Abstract
Protein synthesis in very young leaves utilizes carbon from photosynthesis and from translocated sucrose, and nitrogen translocated in both xylem and phloem. The carbon of young leaf protein is derived mainly from assimilated CO2, while translocated sucrose contributes proportionately more of its carbon to insoluble carbohydrate. Most protein amino-acids become labelled from 14CO2, glutamate being the notable exception. Glutamine or glutamate is synthesized from sucrose in roots, and is translocated to young leaves. It is suggested that a small but significant proportion of the nitrogen requirement of the young leaf is translocated from roots as glutamine, in the phloem. Inorganic nitrogen is translocated in xylem.