Phloem transport of (14)C-labelled assimilates in Ricinus

Exudate can be obtained from incisions made in the bark of the stem of actively growing Ricinus plants. (14)C-labelled assimilates from a fed leaf are rapidly detected in the exudate. This movement was both acropetal and basipetal from the fed leaf, at rates of over 100 cm h(-1). Estimated rates within intact plants were 80-84 cm h(-1).In contrast with xylem sap obtained from the same plant, the exudate obtained had an alkaline pH (8.2), a high dry matter content (10-12.5%), high sugar content (8-10%) which was predominantly sucrose; high potassium content (60-80 mM) and low calcium content (0.5-1.0 mM).It is concluded, on the basis of the present evidence, that the exudate is a true sample of the sieve tube sap undergoing translocation.