Use of secondary ion mass spectrometry to image (44)calcium uptake in the cell walls of apple fruit

Calcium, an important agent in regulating cell wall autolysis during fruit ripening, interacts with pectic acid polymers to form cross-bridges 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 discriminate between isotopes, allowed localization of the exogenously applied 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 and that heterogeneity of Ca-44 distribution occurred in the pericarp. Isotope 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. These data suggest that exogenously applied calcium may influence cell wall autolysis.