The behaviour of Sitobion avenae (F.) was compared on resistant wheat lines of Triticum monococcum (L.) and a susceptible variety of Triticum aestivum (L.). Firstly, stylet penetration activities were monitored with the Electrical Penetration Graph (EPG) technique and subsequently analysed using flow charts combined with correspondence analysis. Plant resistance was shown to be associated with repeated penetrations without access to either the xylem or the phloem, and with numerous failures in starting a sustained sap ingestion (as represented by pattern E2). Access to sieve elements of the phloem did not seem to be much affected on resistant plants but it took the aphid three times as long to produce a sap ingestion pattern when maintained on the resistant line T. monococcum n o 44 (Tm44) as compared with aphids maintained on susceptible plants. As a result the total time spent in ingesting from sieve elements was reduced by 72% on Tm44. Secondly, direct observations of freely-moving apterous adults were performed. Aphids did not discriminate between resistant and susceptible wheat during the first 30 min of access to test leaves, but only 4 out of 25 aphids were still probing after eight hours on resistant Tm44.The relevance of these results to possible location of the resistance factor(s) are discussed. Although detection of plant resistance before sieve elements are reached can not be rigorously excluded, the factors involved in T. monococcum resistance to S. avenae undoubtedly occur within the phloem vessels.
The development and reproduction of 60 clones of Sitobion avenae (Fabricius), collected in the Rennes Basin, were compared on a resistant Triticum monococcum (Linnaeus) line (Tm44) and a susceptible Triticum aestivum (Linnaeus) cultivar (Arminda). All clones had lower larval survival and mean fecundity when reared on Tm44 in comparison with Arminda. They all performed equally well on Arminda whereas there was a marked and continuously distributed variation in performance parameters among clones maintained on Tm44. The plant species, from which clones originated, significantly affected aphid performances on resistant Tm44. A more detailed experiment was carried out with four clones, shown to differ in their level of fitness on Tm44. Their performances were compared on resistant (Tm44 and Tm46) and susceptible (Tm47 and Arminda) wheat genotypes. On the basis of larval development time and intrinsic rate of natural increase (rm), two types of response were distinguished among the four clones: clones Sa1 and Sa39 appeared to be less affected by Tm44 and Tm46 resistance than Sar2 and Sa48. Moreover, Tm46 was much less resistant to clone Sal than was Tm44. These results are discussed in relation to the origins of the variation in performance of S. avenae on resistant wheat, and their implications in plant breeding for resistance to S. avenae.
The stylet penetration activities of Sitobion awnae F. were followed with an electronic method (DC-EPG) on resistant and susceptible wheat genotypes. Discriminant analysis was therefore applied to data evaluated on each electrical penetration graphs obtained (EPGs). The variables which mostly explained the resistance-susceptibility status of the wheat genotypes appeared to be related to the ingestion sap event (as represented by pattern E2), either to the time preceding first occurrence of a sap ingestion event or to the total time spent by the aphid ingesting sap from the sieve elements of the phloem. On the basis of their stylet penetration activities, individuals were separated into three blocks. The first one was made up the individuals tested on susceptible or slightly resistant wheat ('susceptible block'). The second one contained the individuals tested on two resistant genotypes ('resistant block 1') and the third one regrouped the individuals tested on another resistant genotype ('resistant block 2'). These results are discussed in relation to the possible mechanisms of wheat resistance to S. auenae.
A collection of 87 ancient wheat genotypes, 67 Triticum monococcum, 13 Triticum boeoticum, seven Triticum urartu and one cultivar of the modem wheat Triticum aestivum (variety 'Arminda') were evaluated for resistance to the cereal aphid Sitobion avenae, the main damaging aphid pest on winter wheat in Europe. The intrinsic rate of natural increase (r^), which is regarded as a good estimate of the fitness of an aphid population, was used as an indicator for the level of plant resistance. Differentiation ofthe 88 plant genotypes into four distinct groups was achieved with a cluster analysis of the r^ values. The modern wheat 'Arminda' was more susceptible than any of the ancient wheat genotypes tested (r^ = 0.24, i.e. the aphid population doubled every 2,6 days). A second group of 19 plants ranged from relatively susceptible to moderately resistant (0,17 < r^ < 0.21). Fifty-one plants were allocated to a third group and classified as resistant (0.09 < r^ < 0.16). The last group contained 17 genotypes with a high level of resistance where aphid fitness was greatly reduced (0,02 < rn, < 0.09, i.e. the aphid population doubled every 11.4days or 7,7 days, respectively). Clustering of the accessions into the different phenetic groups did not follow the geographical origin of the wheat genotypes or the species to which they belong. These results show that ancient diploid wheats, all characterized by the genome A, present considerable interest for plant breeding for resistance to S. avenae in modern wheat. The potential use of these strong and partial sources of resistance for introduction of a stable and durable form of resistance to S. avenae in wheat is discussed.
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