Nonaqueous Fractionation to Assess the Ionic Composition of the Apoplast during Fruit Ripening

• Published in: Plant physiology (Bethesda) 1995-08, Vol.108 (4), p.1679-1689
• Main Authors: Alistair James Mac Dougall, Parker, Roger, Robert Rasiah Selvendran
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Calcium; Cell walls; Citrates; Development and Growth Regulation; esterification; fractionation; Fructification and ripening; Fruits; Fundamental and applied biological sciences. Psychology; Magnesium; malate dehydrogenase; mannosidases; Organic acids; organic acids and salts; Oxalates; pectins; pericarp; Plant physiology and development; Plants; Ripening; Solanum pimpinellifolium; Uronic acids; Vegetative and sexual reproduction, floral biology, fructification
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.108.4.1679

We have examined the possibility that pectin solubilization and cell separation in fruit may be due to organic acids disrupting calcium bridges between pectic polysaccharides. With fruit from a wild tomato (Lycopersicon pimpinellifolium [Dunal]) we demonstrated the validity of a nonaqueous fractionation method to obtain reliable estimates of the ionic content of the apoplast. In unripe fruit no organic acids were associated with the cell wall, which contained 67% of the total calcium and 47% of the magnesium. In ripe fruit 4% of the malate, 10% of the citrate, and 15% of the oxalate were estimated to be in the cell wall, together with 84% of the calcium and 52% of the magnesium. In contrast to the cultivated tomato, we did not find a consistent decrease in the degree of methyl esterification between unripe and ripe fruit, and an overall average of 75% was observed. In the cell walls of ripe fruit the ratio of calcium:magnesium:organic acid:unesterified uronic acid, on the basis of charge, was 15:4:4:16. The use of a computer program to predict the proportions of different ionic species in complex mixtures suggested that in ripe fruit 70% of the unesterified uronic acid would be complexed with calcium. Our results show that organic acids do not accumulate in the cell wall sufficiently to disrupt calcium cross-linking, nor is the calcium removed from the wall into the cell. We therefore conclude that organic acids do not contribute to cell separation during the ripening of tomato fruit.