Chemical Composition of Bleeding Xylem Sap from Kiwifruit Vines

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Abstract
A study of the chemical composition and charge balance was made of bleeding xylem sap collected from excised one-year-old extension shoots of healthy, Mn-deficient, Mn-toxic and Zn-deficient kiwifruit vines (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson) immediately prior to leafburst. The exudates were analysed for macronutrient cations and anions, trace elements, amino acids, organic acids and sugars. Major charged species measured were Ca (13.3 mM), K (8.9 mM), Mg (5.6 mM), malate (12.5 mM) and phosphate (5.8 mM). Glutamine (12 mM) was the predominant N carrier identified, accounting for 58 per cent of the total N followed by NO2−-N (4.5 per cent), NH4+-N (3.5 per cent) and arginine-N (2.9 per cent). Approximately 22 per cent of the N was in a hydrolysable proteinaceous fraction comprising mainly glutamine and glutamate. Eighteen free proteinaceous amino acids were idetified in sap, the most abundant being glutamine, glutamic acid, valine, isoleucine and phenylalanine. Computer simulation of the chemical composition predicted that in addition to hydrated cations, ion pairs formed between inorganic components (SO42−, HPO42−, H2PO4−) and cations (Ca2+, Mg2+, Mn2+), plus metal-organic ligand complexes (Ca Malate, Zn Malate, FeCit, CuHis, CuGln) are important species involved in translocation. The solubility product of hydroxyapatite was exceeded in all exudates although in vitro precipitation was not observed. To achieve electroneutrality with the components measured, however, formation of precipitate precursors (hydroxyapatite nuclei) had to be assumed. Irregularities in Mn nutrition (but not Zn) were clearly indicated by the elemental composition of exudate suggesting the use of sap analysis as a possible pre-season indicator of nutritional status for this species.

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