Photosynthesizing Leaves and Nodulated Roots as Donors of Carbon to Protein of the Shoot of the Field Pea (Pisum arvense L.)

Abstract

In Pisum arvense, the amides and amino-acids normally supplied to the shoot in the transpiration stream transfer carbon to protein largely throught the amino-acids, aspartic acid (+asparagine), glutarnic acid (+glutamine), threonine, lysine, arginine, and proline. Carbon from carbon dioxide enters the protein of photosynthesizing tissues through an essentially complementary set of amino-acids including glycine, alanine, serine, valine, and the aromatic amino-acids tyrosine, phenylalnine, and histidine. Young tissues of the shoot synthesize certain amino-acids de novo by metabolism of sugars supplied from photosynthesizing leaves. Each mature leaf on a shoot contributes carbon to current synthesis of protein at the shoot apex. Sucrose accounts for more than 90 per cent of the labelled carbon leaving any age of leaf which has been fed with ¹⁴CO₂. Upper leaves supply labelled assimilates directly to the shoot apex, and the radiocarbon from these assimilates is subsequently incorporated into a wide range of amino-acid units of protein. The majority of the labelled assimilates exported from a lower leaf move downwards to the root and nodules and, in consequence, the amino-acids and amides associated with root metabolism are strongly represented among the compounds eventually labelled in the apical region of the shoot.