2017 ASHS Annual Conference

Modern tree fruit production systems depend on rootstocks to control important traits of the scion including plant size, yield and biotic and abiotic stress tolerance. In apple and cherry rootstock studies, part of the USDA NC-140 Regional Rootstock Research Project, rootstocks have been observed to affect aphid populations. Since aphids obtain their nutrition from the phloem of these plants, we explored the effect rootstocks had on the amino acid composition of phloem exudates from apple and cherry scions with the objective of identifying relationships with aphid populations. To conduct this study, phloem exudates were collected from leaves excised in 15 mM EDTA from both ‘Honeycrisp’ apple and ‘Skeena’ sweet cherry trees. Apple leaves were sampled from trees grown on 10 different rootstocks (M9, M26, G11, G41, G202, G935, B9, B7-3-150, B71-7-22, PiAu9-90) from experimental orchards in Nova Scotia, Ontario and British Columbia. Cherry leaves were sampled from trees grown on 3 different rootstocks (Gisela 3, 5, 6) and 3 training systems (Upright Fruiting Offshoots, Tall Spindle Axe, Kym Green Bush) from experimental orchards in Nova Scotia and British Columbia. Free amino acid composition of phloem exudates was analyzed using ultra high performance liquid chromatography (UHPLC) with fluorescence detection following derivatization with o-phthalaldehyde-ethanethiol-9-fluorenylmethyl chloroformate (OPA-ET-FMOC). Of the 20 amino acids analyzed, the relative abundance of 13 and 15 amino acids differed significantly among rootstocks for apple and cherry, respectively. Cherry training systems significantly affected the relative abundance of 6 amino acids. For apple, the top 6 amino acids accounted for 86% of the free amino acids and included proline (Pro), lysine (Lys), glutamate (Glu), aspartate (Asp), alanine (Ala) and valine (Val). For cherry, the top 6 amino acids accounted for 79% and included Glu, Pro, glutamine (Gln), Ala, Asp and serine (Ser). Principle component analysis (PCA) was conducted to identify differences among amino acid profiles between rootstocks. Apple rootstocks B71-7-22 and PiAu9-90, and cherry rootstock Gisela 3 were associated with higher concentrations of Pro in the phloem than other rootstocks. These apple rootstocks were also observed to have consistently low (i.e. none) aphid infestation. The impact of rootstocks on phloem amino acids and their possible impact on aphid resistance will be discussed.