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

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.