Nitrogen metabolism of Picea glauca. I. Seasonal changes of free amino acids in buds, shoot apices, and leaves, and the metabolism of uniformly labelled 14C-L-arginine by buds during the onset of dormancy

Buds, shoot apices, and leaves from terminal shoots of white spruce saplings accumulated high levels of alcohol-soluble nitrogen in spring, late summer, and early winter. Major components, e.g. arginine, glutamine and proline, of the soluble nitrogen showed patterns complementary to each other. These changes represented the storage and mobilization of nitrogenous compounds during the onset of dormancy or the growth of shoots. Leaves contained less total soluble nitrogen than buds or shoot apices. Soluble nitrogen and arginine content of leaves resembled buds in their seasonal patterns but changes in aspartic acid, glutamic acid, and alanine were much greater than in buds, especially in late summer.When the first frost appeared, uniformly labelled 14C-arginine, applied to the apices of buds, readily entered newly synthesized protein, and free arginine was converted to proline via ornithine. Proline with carbon derived from arginine also entered proteins that were metabolized at different rates. A fraction of the proline in protein was hydroxylated to hydroxyproline. Although traces of 14C-citrulline were detected, more carbon was metabolized to free guanidino compounds, e.g. α-keto-δ-guanidinovaleric acid, γ-guanidinobutyric acid, and several monosubstituted guanidines. After 24 hours, labelled arginine, proline, and γ-guanidinobutyric acid moved down the shoot to the leaves. These metabolic changes in buds show that many of the seasonal changes in amino acids are intimately related to the carbon and nitrogen metabolism of arginine.