Effect of Phloem Interruption on Endopeptidase and Aminopeptidase Activities in Flag Leaves of Field-grown Wheat

Leaf senescence and nitrogen remobilization in field grown wheat can be affected by steam-girdling (phloem interruption). The peduncle (below the ear) or the base of the flag leaf was steam-girdled 1 or 10 d after anthesis. Protein contents, free amino acids and peptide hydrolase activities were analyzed throughout the maturing period in extracts of the flag leaf lamina from treated plants and untreated controls. Phloem interruption at the leaf base initiated a rapid net degradation of proteins and an accumulation of free amino acids (mainly glutamine and proline). Azocaseinase activity at pH 5.0 increased during the first week after this treatment and declined afterwards, while the activity at pH 9.0 peaked late during senescence when most of the leaf proteins were already hydrolyzed. Aminopeptidase activities (measured with 8 different substrates) remained at the original level for about one week before they decreased. From this time course it appears possible that aminopeptidases contributed to the complete degradation of leaf proteins. Steam-girdling below the ear slightly delayed the remobilization of proteins in the flag leaf lamina, the increase in endopeptidase and the decrease in aminopeptidase activities. The contents of free amino acids remained higher than in untreated controls. The catabolism of leaf proteins and the pattern of peptide hydrolases were differently influenced by the two treatments