The stiffening of the cell walls observed during physiological softening of pears.

Artur Zdunek,Arkadiusz Kozioł,Małgorzata Lekka,Justyna Cybulska,Piotr M Pieczywek

doi:10.1007/s00425-015-2423-0

Abstract

Main conclusionThe Young’s modulus of the primary cell walls of pears decreases linearly during the pre-harvest on-tree maturation and increases during postharvest storage, and does not correlate with firmness of fruit.The determination of mechanical properties of cell walls is indispensable for understanding the mechanism of physiological softening and deterioration of quality of fruits during postharvest storage. The Young’s modulus of the primary cell walls from pear fruit (Pyrus communis L., cultivars ‘Conference’ and ‘Xenia’) during pre-harvest maturation and postharvest storage in an ambient atmosphere at 2 °C followed by shelf life was studied using atomic force microscopy (AFM). The results were related to the firmness of fruits, galacturonic acid content in water, chelator, sodium carbonate and insoluble pectin fractions, polygalacturonase and pectin methylesterase activities. The Young’s modulus of the primary cell walls decreased linearly during the last month of pre-harvest maturation from 3.2 ± 1.8 to 1.1 ± 0.7 MPa for ‘Conference’ and from 1.9 ± 1.2 to 0.2 ± 0.1 MPa for ‘Xenia’ which correlated with linear firmness decrease. During postharvest storage the cell wall Young’s modulus increased while firmness continued to decrease. Correlation analysis for the entire period of the experiment showed a lack of straightforward relation between the Young’s modulus of primary cell walls and fruit firmness. The Young’s modulus of cell walls correlated negatively either with galacturonic acid content in sodium carbonate soluble pectin (‘Conference’) or with insoluble pectin fractions (‘Xenia’) and positively with polygalacturonase activity. It was therefore evidenced that covalently linked pectins play the key role for the stiffness of fruit cell walls. Based on the obtained results, the model explaining the fruit transition from firm and crispy to soft and mealy was proposed.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-015-2423-0) contains supplementary material, which is available to authorized users.

Highlights

Cell walls determine macroscopic mechanical properties of fruit (Jarvis 2011; Cybulska et al 2013; Gwanpua et al 2014), as well as water transport and shrinkage (Fanta et al 2014)
Our studies revealed that the Young’s modulus of the primary cell wall in pears decreases during pre-harvest maturation and increases when fruit continues softening during postharvest storage
This discrepancy during postharvest period may be due to degradation of pectins in middle lamella causes decreasing of a cell-to-cell adhesion whereas in the primary cell wall causes its stiffening

Summary

Introduction

Cell walls determine macroscopic mechanical properties of fruit (Jarvis 2011; Cybulska et al 2013; Gwanpua et al 2014), as well as water transport and shrinkage (Fanta et al 2014). Material properties of cell walls in plants change during growth and development due to biosynthesis and degradation of its constituents (Albersheim et al 2011). It is generally believed that during fruit ripening, the cell walls loosen and become weaker, neither the structural bases of these changes (Vicente et al 2007) nor experimental evidences have been provided, so far. This is largely due to problems with the evaluation of cell wall structure and mechanical properties in conditions close to natural ones

Methods

Results

Discussion

Conclusion