Decrease in Galactose Residue in Cell Wall Polysaccharides of 'Andesu' Netted Melon Fruit Influences the Formation of the Water-Core

摘要

Water-core symptoms of the netted melon 'Andesu' (Cucumis melo L.) were observed frequently in open field cultivation on sand dunes in Yamagata Prefecture, Japan. Previously we reported that shading treatment resulted in accelerated fruit softening, the development of a water-core during the ripe to over-ripe stages, and shortening of the shelf life. In this study, 'Andesu' plants were grown under shaded conditions during the fruit developing stages, 14 days before harvest. Cell wall polysaccharides of the inner mesocarp of pre-ripe, normally-ripened, over-ripe, and water-cored fruits induced by shading treatments were extracted sequentially with water, CDTA and Na_2CO_3. Uronic acid and neutral sugar contents were higher in the Na_2CO_3-soluble fraction than in the CDTA-soluble fraction in all fruits, in the Na_2CO_3-soluble fraction, normally-ripened, water-cored and over-ripe fruits were characterized by lowered neutral sugar contents than pre-ripe fruit. In the CDTA-soluble fraction, distribution profiles of polysaccharides by gel filtration chromatography showed a decrease in molecular size in normally-ripened, over-ripe and water-cored fruit. Among the neutral sugar residues, marked decreases were observed in the galactose residue in both of the CDTA- and Na_2CO_3-soluble fractions. The decreases in galactose residue in the Na_2CO_3-soluble fraction were more significant in normally-ripened and water-cored fruits, particularly in the high molecular weight pectic fractions, and were observed in both high and low molecular weight fractions in over-ripe fruits. β-Galactosidase activity was higher in water-cored fruits than in over-ripe fruits. Water-core symptoms induced by shading treatment of plants before fruit harvesting are related to the degradation of cell wall polysaccharides, particularly a reduction in the galactose residue on the side chain of pectic polysaccharides.