A novel, repeatable, and non-destructive method to collect root xylem exudate from mature apple trees in an orchard and to compare its constituents with shoot exudate

Abstract

SUMMARYA method is described in detail to allow repeatable and non-destructive extraction of xylem exudate from the trunk of mature apple trees (Malus domestica Borkh.) by the use of reduced air pressure. The method allows extraction of sufficient uncontaminated exudate to determine and quantify constituents such as plant hormones, sugars, amino acids, and minerals. Analytical measurements from the first experimental year were conducted to determine the minimum volume of exudate required to obtain reliable values for various exudate constituents. Some constituents of root exudates differed compared to shoot exudates obtained by traditional destructive methods. Thus, shoot exudates may be less suitable if the influence of roots on the aerial parts of a tree is being studied (e.g., in relation to environmental conditions or horticultural practices such as root pruning, girdling, irrigation or fertilisation). The extraction method worked well between Spring and mid-Summer over 3 years. However, the procedure failed from the end of June, onwards. Possible reasons for this failure are discussed, and experiments which may improve lateseason sap recovery are suggested. Small modifications to the method should allow simultaneous, non-destructive extraction of root and shoot-exudates, permitting a more coherent picture of long-distance signals from root-to-shoot, and vice versa, and their co-ordinating significance.