Ethephon-Induced Gummosis in Sour Cherry (Prunus cerasus L.)

Abstract

Flow of sour cherry (Prwuns cerasus L. cv. Montmorency) gum solutions through a glass capillary was Newtonian for pressure gradients from 0 to 1.8 megapascals per meter, and hydraulic conductance was inversely proportional to solution viscosity in this range. However, flow became plastic at pressure gradients above 1.8 megapascals per meter, resulting in a decrease in solution viscosity. The magnitude of this effect diminished as gum concentration increased. Flow of water, a solution of the component sugar monomers of sour cherry gum, and sucrose solutions remained Newtonian over the entire pressure gradient range examined (0-4 megapascals per meter). Plastic flow of gum solutions in the vessels of intact sour cherry shoots is possible under pressure gradients induced by transpiration when high resistance to flow occurs over short distances.Sour cherry gum is a high mol wt, hydrophilic, weakly acidic arabinogalactan polysaccharide (19). Gum synthesis and accumulation in Prunus is promoted by stress and wounding (2, 3, 6, 7) or more directly by ethylene (1, 10). The gum formed can enter the xylem vessels and interfere with flow through the vessels both by increasing vessel sap viscosity and by occlusion of vessels (14).Laminar flow through capillaries is described by the Hagen- (5,8,11,16,17) and a summary is provided in Figure 1 The objective of this study was to establish the effect of gum concentration and applied pressure gradient on the flow characteristics of aqueous sour cherry gum solutions, and to relate these effects to conduction of sap through xylem vessels of sour cherry shoots.MATERIALS AND METHODS Test Solutions. Clear, nonpigmented gum exudates were collected from branches of mature sour cherry Prunus cerasus L. cv. Montmorency) trees and stored frozen until needed. Gum exudation was induced by a foliar spray of ethephon, (2-chloroethyl)phosphonic acid, at concentrations up to 69.2 mm. Gum solutions of 1.90o and 3.81% (w/v) were prepared by dissolving gum in warm, deionized, distilled H20. The solutions were then centrifuged to remove debris and stored at 4°C. Gum concentration was determined from the residue dry weight of a 2-ml aliquot of solution. Gum solutions of 1.09% and 1.91% were prepared by dilution of the 3.8 1% solution. Solution density was determined at 25°C with a glass pycnometer (25 ml) and kinematic viscosity with an Ostwald viscometer at the same temperature. Kinematic viscosity was multiplied by solution density to obtain solution viscosity. Deionized, distilled H20, solutions of 44% and 66% (w/ v) sucrose, and a solution of the component sugar monomers of sour cherry gum at concentrations equivalent to a gum solution of 1.90o were also prepared for comparison with gum solutions. The composition of the solution of gum monomers was based on previous gas-liquid chromatographic assay (13) of the monomer composition of exuded 'Montmorency' sour cherry gum. The gum contains 55.0o (w/w) L-Ara, 32% D-Gal, 6.3% D-Man, 6.1% DXyl, and 0.6% D-Glu. Concentrations, densities, and...

Ethephon-Induced Abscission of “Redhaven” Peach

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