The Effect of Changes in the Diffusion Potential of the Xylem Water on Sieve-tube Exudation from Isolated Stem Segments

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Abstract
14CO2, was supplied to rooted leafy cuttings of Salix viminalis. Several hours after activity had reached a point on the stem 50 cm below the leafy shoots (indicated by activity being detectable in honeydew collected from a colony of the aphid Tuberolachnus salignus) the stem was severed at this point, and also some 10 cm below the shoots. This gave a segment approximately 40 cm in length, in which there was a gradient of 14C activity, this being highest at the apical end, and lowest at the basal end. An aphid colony was then sited at the low-activity end of the segment and the effect of changes in the diffusion potential of the xylem water on the specific activity of honeydew from this colony was investigated. It was shown that gradients of hydrostatic pressure affected the specific activity. With the colony at the low-pressure end of the segment the specific activity increased, the converse being found with the colony at the high-pressure end. Uniform changes in diffusion potential also produced changes in specific activity: an increase in hydrostatic pressure causing a rise, and a lowering of the diffusion potential, using an osmoticum, a fall. These changes in specific activity are interpreted as being due to changes in the length of segment supplying solutes to the aphid colony.

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