Dick van der Wal-1, Maarten van Helden-2,Teris A.
van Beek-1, & Freddy Tjallingii-2
An absolute unonogenic resistance of lettuce (Lacruca sativa L.) to the aphid Nasonovia ribisnigri (Mosley) (Homoptera. Aphididae) is based on a chemical difference in the phloem sap between resistant and susceptible plants.
Phloem sap can be collected by (1) amputation of aphid styiets (2) EDTA facilitated exudation or (3) as honeydew. Using total phloem sap samples. collected by method (a), we were unable to show differences between near isogenic susceptible and resistant lettuce lines for sugars, amino acids, proteins and major UV absorbing compounds.
Therefore, a bioassay, using EDTA collected phloem sap, was developed. After the removal of the EDTA. the phloem extract was dissolved in artificial diet and offered to the aphids in a choice siltation. N. ribisnigri clearly prefers the susceptible phloem sap, possibly due to the higher concentration of a feeding deterrent in the phloem sap of the resistant plant. This bioassay is now used in a classical bioassay guided fractionation to isolate the allomones involved in the resistance. After several chemical fractionations we are now approaching the isolation and identification of individual compounds which show a clear biological activity. These compounds, probably glycosides, might be involved in both recognition of lettuce as a host plant and the deterrence of the aphids from the resistant plants.