Sweet Cherry Fruit: Ideal Osmometers?

Abstract

Osmotic water uptake through the skin is an important factor in rain cracking of sweet cherries. The objective was to establish whether a sweet cherry behaves like an ideal osmometer, where: (1) water uptake rates are negatively related to fruit osmotic potential, (2) a change in osmotic potential of the incubation solution results in a proportional change in water uptake rate, (3) the osmotic potential of the incubation solution yielding zero water uptake is numerically equal to the fruit water potential (in the absence of significant fruit turgor), and (4) the fruits' cuticular membrane is permeable only to water. The fruits' average osmotic potential and the rate of water uptake were related only weakly. Surprisingly, incubating a fruit in (a) the expressed juice from fruit of the same batch or (b) an isotonic artificial juice composed of the five major osmolytes of expressed juice or (c) an isotonic glucose solution-all resulted in significant water uptake. Decreasing the osmotic potential of the incubation solution decreased the rate of water uptake, while decreasing it still further resulted in water loss to the incubation solution. Throughout fruit development, the "apparent" fruit water potential was always more negative than the fruits' measured average osmotic potential. Plasmolysis of epidermal cells indicates the skin's osmotic potential was less negative than that of the flesh. When excised flesh discs were incubated in a concentration series of glucose solutions, the apparent water potential of the discs matched the osmotic potential of the expressed juice. Significant penetration of ¹⁴C-glucose and ¹⁴C-fructose occurred through excised fruit skins. These results indicate a sweet cherry is not an ideal osmometer. This is due in part to the cuticular membrane having a reflection coefficient for glucose and fructose less than unity. As a consequence, glucose and fructose were taken up by the fruit from the incubation solution. Furthermore, the osmotic potential of the expressed fruit juice is not uniform. The osmotic potential of juice taken from the stylar scar region is more negative than that from the pedicel region and that from the flesh more negative than that from the skin.

Keywords: cuticle; osmotic potential; prunus avium; reflection coefficient; water potential; water uptake.

Figure 1

Relationship between the rate of water uptake [F_H2O; (A,C)] or the flux in water uptake [J; (B,D)] and the surface area (A) or the osmotic potential (${\text{Ψ}}_{\Pi }^{fruit}$) of mature sweet cherry fruit. The equations of the linear regression lines were: F_H2O (g h⁻¹) = 1.71 (±0.24) × A (cm²) – 25.86 (±5.47); r² = 0.23 (A); J (× 10⁻⁶ kg m⁻² s⁻¹) = 0.15 (±0.03) × A (cm²) – 1.75 (± 0.66); r² = 0.13 (B); F_H2O (g h⁻¹) = −4.78 (± 0.20) × ${\text{Ψ}}_{\Pi }^{fruit}$ (MPa); r² = 0.14 (C); J (× 10⁻⁶ kg m⁻² s⁻¹) = −0.57 (± 0.02) × ${\text{Ψ}}_{\Pi }^{fruit}$ (MPa); r² = 0.12 (D). The intercept terms for (C,D) were not significant. Data symbols represent individual fruit.

Figure 2

Effect of the osmotic potential (${\text{Ψ}}_{\Pi }^{solution}$) of polyethylene glycol 6000 (PEG 6000) (A) and artificial or natural sweet cherry juice (B) on the time course of water uptake and rates of water uptake (F_H2O) (C). Artificial cherry juice was prepared using the five most abundant osmolytes of sweet cherry fruit, i.e., glucose, fructose, sorbitol, malic acid, and potassium malate. The open square represents the water uptake from juice extracted from the same batch of fruit. The apparent fruit water potential (Ψ^fruit) equals the ${\text{Ψ}}_{\Pi }^{solution}$ causing no change in fruit mass. The apparent Ψ^fruit is indicated by arrows. The dashed vertical line represents the ${\text{Ψ}}_{\Pi }^{fruit}$.

