Carbon and Nitrogen Sources for Protein Synthesis and Growth of Sugar-beet Leaves

Protein synthesis in very young leaves utilizes carbon fromphotosynthesis and from translocated sucrose, and nitrogen translocatedin both xylem and phloem. The carbon of young leaf protein isderived mainly from assimilated CO₂, while translocated sucrosecontributes proportionately more of its carbon to insolublecarbohydrate. Most protein amino-acids become labelled from¹⁴CO₂, glutamate being the notable exception. Glutamine or glutamateis synthesized from sucrose in roots, and is translocated toyoung leaves. It is suggested that a small but significant proportionof the nitrogen requirement of the young leaf is translocatedfrom roots as glutamine, in the phloem. Inorganic nitrogen istranslocated in xylem.