PLANT PHYSIOLOGY , Vol 108, Issue 4 1679-1689, Copyright © 1995 by American Society of Plant Biologists
|
DEVELOPMENT AND GROWTH REGULATION |
The Use of Nonaqueous Fractionation to Assess the Ionic Composition of the Apoplast during Fruit Ripening
A. J. MacDougall, R. Parker and R. R. Selvendran
Food Molecular Biochemistry Department, Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich, NR4 7UA United Kingdom
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.
