Abstract
Summary The exudation of phloem sap from the cut hypocotyl of Ricinus seedlings was increased two-fold by incubation of the cotyledons in phosphate buffer. The analysis of osmotic potential differences revealed that this stimulation of the exudation rate was due to a five-fold increase in sieve tube conductance. Parallel measurements of sieve-tube metabolites showed that the observed changes of UDP-glucose- and ATPlevels were probably not causal for the conductance increase. The levels of calcium ions, both bound and free, increased strongly during phosphate incubation and at the same time the pH of sieve-tube sap decreased to pH 5.7 Since a similar pH-shift had been observed in springtime phloem flow of trees, concomitant with removal of dormancy callose, it is suggested that a flush of phosphate transport is responsible for the latter.
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