TY - JOUR

T1 - Studies on water transport through the sweet cherry fruit surface. 10. Evidence for polar pathways across the exocarp

AU - Weichert, Holger

AU - Knoche, Moritz

PY - 2006/5/31

Y1 - 2006/5/31

N2 - Water uptake through the fruit surface is considered as an important factor in cracking of sweet cherry (Prunus avium L.) fruit. Uptake may occur by diffusion and/or viscous flow along a polar pathway. To establish the mechanism of water uptake, the effects of viscosity and molecular weight of selected osmotica on water uptake into detached sweet cherry fruit were investigated. In addition we investigated the effect of temperature on penetration of 2-(1-naphthyl)[1-14C]acetic acid ([14C]NAA; pKa = 4.2) as a molecular probe in the nondissociated (pH 2.2) and dissociated (pH 6.2) forms. Rates of water uptake were linearly related to the inverse viscosity of gum arable solutions (range of concentrations and dynamic viscosities 10-300 g L-1 and 1.3 × 10-3 to 115.9 × 10 -3 Pa s, respectively). When fruit was incubated in solutions of osmotica of differing molecular weight that were isotonic to the fruit's water potential, water uptake depended on the molecular weight of the osmoticum [range 58-6000 for NaCl to poly(ethylene glycol) 6000 (PEG 6000)]. There was no uptake from PEG 6000 solutions, but rates of water uptake increased as the molecular weight of the osmotica decreased. Apparent water potentials of sweet cherry fruit, determined by incubating fruit in concentration series of selected osmotica, increased as the molecular weight of the osmotica increased up to 1500 and remained constant between 1500 and 6000. Reflection coefficients (σ) estimated from this relationship were closely related to hydrodynamic radii (r) of the osmotica [σ = 1.0(±0.0) - [10.9(±0.9) × 10 -11][r-1 (m-1)], R2 = 0.97, P < 0.0001]. The permeability of the sweet cherry fruit exocarp to NAA (pK a = 4.2) and temperature dependence of NAA permeability (P d) as indexed by the energy of activation (Ea, temperature range 5-35°C) were significantly higher for the nondissociated NAA (pH 2.2, Pd = 10.2(±0.8) × 10-8 m s-1, Ea = 67.0 ± 1.7 kJ mol-1) than for the dissociated NAA (pH 6.2, Pd = 1.1 (±0.2) × 10-8 m s -1, Ea = 51.8 ± 1.9 kJ mol-1). The activation energy for penetration of the dissociated NAA was closely related to the stomatal density (R2 = 0.84***, P < 0.0001) but less so for the nondissociated NAA (R2 = 0.30*, P < 0.03). These data provide evidence for the presence of polar pathways through the sweet cherry fruit exocarp that allow water uptake by viscous flow. These pathways offer a potentially useful target for strategies to reduce water uptake and fruit cracking, provided that a technique is identified that selectively "plugs" these pathways.

AB - Water uptake through the fruit surface is considered as an important factor in cracking of sweet cherry (Prunus avium L.) fruit. Uptake may occur by diffusion and/or viscous flow along a polar pathway. To establish the mechanism of water uptake, the effects of viscosity and molecular weight of selected osmotica on water uptake into detached sweet cherry fruit were investigated. In addition we investigated the effect of temperature on penetration of 2-(1-naphthyl)[1-14C]acetic acid ([14C]NAA; pKa = 4.2) as a molecular probe in the nondissociated (pH 2.2) and dissociated (pH 6.2) forms. Rates of water uptake were linearly related to the inverse viscosity of gum arable solutions (range of concentrations and dynamic viscosities 10-300 g L-1 and 1.3 × 10-3 to 115.9 × 10 -3 Pa s, respectively). When fruit was incubated in solutions of osmotica of differing molecular weight that were isotonic to the fruit's water potential, water uptake depended on the molecular weight of the osmoticum [range 58-6000 for NaCl to poly(ethylene glycol) 6000 (PEG 6000)]. There was no uptake from PEG 6000 solutions, but rates of water uptake increased as the molecular weight of the osmotica decreased. Apparent water potentials of sweet cherry fruit, determined by incubating fruit in concentration series of selected osmotica, increased as the molecular weight of the osmotica increased up to 1500 and remained constant between 1500 and 6000. Reflection coefficients (σ) estimated from this relationship were closely related to hydrodynamic radii (r) of the osmotica [σ = 1.0(±0.0) - [10.9(±0.9) × 10 -11][r-1 (m-1)], R2 = 0.97, P < 0.0001]. The permeability of the sweet cherry fruit exocarp to NAA (pK a = 4.2) and temperature dependence of NAA permeability (P d) as indexed by the energy of activation (Ea, temperature range 5-35°C) were significantly higher for the nondissociated NAA (pH 2.2, Pd = 10.2(±0.8) × 10-8 m s-1, Ea = 67.0 ± 1.7 kJ mol-1) than for the dissociated NAA (pH 6.2, Pd = 1.1 (±0.2) × 10-8 m s -1, Ea = 51.8 ± 1.9 kJ mol-1). The activation energy for penetration of the dissociated NAA was closely related to the stomatal density (R2 = 0.84***, P < 0.0001) but less so for the nondissociated NAA (R2 = 0.30*, P < 0.03). These data provide evidence for the presence of polar pathways through the sweet cherry fruit exocarp that allow water uptake by viscous flow. These pathways offer a potentially useful target for strategies to reduce water uptake and fruit cracking, provided that a technique is identified that selectively "plugs" these pathways.

KW - Cuticle

KW - Cuticular membrane

KW - Fruit cracking

KW - Prunus avium L.

KW - Stomata

KW - Water permeability

UR - http://www.scopus.com/inward/record.url?scp=33745489427&partnerID=8YFLogxK

U2 - 10.1021/jf053220a

DO - 10.1021/jf053220a

M3 - Article

C2 - 16719520

AN - SCOPUS:33745489427

VL - 54

SP - 3951

EP - 3958

JO - Journal of Agricultural and Food Chemistry

JF - Journal of Agricultural and Food Chemistry

SN - 0021-8561

IS - 11

ER -