Studies in Sieve-tube Exudation through Aphid Mouth-parts: The Effects of Light and Girdling1

Abstract

Exudation from the mouth-parts of the willow aphid Tuberolachnus salignus (Gmelin) inserted in the phloem of Salix sp. was studied. Leafy rooted cuttings 80–100 cm. in length were grown in artificial light. Exuding mouth-parts were located on the stem below the crown of leaves and the rates of exudation and concentration of sucrose in the exudate measured whilst the leaves were alternately illuminated (600–800 f.c.) and darkened. In darkness the rate of sap and sucrose exudation increased and the concentration fell, relative to the values in the light. Similar effects were produced by girdling the stem just beneath the crown of leaves. These were shown to be secondary effects due to a lessening of transpirational tensions during darkness or after girdling. They were eliminated if changes in water tension were avoided. Direct effects of light and girdling on exudation were demonstrated by allowing the leaves to assimilate ¹⁴CO₂ and measuring the specific activity of the exudate. These changes in radioactivity seen against the constancy in total sugar exudation demonstrate a switch in source from leaf to stem. An attempt is made to explain the pattern of changes in specific activity of the exudate during light and dark periods in terms of shifts in the location of contributing sources. The rate of translocation between two aphid colonies situated a measured distance apart was estimated by noting the times when the honeydew from each reached a certain level of radioactivity. A mean figure of 28 cm./hr. was obtained.