Ripening‐related changes in the cell walls of Spanish pear (Pyrus communis)

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Abstract
Unripe Spanish pears (Pyras commanis L. ev. Blanquilla) were ripened at 18°C for 5 and 10 days. Softening of the cortical tissues was associated with swelling of parenchyma cell walls from 1 to more than 5 μm in 10 day ripe pears, by which time the pears were over ripe. However, there was little indication of cell separation and the middle lamella could be detected between most cell walls. Furthermore, cell separation was constrained by regions rich in plasmodesmata where wall swelling was prevented. Parenchyma cells in the 500 μm of tissue underlying the epidermis did not undergo ripening‐related changes to the same extent as those of the cortex. These cells, in combination with a sub‐epidermal layer of lignified sclereid clusters, constituted a relatively tough and protective skin. Ripening of the cortical tissues was associated with a depletion of alcohol‐insoluble pectic polysaccharides, as indicated by the decrease in arabinose and uronic acid. Analysis of alcohol‐insoluble cell wall preparations enriched in either parenchyma or sclereid cell walls indicated that this change was predominantly associated with the parenchyma walls. Such changes were less prominent in the peel. The decrease in pectic polysaccharides was accompanied by an increase in their solubility. During ripening, the sclereid clusters of the cortex continued in develop, as indicated by an increase in their size and yield of cell wall xylose and glucose. Cortical parenchyma cells radiating from the sclereids were firmly attached to the lignified cells. This was due to lignification extending from the sclereids into the primary walls of the parenchyma cells. We conclude that dissolution of pectic polysaccharides is one of several factors which determine softening during ripening of Spanish pears.