The resistance of lettuce to the aphid Nasonovia ribisnigri

The resistance of lettuce to the aphid Nasonovia ribisnigri is based on a single, dominant gene, the Nr-gene. On the resistant plant aphids died within a few days, without any honeydew production. Transfer-experiments with a short stay on a resistant plant followed by a relocation to a susceptible plant showed that no weight increase occurred on the resistant plant, but weight gain immediately resumed on a susceptible plant (Chapter 2). So no intoxication seemed to occur on the resistant plant. Behavioral observations of (tethered) aphids using Electrical Penetration Graphs (EPG) (Chapter 3) and comparison with free living aphids (Chapter 4) showed that on the resistant plant the aphid's stylets did penetrate to the phloem sieve elements. On the resistant plants hardly any E2 pattern (phloem sap ingestion) occurred and the aphid finally died of malnutrition, or left the plant (Chapter 4). It was concluded that the resistance is based on a factor in the phloem. This can be either a chemical factor like a feeding deterrent or a mechanical factor which obstructs ingestion. Experiments concentrated on chemical differences in the phloem sap of resistant and susceptible (isogenic apart from the resistance gene) lettuce lines. Amputation of aphid stylets (stylectomy) during feeding can yield small but pure phloem sap samples. However, on the resistant and susceptible isogenic lines no phloem sap samples could be collected because outflow from the stylet stump stopped after a few seconds. Larger phloem sap samples were obtained by EDTA chelation and honeydew collection (Chapter 5).Chemical analysis of the composition of the phloem sap samples showed no differences in sugars, amino acids, proteins and UV absorbing compounds (Chapter 6). Exhaustive analysis of phloem sap compounds was not feasible, not only because of the number of possible compounds, but also because the sample size and quantity was limited.In a bioassay aphids were offered a choice between EDTA collected phloem sap samples of resistant and susceptible plants, added to a complete artificial diet (chapter 7). Aphids showed a clear aversion to the extract of the resistant plant. This suggests that the resistance is based on the presence of feeding deterrents in the phloem sap of the resistant plant. Hopefully, these substances can be isolated and identified with the help of the bioassay.