Abstract
Susceptibility of sweet cherry (Prunus avium L.) fruit to rain cracking increases toward maturity and is thought to be related to increases in both tissue pressure (
Many soft and fleshy fruit crack on exposure of the fruit to surface water. Osmotic water uptake that occurs along a gradient in water potential between the surface water on the fruit and the water inside the fruit is thought to be causal. The water potential of the fruit
For a cherry grown under irrigated conditions and examined just before dawn, one might anticipate near-equilibrium water potential conditions within the fruit and so fruit (
Both lines of arguments imply that we should expect to record increases in
Although this logic would seem to offer a satisfactory conceptual framework within which to understand fruit cracking, recent measurements of
To provide a more useful understanding of fruit water relations in cherry,
Materials and Methods
Plant material.
Developing fruit of the ‘Regina’ sweet cherry of uniform size and color were sampled at regular intervals from greenhouse-grown trees grafted on ‘Gisela 5’ rootstocks (Prunus cerasus L. × Prunus canescens Bois) at the Horticultural Research Station of the Leibniz University Hannover at Sarstedt/Ruthe, Germany (lat. 52°14′ N, long. 9°49′ E). Unless otherwise specified, fruit was processed on the day of sampling. Fruit mass was determined [n = 20 (CPA 225D and BP 211D; Sartorius, Göttingen, Germany)] and fruit color quantified in the CIE 1976 (L*, a*, b*) scale as defined by the Commission Internationale de l’É-clariage (McGuire, 1992) using a chromometer (CR-200; Minolta, Osaka, Japan). To describe the change in color of the developing fruit, the hue angle was calculated as described by McGuire (1992). Three readings were taken per fruit on a total of 20 fruit per sampling date.
Measuring 
, 
, and 
using VPO.



Values of
Measuring 
using water uptake and bending assays.

Tissue water potential
For the water uptake assay, flesh discs (8 mm diameter, 2 mm high) were excised as described previously and incubated at 22 °C for 2 h in a series of aqueous sucrose solutions having concentrations ranging from 0 to 1.5 M (equivalent to 0 to 3.6 MPa at 22 °C). Control discs were incubated in silicone oil (AK 10; Wacker Chemie, Munich, Germany) to minimize water gain/loss. Except for the first sampling date, all tissue discs comprised flesh only. Discs were photographed within 5 min of the start of incubation and then again 2 h later using a digital camera (Altra 20; Olympus Europa, Hamburg, Germany). The cross-sectional areas [A (square millimeters)] of the discs were quantified by image analysis (cellˆp; Olympus Europa) at 0 h (A0h) and at 2 h (A2h) and the changes in area [ΔA (square millimeters)] calculated by the difference. The 2-h interval was chosen after a preliminary study, which established that this was sufficient to allow significant ΔA values to be recorded (C. Schumann, unpublished data). The
For the bending assay, skin strips comprising cuticle, epidermis, hypodermis, and adhering flesh were prepared using parallel razorblades. Strips were excised from the cheek of the fruit parallel to the pedicel/stylar-scar axis. Strips were 2 mm wide, 2 mm high, and 10 mm long for fruit from 27 to 55 DAFB and 17 mm long for fruit from 62 to 97 DAFB. Strips were incubated at 22 °C for 2 h in aqueous sucrose solutions of a series of concentrations from 0 to 1.5 M or in silicone oil (control). Strips were photographed within 5 min of incubation and again after 2 h. The angle of curvature [C (degrees)] was quantified by image analysis (cellˆp; Olympus Europa) at 0 h (C0h) and at 2 h (C2h). The change in curvature (ΔC) was calculated by subtracting C2h from C0h. The
Quantifying 
by CP.

Values of
Determining 
using the CPP.

A CPP as described by Steudle (1993) was used to measure the pressure of parenchyma cells of the outer flesh in the cheek region ≈200 to 400 μm below the epidermal surface. The CPP comprised a glass capillary (tip diameter 30 to 60 μm) filled with silicone oil (AS 4; Wacker Chemie, Munich, Germany) and connected to a pressure transducer (26PCGFA6D; Honeywell Sensing and Control, Golden Valley, MN). The CPP was mounted on the motorized stage of a micromanipulator positioned on a damping table. The capillary of the CPP was carefully inserted into the fruit under a horizontal microscope. Following volume correction, the peak pressure of the system was recorded and the apparatus subsequently checked for plugging and leakage artefacts as described previously (Knoche et al., 2014). As described earlier, there was no significant difference (P = 0.55, paired observation t test) in
Data analyses.
Data were subjected to linear and non-linear regression using Proc REG and Proc NLIN (SAS Version 9.1.3; SAS Institute, Cary, NC).
Results
Fruit mass and color.
Fruit mass increased between 27 and 97 DAFB following a sigmoid pattern (Fig. 1A). The transition from Stage II to Stage III development occurred at ≈55 DAFB when fruit color began to change from green to yellow and later to red as indicated by the decrease in hue angle (Fig. 1B). The maximum rate of growth averaged 0.28 g·d−1 at 66 DAFB and happened to coincide with the maximum rate of change in color.

Time course of change in mass, growth rate (inset A) and color of developing ‘Regina’ sweet cherry (B). Color was indexed by the hue angles that between 100° and 115° represent green and between 0° and 10° represent red. The vertical dashed line indicates the transition from Stage II to Stage III at ≈55 d after full bloom (DAFB).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

Time course of change in mass, growth rate (inset A) and color of developing ‘Regina’ sweet cherry (B). Color was indexed by the hue angles that between 100° and 115° represent green and between 0° and 10° represent red. The vertical dashed line indicates the transition from Stage II to Stage III at ≈55 d after full bloom (DAFB).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
Time course of change in mass, growth rate (inset A) and color of developing ‘Regina’ sweet cherry (B). Color was indexed by the hue angles that between 100° and 115° represent green and between 0° and 10° represent red. The vertical dashed line indicates the transition from Stage II to Stage III at ≈55 d after full bloom (DAFB).
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
Quantifying 
, 
, and 
by VPO.



