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

In Pisum arvense, the amides and amino-acids normally suppliedto the shoot in the transpiration stream transfer carbon toprotein largely throught the amino-acids, aspartic acid (+asparagine),glutarnic acid (+glutamine), threonine, lysine, arginine, andproline. Carbon from carbon dioxide enters the protein of photosynthesizingtissues through an essentially complementary set of amino-acidsincluding glycine, alanine, serine, valine, and the aromaticamino-acids tyrosine, phenylalnine, and histidine. Young tissuesof the shoot synthesize certain amino-acids de novo by metabolismof sugars supplied from photosynthesizing leaves. Each mature leaf on a shoot contributes carbon to current synthesisof protein at the shoot apex. Sucrose accounts for more than90 per cent of the labelled carbon leaving any age of leaf whichhas been fed with ¹⁴CO₂. Upper leaves supply labelled assimilatesdirectly to the shoot apex, and the radiocarbon from these assimilatesis subsequently incorporated into a wide range of amino-acidunits of protein. The majority of the labelled assimilates exportedfrom a lower leaf move downwards to the root and nodules and,in consequence, the amino-acids and amides associated with rootmetabolism are strongly represented among the compounds eventuallylabelled in the apical region of the shoot.