CELL-WALL CHANGES IN SPANISH PEAR DURING RIPENING

Unripe Spanish pears (Pyrus communis, c.v. Blanquille) were allowed to ripen at 18 degrees C for 1 week. Cell walls were prepared as alcohol-insoluble residues (AIRs) which were free of starch. On a fresh weight basis, ripening was accompanied by a decrease in cell wall arabinose and uronic acid, a proportion of which were metabolised, and an increase in xylose and glucose. To investigate the origin of the changes in cell wall polymers, the AIRs of unripe and ripe fruit were extracted sequentially with water, cyclohexane-trans-1,2-diamine-N, N, N', N'-tetraacetate (CDTA), Na2CO3 at 1 degrees C and 20 degrees C, 0.5, 1 and 4M KOH to leave a cellulose-rich residue. The extracts and residues were analysed for their carbohydrate composition and, where appropriate, degree of pectin-methyl esterification. The water, CDTA and Na2CO3 extracted polymers were rich in pectic polysaccharides. These exhibited heterogeneity in the ratios of neutral and acidic components and contained small quantities of xylose. The KOH-extracted polymers were rich in hemicelluloses, particularly xylans, but also contained sugars common to pectic polysaccharides. The cellulose-rich residues were rich in cellulosic glucose and xylose which probably originated from the highly-lignified sclereids. Ripening resulted in an increase in water- and CDTA-soluble pectic polysaccharides which exhibited a lower degree of pectin-methyl esterification. These results and the changes in carbohydrate composition are discussed in the light of corresponding decreases in the yields of pectic polysaccharides of the Na2CO3 extracts and cellulose-rich residues during ripening.