Sensing pH and sucrose concentration (with a preference for pH values of 7.0-7.5 and sucrose concentrations of approximately 400. mmol l(-1)) enables aphids to recognise sieve tubes inside vascular bundles. However, it is still unclear how aphids find their way to the vascular bundles. Membrane potentials in the cortex of Vicia faba stems were measured along a radial transect from the epidermis to the sieve elements and there was no gradient detected that could be used by aphids to guide their stylets to the sieve elements. Additionally, aphids did not demonstrate a preference between artificial diets with low or high levels of dissolved oxygen, making it unlikely that oxygen gradients in the cortex assist orientation towards the phloem. Tracks of salivary sheaths indicate that aphids search for vascular bundles in a radial direction (perpendicular from the stem surface to the vascular bundle) with regular side punctures in a pre-programmed fashion. Optical examination and electrical penetration graph (EPG) recordings suggest that aphids (Megoura viciae) probe the vacuolar sap of cortex cells. Acidic pH (5.0-5.5) and low sucrose concentrations in vacuoles, therefore, may provoke aphids to retract their stylets and probe the next cell until a favourable cell sap composition is encountered. The importance of sucrose as a cue was demonstrated by the experimental manipulation of Ricinus communis plants that cause them to transport hexoses instead of sucrose. Aphids (Aphis fabae) ingested less phloem sap of plants transporting hexoses compared with plants transporting the normal sucrose. The proposed rejection-acceptance behaviour provides a universal plant-directed mode of how aphids orientate their stylets towards the phloem.

SUMMARY The ability of aphids to detect and find sieve tubes suggests that aphids receive cues for sieve-tube recognition by taking samples. Specific natural conditions such as pH value, sugar species and concentration, viscosity, and oxygen pressure may enable sieve-tube detection. We tested the preference of Megoura viciae and Myzus persicae for potential plant-borne orientation parameters in artificial choice-chamber systems. Both species preferred sucrose (in comparison with fructose, glucose, raffinose or sorbitol) at concentrations of 15–22.5% (over a tested range of 0–22.5%) and at approximately pH 7 (over a tested range of pH 5–8). This preference matches the composition of the sieve-tube sap of their host plants. Likewise, Rhopalosiphum padi (normally found on barley plants with sucrose in the phloem sap) and Macrosiphum euphorbiae (normally found on pumpkin plants with raffinose-family oligosaccharides in the phloem sap) showed a significant preference for sucrose. In the absence of sucrose, however, M. euphorbiae strongly preferred raffinose. No clear preference for any carbohydrate was observed for Macrosiphum rosae and Aphis pomi (both normally found on plants with various amounts of sorbitol in the phloem sap). Electrical penetration graph (EPG) measurements of M. persicae feeding on artificial diets confirmed that sieve tubes are recognized solely by a combination of carbohydrate abundance and a neutral to slightly alkaline pH.

Entomologia Experimentalis et ApplicataVolume 127, Issue 3 p. 237-245 A novel perfusion system shows that aphid feeding behaviour is altered by decrease of sieve-tube pressure Torsten Will, Torsten Will Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this authorAngela Hewer, Angela Hewer Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this authorAart J.E. Van Bel, Aart J.E. Van Bel Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this author Torsten Will, Torsten Will Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this authorAngela Hewer, Angela Hewer Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this authorAart J.E. Van Bel, Aart J.E. Van Bel Plant Cell Biology Research Group, Department of General Botany, Justus Liebig University, Senckenbergstrasse 17-21, D-35390 Giessen, GermanySearch for more papers by this author First published: 16 April 2008 https://doi.org/10.1111/j.1570-7458.2008.00687.xCitations: 20 *Correspondence: E-mail: aart.v.bel@bot1.bio.uni-giessen.de Torsten Will and Angela Hewer contributed equally to this work. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume127, Issue3June 2008Pages 237-245 RelatedInformation