USE OF SECONDARY-ION MASS-SPECTROMETRY TO IMAGE (44)CALCIUM UPTAKE INTHE CELL-WALLS OF APPLE FRUIT

Calcium, an important agent in regulating cell wall autolysis during f ruit ripening, interacts with pectic acid polymers to form cross-bridg es that influence cell separation. In the present study, secondary ion mass spectrometry (SIMS) was used to determine whether the cell walls of apple fruit were able to rake up exogenously applied Ca-44, which was infiltrated into mature fruit. SIMS, which has the ability to disc riminate between isotopes, allowed localization of the exogenously app lied Ca-44 and the native Ca-40. The results indicated that the total amount oi calcium present in the cell walls was enriched with Ca-44 an d that heterogeneity of Ca-44 distribution occurred in the pericarp. I sotope ratio images showed microdomains in the cell wall, particularly in the middle lamella intersects that oppose the intercellular spaces . These domains may be the key areas that control cell separation. The se data suggest that exogenously applied calcium may influence cell wa ll autolysis.