Maarten van HELDEN-1, Dick van der WAL-2, Teris A. van BEEK-2 &
W. Freddy TJALLINGII-1
Wageningen Agricultural University, 1-Department of Entomology
(P.O.Box 8031, 6700 EH Wageningen) and 2-Department of Organic Chemistry
(Phytochemical section, Dreijenplein 8, 6703 HB Wageningen), the Netherlands.
Email:Maarten.vanhelden@medew.ento.wau.nl
The absolute resistance of lettuce (Lactuca sativa L.) to the
monophagous aphid Nasonovia ribisnign (Mosley) (Homoptera,
Aphididae) is based on a single dominant gene (Nr gene). The
analysis of aphid feeding behaviour using electrical penetration graphs
showed that the aphids do reach the phloem sap on the resistant plants,
but no ingestion occurs. Therefore it was conluded that the resistance is
based on a chemical or mechanical difference of the phloem between
resistant and susceptible plants.
Phloem sap can be collected by amputation of aphid stylets, EDTA
facilitated exudation or, bioprocessed, as honeydew. Chemical comparison
of phloem sap samples for sugars, amino acids, proteins and major UV
absorbing secondary plant compounds showed no consistent differences
between near-isogenic resistant and susceptible lettuce lines.
Therefore, a bioassay using EDTA collected phloem sap was developed. The EDTA was removed and the remaining extract was dissolved in artificial diet and offered to tne aphids in a choice situation. N. ribisnign was able to distinguish between resistant and susceptible phloem sap. showing that there is indeed a chemical difference.
This choice test is now used in a bioassay guided fractionation to isolate the ailomones involved in the resistance. We are now approaching the isolation and identification of individual compounds which show a clear biological activity. It is possible that active components are involved in both recognition of lettuce as a nost plant as well as the deterrence of the aphids from the resistant plants.