INFLUENCE OF AMMONIUM AND NITRATE ON THE PROTEIN- AND FREE AMINO ACIDS IN SHOOTS OF WHEAT SEEDLINGS

WEISSMAN, GERARD S. (Rutgers U., Camden, N. J.) Influience of ammonium and nitrate on the proteinand amino acids in shoots of wheat seedlings. Amer. Jour. Bot. 46(5): 339-346. 1959.-Total and protein nitrogen per shoot of wheat seedlings grown with endosperm attached increased at a steady rate during a 96-hr. growth period, and protein nitrogen, as a percentage of total nitrogen, remained constant at about 53%. Total and protein nitrogen concentration was greatest for 24-hr. shoots and declined as the shoots became older. Total and protein nitrogen were determined in 96-hr. shoots of seedlings grown with endosperm attached but also supplied with ammonium, nitrate, or both in the culture solution. Total nitrogen was greatest in shoots supplied with ammonium, but only 38% was in the form of protein. Maximum protein synthesis occurred in shoots grown in both ammonium and nitrate and protein nitrogen as a percentage of total nitrogen approximated that achieved in shoots lacking nitrogen in the culture solution. The protein amino acid composition of 48-, 72-, and 96-hr. shoots was very similar but differed from 24-hr. shoots which contained higher percentages of arginine and lysine and lower percentages of alanine and threonine. This may be correlated with the higher proportion of meristematic cells in 24-hr. shoots. The protein amino acids in shoots grown with ammonium resembled that of shoots lacking nitrogen in the culture solution, but nitrate shoot protein contained a higher percentage of arginine and a lower percentage of lysine. Nitrate may stimulate the formation of enzymes, possibly of a nitrate-reducing system, with high argininelow lysine content. Free asparagine and glutamine were both at a maximum in ammonium shoots and at a minimum in nitrate shoots, but asparagine predominated in shoots supplied with ammonium while glutamine was greatest in nitrate shoots. Aspartic acid, asparagine, and glutamine appeared to have ammonia-storage functions, but glutamic acid appeared to be primarily concerned with protein synthesis. Amino acid accumulation was greatest in shoots supplied with both ammonium and nitrate. Protein synthesis in these appeared to be limited by inadequate concentrations of glutamic acid and proline. A hypothesis is proposed in explanation of the high glutamic acid concentration in shoots provided with ammonium and nitrate. MANY INVESTIGATORS have noted the different effects of ammonium and nitrate supply on the growth and chemical composition of plants (Clark, 1936; Vickery et al., 1940; Sideris and Young, 1946; Gilbert et al., 1951). Previous work by the author (Weissman, 1951) indicated that wheat seedlings grown for 4 days in darkness and supplied with both ammonium and nitrate nitrogen in the culture solution produced a level of protein higher than that achieved by seedlings receiving either ammonium or nitrate alone. It was suggested that the limited protein synthesis in shoots supplied with ammonium and nitrate was due to the absence of the proper proportions of free amino acids. In the present investigation, a quantitative study has been made of the free amino acids in the shoots of 96-hr. seedlings grown with varying levels of ammonium and nitrate in order to determine whether any amino acids might be limiting protein synthesis. A proper interpretation of the role played by the free amino acid pool in the synthesis of protein would require some information on the quantity of specific 1Received for publication September 22, 1958. This investigation was conducted with the aid of research grant A-200 provided by the Research Council of Rutgers University. amino acids in the shoot protein. In addition, examination of the protein amino acids in wheat shoots would afford an opportunity for comparison with the data of Lugg (1949) and Yemm and Folkes (1953) on the mixed protein of other Gramineae. Accordingly, a quantitative study was made of the amino acid content of the mixed protein of 96-hr. wheat shoots grown with varying amounts of ammonium and nitrate. In the present study, protein in the wheat shoots has its origin in the nitrogen derived from both the culture solution and the endosperm. In order to understand the influence of exogenous ammonium and nitrate on the protein amino acids of the shoots, information on the amino acid content of the protein in the shoots of seedlings provided with no nitrogen source other than that of the attached endosperm is necessary. Therefore, an examination was made of the protein amino acids of 96-hr. shoots grown in solutions lacking nitrogen. A similar examination was made on the shoots of seedlings grown for 24, 48, and 72 hr. in solutions lacking nitrogen, in order to determine whether any changes occurred in the amino acid composition of the mixed protein during these stages of development. MATERIALS AND METHODS.-The culture methods