Figure 3

Effect of the osmotic potential of glucose incubation solutions (${\text{Ψ}}_{\Pi }^{solution}$) on the rates of water uptake (F_H2O) and the apparent fruit water potential (Ψ^fruit) of developing sweet cherry fruit [(A) 32, (B) 52, (C) 71, and (D) 91 days after full bloom (DAFB)]. The open circle [see arrow in (D)] represents water uptake from juice extracted from the same batch of fruit. (E) Apparent Ψ^fruit and osmotic potentials (${\text{Ψ}}_{\Pi }^{fruit}$) in the course of development. (F) Difference between apparent Ψ^fruit and ${\text{Ψ}}_{\Pi }^{fruit}$ in the course of development. The apparent Ψ^fruit equals the ${\text{Ψ}}_{\Pi }^{solution}$ causing no change in fruit mass and is indicated by vertical arrows (A–D). Vertical dashed lines in (A–D) indicate the ${\text{Ψ}}_{\Pi }^{fruit}$.

Figure 4

Effect of the osmotic potential of sucrose solutions (${\text{Ψ}}_{\Pi }^{solution}$) on plasmolysis of epidermal cells. Arrows indicate the onset of plasmolysis (A). Effect of ${\text{Ψ}}_{\Pi }^{solution}$on the percentage of plasmolyzed epidermal cells of sweet cherry fruit held under non-transpiring conditions (B) or transpiring conditions (C). Non-transpiring and transpiring conditions were imposed on the fruit to equilibrate (non-transpiring) or to induce a gradient (transpiring) in the osmotic potential between fruit and skin. Fruit was held under non-transpiring conditions (~100% RH) for up to 48 d or under transpiring conditions (~0% RH) for 4 d.

Figure 5

Osmotic potential of different parts of the sweet cherry fruit. Fruit was cut longitudinally along the pedicel stylar scar axis in half. Osmotic potentials were determined for juice extracted from one of the halves (A) and for four horizontal sections of the other half, i.e., sections comprising the regions of the pedicel end (B), the pedicel center (C), the stylar scar center (D), or the stylar scar (E).

Figure 6

Time course of water uptake into discs excised from the flesh of mature sweet cherry fruit (A) and whole fruit (B). (C) Rates of water uptake (F_H2O) as affected by the osmotic potentials of the glucose incubation solutions (${\text{Ψ}}_{\Pi }^{solution}$). The apparent fruit water potential equals the water potential of a solution causing no change in fruit mass and is indicated by arrows. The dashed vertical line represents the osmotic potential of the fruits' juice.

Figure 7

Time course of cumulative uptake of ¹⁴C-glucose and ¹⁴C-fructose through the sweet cherry fruit skin (A) and of the change in skin permeance (P) with time (B). (C–F). Simulated uptake of glucose and fructose into the fruit (C,D) and into the skin compartment (E,F). (C,D) Cumulative uptake and rate of uptake into an intact sweet cherry that does not contain any glucose and fructose, i.e. the internal concentrations of glucose and fructose (c_i) equal zero (c_i = 0). (E,F) Cumulative uptake and rate of uptake into the skin compartment of a sweet cherry fruit. The skin was assumed to contain no glucose or fructose (c_i = 0) or glucose and fructose at concentrations corresponding to those at the skin water potentials estimated by plasmolysis (c_i = 41.9 g l⁻¹ for glucose, c_i = 38.2 g l⁻¹ for fructose). The simulations were performed using the following parameters: P_glucose: 1.6 × 10⁻⁹ m s⁻¹, P_fructose: 2.1 × 10⁻⁹ m s⁻¹, fruit mass 10 g, fruit surface area 22.4 cm², skin thickness 100 μm, density 1 kg l⁻¹, no pit, concentration of incubation solution: c_o = 77.8 g l⁻¹ for glucose, c_o = 70.9 g l⁻¹ for fructose.