ANALYSIS OF WHEAT RESISTANCE TO THE CEREAL APHID SITOBION-AVENAE USING ELECTRICAL PENETRATION GRAPHS AND FLOW-CHARTS COMBINED WITH CORRESPONDENCE-ANALYSIS

The behaviour of Sitobion avenae (F.) was compared on resistant wheat lines of Triticum monococcum (L.) and a susceptible variety of Triticu m aestivum (L.). Firstly, stylet penetration activities were monitored with the Electrical Penetration Graph (EPG) technique and subsequentl y analysed using flow charts combined with correspondence analysis. Pl ant resistance was shown to be associated with repeated penetrations w ithout access to either the xylem or the phloem, and with numerous fai lures in starting a sustained sap ingestion (as represented by pattern E2). Access to sieve elements of the phloem did not seem to be much a ffected on resistant plants but it took the aphid three times as long to produce a sap ingestion pattern when maintained on the resistant li ne T. monococcum n degrees 44 (Tm44) as compared with aphids maintaine d on susceptible plants. As a result the total time spent in ingesting from sieve elements was reduced by 72% on Tm44. Secondly, direct obse rvations of freely-moving apterous adults were performed. Aphids did n ot discriminate between resistant and susceptible wheat during the fir st 30 min of access to test leaves, but only 4 out of 25 aphids were s till probing after eight hours on resistant Tm44. The relevance of the se results to possible location of the resistance factor(s) are discus sed. Although detection of plant resistance before sieve elements are reached can not be rigorously excluded, the factors involved in T. mon ococcum resistance to S. avenae undoubtedly occur within the phloem ve ssels.