various times in this laboratory on maturing corn have failed to indicate even traces of cyanide nitrogen. MEvius (19) found that corn supplied with ammonium salts grew well if the culture solution pH was maintained between 3.5 to 7.0, but that above this range the internal reaction became alkaline, excessive quantities of ammonia were absorbed, and growth was depressed. 1Published with the approval of the Director of the South Dakota Agricultural Experiment Station as Journal Paper no. 201.