Swelling of cell walls in mature sweet cherry fruit: factors and mechanisms

Christine Schumann,Moritz Knoche

doi:10.1007/s00425-020-03352-y

Abstract

Main conclusionSwelling of sweet cherry cell walls is a physical process counterbalanced by turgor. Cell turgor prevents swelling in intact cells, whereas loss of turgor allows cell walls to swell.Swelling of epidermal cell walls precedes skin failure in sweet cherry (Prunus avium) cracking. Swollen cell walls lead to diminished cell:cell adhesions. We identify the mechanism of cell wall swelling. Swelling was quantified microscopically on epidermal sections following freeze/thaw treatment or by determining swelling pressure or swelling capacity of cell wall extracts. Releasing turgor by a freeze/thaw treatment increased cell wall thickness 1.6-fold within 2 h. Pressurizing cell wall extracts at > 12 kPa prevented swelling in water, while releasing the pressure increased swelling. The effect was fully reversible. Across cultivars, cell wall thickness before and after turgor release in two subsequent seasons was significantly correlated (before release of turgor: r = 0.71**, n = 14; after release of turgor: r = 0.73**, n = 14) as was the swelling of cell walls upon turgor release (r = 0.71**, n = 14). Close relationships were also identified for cell wall thickness of fruit of the same cultivars grown in the greenhouse and the field (before release of turgor: r = 0.60, n = 10; after release of turgor: r = 0.78**, n = 10). Release of turgor by heating, plasmolysis, incubation in solvents or surfactants resulted in similar swelling (range 2.0–3.1 µm). Cell wall swelling increased from 1.4 to 3.0 µm as pH increased from pH 2.0 to 5.0 but remained nearly constant between pH 5.0 and 8.0. Increasing ethanol concentration decreased swelling. Swelling of sweet cherry cell walls is a physical process counterbalanced by turgor.

Highlights

Rain cracking severely limits the production of many soft, fleshy, fruit species in all regions of the world where rain occurs during the harvest period
A time course study of cell wall swelling following loss of turgor caused by a freeze/thaw treatment established that swelling was rapid
Our results demonstrate that cell wall swelling in mature sweet cherry fruit is substantially a physical process

Summary

Introduction

Rain cracking severely limits the production of many soft, fleshy, fruit species in all regions of the world where rain occurs during the harvest period. The primary cause of rain cracking in a fleshy fruit has for many years been thought to be an excessive internal (tissue) pressure (Considine and Kriedemann 1972; Sekse et al 2005; Measham et al 2009), where this excessive pressure is a direct result of osmotic water uptake through the fruit’s rain-wetted skin. The fruit has been assumed to resemble a thin-walled pressure vessel containing a solution rich in osmotically active carbohydrates. The increase in fruit volume and surface area occasioned by water uptake increases the tissue pressure (‘turgor’) inside the fruit. The fruit is believed to crack when certain critical thresholds of turgor and/or of skin strain are exceeded

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