Sweet cherry flesh cells burst in non-random clusters along minor veins

Abstract

Sweet cherry flesh cells burst when exposed to water but they do so in clusters indicating heterogeneity with respect to osmotic concentration, which depends on proximity to a minor vein. Water plays a key role in cracking in sweet cherry fruit. Magnetic resonance imaging has previously indicated preferential partitioning of water along veins. A more negative osmotic potential along veins seems the likely explanation. Here we establish if cell bursting in mature sweet cherry fruit is also associated with the veins. Cell bursting was identified by a novel light microscope technique involving exposure of a cut fruit surface to water or to sucrose solutions. Upon exposure to water there was no bursting of skin cells but for cells of the flesh (mesocarp) bursting increased with time. When the cut surface was exposed to sucrose solutions of decreasing osmotic concentrations (increasing water potentials) the incidence of cell bursting increased from hypertonic (no bursting), to isotonic, to hypotonic. Cell bursting in the outer mesocarp occurred primarily in the vicinity of minor veins that in the inner mesocarp was primarily between radial veins. The median distance between a minor vein and a bursting cell (mean diameter 0.129 mm) was about 0.318 mm that between a radial vein and a bursting cell was about 0.497 mm. In contrast, the distance between adjacent minor veins averaged 2.57 mm, that between adjacent radial veins averaged 0.83 mm. Cell bursting tends to occur in clusters. Mapping of cell bursting indicates (1) that a seemingly uniform population of mesocarp cells actually represents a heterogeneous population with regard to their cell osmotic potentials and (2) cell bursting afflicts clusters of neighbouring cells in the vicinities of minor veins.

Keywords: Cracking; Osmotic potential; Phloem; Prunus avium; Solute potential; Water uptake.

Fig. 1

a–c Light micrographs taken from above the cut surface of a ripe ‘Fabiola’ sweet cherry fruit exposed to deionised water. White dots indicate burst cells detected by inspection of a temporal sequential of light micrographs. Bar 1 = mm. d Short period of exposure to deionised water. Inset: longer period of exposure to deionised water. Cell bursting was detected by observing differences between sequential light micrographs. Data represent means ± SE of three (main graph) and four replicates (inset). For details see text

Fig. 2

a–c Light micrographs taken from above the cut surface of ripe ‘Regina’ sweet cherry fruits exposed to sucrose solutions of decreasing osmotic concentration (less negative osmotic potential). Bar = 1 mm. d Effect of osmotic potential on the incidence of cell bursting. Vertical dashed lines indicate the osmotic potential of expressed sweet cherry juice. Cell bursting was detected by light microscopy. Cell bursting was recorded for experimental designs employing either ‘single-measures’ or ‘repeated-measures’. Data represent means ± SE of three replicates. For details see text

Fig. 3

a–d Frequency distributions of the distances between (a, b) burst cells and minor veins of the outer mesocarp or (c, d) burst cells and the radial veins of the inner mesocarp of ripe sweet cherries. The vertical dashed lines in a and c indicate the mean half-distance [‘(vein–vein)/2’] between two minor veins (a) and between two radial veins (c). b, d Same data as in a and c but redrawn as cumulative frequency distributions of log transformed distances. Note that the y-axis is plotted on a probability scale. Linearity indicates distances are log normal distributed. e Frequency distribution of minimum distances between burst cells (‘cell–cell distance’) in the outer mesocarp of ripe sweet cherries. The vertical dashed line in e indicates mean diameter of the outer mesocarp cells (‘cell diameter’) (n = 162). f Cumulative frequency distributions of log transformed minimum distances between burst mesocarp cells (○) and of mesocarp cell diameters (●)