Loading and source of sugars for the sieve elements in stems of willow

Uniformly labelled [(14)C]glucose was introduced into the xylem of segments of willow stem. Forty-eight hours later sieve-tube sucrose was collected via servered aphid stylets, and the distribution of radioactivity in the hexose moieties of this was compared with the distribution in those of sucrose extracted from the segment. Very little correlation was found between the two sets of values, indicating possible inversion during loading. This lack of correlation could not be attributed to contributions to the sieve-tube sucrose from pools of labelled hexoses in the segment. Further experiments, however, showed quite high degrees of correlation between sieve-tube sucrose and sucrose extracted from the wood, indicating that the latter tissue was a major source of sieve-tube sucrose. This conclusion was substantiated in experiments in which sieve-tube exudate, obtained from stem segments, was compared with exudate obtained from the isolated bark of the segment. In other experiments, stylets were established on stem segments, then on isolated pieces of bark obtained from these segments. Sucrose and potassium exudation rates fell by as much as 50% on removing the bark from the segment. It was not possible to formulate a precise figure for the contribution of the wood to stylet exudation owing to injury effects and the complexity of the experimental system. No firm evidence could be found in support of the view that sucrose is inverted during loading of sieve elements from the storage cells of the stem.