The

(A) Time course of change in water potential of excised flesh discs (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

(A) Time course of change in water potential of excised flesh discs (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
(A) Time course of change in water potential of excised flesh discs (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
Quantifying 
using bending and water uptake assays.

The sucrose concentrations, and hence the

Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
Effect of the osmotic potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

(A) Change in tissue water potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

(A) Change in tissue water potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349
(A) Change in tissue water potential (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 139, 4; 10.21273/JASHS.139.4.349

and 
by CP and CPP.


After a transient peak at ≈41 DAFB, the values of
The changes in
Discussion
Our results establish the following: 1) the values of
Decreases in 
and 
at the onset of Stage III.


The recorded decreases in
It is interesting that the different methods used for estimating
The discrepancy between the

and 
are numerically low relative to 
.



The
This relationship has important practical consequences. A catastrophic increase in
If solute accumulation in the cell wall space balanced the decrease in
Although the general observation of decreasing
That the estimates of
Conclusions
The decrease in
Literature Cited
Bernstein, Z. & Lustig, I. 1981 A new method of firmness measurement of grape berries and other juice fruits Vitis 20 15 21
Bernstein, Z. & Lustig, I. 1985 Hydrostatic methods of measurement of firmness and turgor pressure of grape berries (Vitis vinifera L.) Sci. Hort. 25 129 136
Christensen, J.V. 1996 Rain-induced cracking of sweet cherries. Its causes and prevention, p. 297–327. In: Webster, A.D. and N.E. Looney (eds.). Cherries. CAB Intl., Wallingford, UK
Considine, J.A. & Kriedemann, P.E. 1972 Fruit splitting in grapes. Determination of the critical turgor pressure Aust. J. Agr. Res. 23 17 24
Glenn, G.M. & Poovaiah, B.W. 1989 Cuticular properties and postharvest calcium applications influence cracking of sweet cherries J. Amer. Soc. Hort. Sci. 114 781 788
Grimm, E., Peschel, S., Becker, T. & Knoche, M. 2012 Stress and strain in the sweet cherry fruit skin J. Amer. Soc. Hort. Sci. 137 383 390
Knoche, M., Beyer, M., Peschel, S., Oparlakov, B. & Bukovac, M.J. 2004 Changes in strain and deposition of cuticle in developing sweet cherry fruit Physiol. Plant. 120 667 677
Knoche, M., Grimm, E. & Schlegel, H. 2014 Mature sweet cherries have low turgor J. Amer. Soc. Hort. Sci. 139 3 12
Kondo, S. & Danjo, C. 2001 Cell wall polysaccharide metabolism during fruit development in sweet cherry ‘Satohnishiki’ as affected by gibberellic acid J. Jpn. Soc. Hort. Sci. 70 178 184
Lang, A. & Düring, H. 1991 Partitioning control by water potential gradient: Evidence for compartmentation breakdown in grape berries J. Expt. Bot. 42 1117 1122
Lilleland, O. & Newsome, L. 1934 A growth study of the cherry fruit Proc. Amer. Soc. Hort. Sci. 32 291 299
Matthews, M.A. & Shackel, K.A. 2005 Growth and water transport in fleshy fruit, p. 181–197. In: Holbrook, N.M. and M.A. Zwieniecki (eds.). Vascular transport in plants. Elsevier Academic Press, Amsterdam, The Netherlands
McGuire, R.G. 1992 Reporting of objective color measurements HortScience 27 1254 1255
Measham, P.F., Bound, S.A., Gracie, A.J. & Wilson, S.J. 2009 Incidence and type of cracking in sweet cherry (Prunus avium L.) are affected by genotype and season Crop Pasture Sci. 60 1002 1008
Sekse, L. 1995 Fruit cracking in sweet cherries (Prunus avium L.). Some physiological aspects—A mini review Sci. Hort. 63 135 141
Sekse, L., Bjerke, K.L. & Vangdal, E. 2005 Fruit cracking in sweet cherries—An integrated approach Acta Hort. 667 471 474
Steudle, E. 1993 Pressure probe techniques: Basic principles and application to studies of water and solute relations at the cell, tissue and organ level, p. 5–36. In: Smith, J.A.C. and H. Griffiths (eds.). Water deficits: Plant responses from cell to community. Bios Scientific Publishers, Oxford, UK
Tilbrook, J. & Tyerman, S.D. 2008 Cell death in grape berries: Varietal differences linked to xylem pressure and berry weight loss Funct. Plant Biol. 35 173 184
Tukey, H.B. 1934 Growth of the embryo, seed, and pericarp of the sour cherry (Prunus cerasus) in relation to season of fruit ripening Proc. Amer. Soc. Hort. Sci. 31 125 144
Tyerman, S.D., Tilbrook, J., Pardo, C., Kotula, L., Sullivan, W. & Steudle, E. 2004 Direct measurement of hydraulic properties in developing berries of Vitis vinifera L. cv. Shiraz and Chardonnay Austral. J. Grape Wine Res. 10 170 181
Wada, H., Matthews, M.A. & Shackel, K.A. 2009 Seasonal pattern of apoplastic solute accumulation and loss of cell turgor during ripening of Vitis vinifera fruit under field conditions J. Expt. Bot. 60 1773 1781
Wada, H., Shackel, K.A. & Matthews, M.A. 2008 Fruit ripening in Vitis vinifera: Apoplastic solute accumulation accounts for pre-veraison turgor loss in berries Planta 227 1351 1361