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 B liss, was identified as aphid-resistant, and the other, Malling Jewel, as a phid-susceptible following bioassay with the large raspberry aphid. Amphoro phora idaei, just prior to collection of the wax. Biological activity was p rimarily associated with the more mature leaves. Epicuticular wax eaters co nsisted predominantly of long-chain aliphatic compounds in which even-carbo n-number acids were esterified to even-carbon-number alcohols. Lesser amoun ts of odd-carbon-number esters were also present. The: acid : alcohol combi nations 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, 22:24 20;C-46:20:26,22 24,24:22C(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 alpha- and beta- amyrin and cycloartenol esterified to C-16 C-18 and C-20 acids. Composition al differences between the more mature leaves which may relate to resistanc e to A. idaei were higher levels of cycloartenyl esters and alpha-amyryl es ters in wax from the resistant cultivar Bliss. There were also differences between the cultivars in the distribution of individual alkyl esters and th eir component acids and alcohols. Esters with longer acid : shorter alcohol combinations were more abundant in Jewel than Bliss. There were compositio nal 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 triacylglyc erol were found only on leaves of the aphid-susceptible cultivar, Jewel, wh ich had been subject to bioassay with raspberry aphid. These compounds, whi ch have a C-6 acid at C-2 of the glycerol backbone, were derived from the a phid, and are the major component in the insect's cornicle secretions. The triacylglycerols probably arise from the presence on the leaf surface of sh ed aphid skins, or by incorporation of cornicle fluid into the leaf wax. Th e abundance of aphid triacylglycerols on the leaf surface may provide a mea sure of aphid-susceptibility. (C) 1999 Elsevier Science Ltd. All rights res erved.