Ripeness Effects on Strawberry Water Transport

The effects of ripeness on osmotic water uptake and transpirational water loss were identified using six stages of ripeness (Fig. 2). The ripeness stages were: white, 1/2 light red, 3/4 light red, 1/2 red, 3/4 red, dark red^{21 (link)}. Fruits of ‘Florentina’ were selected based on color (CM-2600 d, orifice 3 mm diameter; Konica Minolta, Tokyo, Japan). The fruit selected ranged from white to dark red. Color was expressed as the hue angle. Rates of water uptake and transpiration were determined as indicated above. Additionally, juice was extracted from the fruit using a garlic press and its osmotic potential quantified by vapor pressure osmometry (VAPRO 5600; Wescor, Utah, USA). The skin permeances for transpiration (P_t; m s⁻¹) and osmotic water uptake (P_f, m s⁻¹) were determined from rates of water movement^{22 (link)}. Briefly, P_t was calculated from Eq. (1). The rate of transpiration (F_t; kg s⁻¹) was divided by the product of the fruit surface area (A; m²), the density of water (ρ_w; kg m⁻³), and the gradient in water activity (Δɑ_w; dimensionless) across the fruit skin²³ . Since the humidity above dry silica is practically zero, Δɑ_w equals the water activity of the strawberry juice, which is approximately one.
The value of P_f (m s⁻¹) was determined using the filtration permeability relation in Eq. (2); where F_f (kg s⁻¹) represents the rate of osmotic uptake, A_fruit the fruit surface area (m²), R (m³ MPa mol⁻¹ K⁻¹) the universal gas constant, T (K) the absolute temperature, V_w (m³ mol⁻¹) the molar volume of water and ρ_w (kg m⁻³) the density of water and ΔΨ (MPa) the difference in water potential between the water potential of the fruit (Ψ_fruit) and that of the incubation solution (Ψ)²⁴ . For fruit incubated in water (Ψ = 0) the driving force for osmotic uptake is essentially equal to the water potential of the fruit (Ψ_fruit). The fruit water potential equals the sum of the fruit’s turgor and the osmotic potential of the expressed juice (Ψ_Π). Because the fruit turgor is negligibly low in strawberry^{6 (link)}, the value of Ψ_Π essentially equals the Ψ_fruit. $$P_t=\frac{F_t}{A_{\mathit{fruit}}·{\rho }_w·\mathrm{\Delta }{a}_w}$$ $$P_f=\frac{F_f}{A_{\mathit{fruit}}·\mathrm{\Delta }\mathrm{\Psi }}·\frac{\mathit{RT}}{\rho ·\overline{V_w}}$$
Fruit surface area was calculated from a solid geometrical model comprising a truncated cone capped by two halves of rotational prolate ellipsoids^{6 (link)}. The respective dimensions were estimated from calibrated photographs by image analysis (cellSens Dimension 1.7.1; Olympus Soft Imaging Solutions, Münster, Germany). The relationship between mass and the measured surface area was plotted and an empirical regression model was fitted. Data from a compilation between different cultivars and development stages ranging from green fruitlets to fully mature fruit were used (see supplementary information). The total number of individual fruit replications was 200.