Epicuticular wax ester and triacylglycerol composition in relation to aphid infestation and resistance in red raspberry ( Rubus idaeus L.)

Epicuticular waxes from two cultivars of red raspberry ( Rubus idaeus) were collected from the newly emerging crown leaves, and also from the group of four more mature leaves immediately below the crown. One cultivar, Autumn Bliss, was identified as aphid-resistant, and the other, Malling Jewel, as aphid-susceptible following bioassay with the large raspberry aphid, Amphorophora idaei, just prior to collection of the wax. Biological activity was primarily associated with the more mature leaves. Epicuticular wax esters consisted predominantly of long-chain aliphatic compounds in which even-carbon-number acids were esterified to even-carbon-number alcohols. Lesser amounts of odd-carbon-number esters were also present. The acid : alcohol combinations of the major esters were C 38: 14 : 24, 16 : 22, 20 : 18; C 40: 14 : 26, 16 : 24, 18 : 22, 20 : 20; C 42: 16 : 26, 20 : 22; 22 : 20, C 44: 20 : 24, 22 : 22, 24 : 20; C 46: 20 : 26, 22 : 24, 24 : 22; C 48: 20 : 28, 22 : 26, 24 : 24, 26 : 22; C 50: 20 : 30, 22 : 28, 24 : 26, 26 : 24, 28 : 22 and C 52: 22 : 30, 24 : 28, 26 : 26. Terpenyl esters were also present and these consisted of α- and β-amyrin and cycloartenol esterified to C 16, C 18 and C 20 acids. Compositional differences between the more mature leaves which may relate to resistance to A. idaei were higher levels of cycloartenyl esters and α-amyryl esters in wax from the resistant cultivar Bliss. There were also differences between the cultivars in the distribution of individual alkyl esters and their component acids and alcohols. Esters with longer acid : shorter alcohol combinations were more abundant in Jewel than Bliss. There were compositional differences between leaves at the different developmental stages. Alkyl esters were more abundant and cycloartenyl esters were not detected in wax from the immature leaves. Small amounts of an unusual class of triacylglycerol were found only on leaves of the aphid-susceptible cultivar, Jewel, which had been subject to bioassay with raspberry aphid. These compounds, which have a C 6 acid at C-2 of the glycerol backbone, were derived from the aphid, and are the major component in the insect’s cornicle secretions. The triacylglycerols probably arise from the presence on the leaf surface of shed aphid skins, or by incorporation of cornicle fluid into the leaf wax. The abundance of aphid triacylglycerols on the leaf surface may provide a measure of aphid-susceptibility.