Ca2+ in Cell Walls of Ripening Tomato and Peach

Abstract

Ca 2+ content in cell wall-middle lamella (CW–ML) areas of outer and inner pericarp, placenta, and gel parenchyma of ripening tomato ( Lycopersicon esculentum Mill. cvs. Celebrity and Jumbo) and mesocarp of ripening peach [ Prunus persica (L.) Batsch cv. Georgia Belle] was determined by energy dispersive (EDS) X-ray microanalysis. Ca 2+ increased from 1.50 to 6.95 mg·g –1 dry weight in CW–ML of outer pericarp and 0.98 to 2.60 mg·g –1 dry weight in CW–ML of inner pericarp during ripening. Ca 2+ content remained constant in tomato placenta and peach mesocarp, and was undetectable in tomato gel parenchyma throughout ripening. CW–ML of peach mesocarp had lower Ca 2+ content than tomato pericarp and placenta at all ripening stages, but total peach uronic acid content was 2.5 times greater. Pectin methylesterase (PME) activity increased in tomato pericarp as fruit ripened, but remained low and unchanged in placenta and gel parenchyma. PME treatment of pericarp increased amounts of CW–ML Ca 2+ in the breaker stage but not in the green mature stage. The results indicate that increased amounts of Ca 2+ are bound to CW–ML of tomato pericarp as ripening occurs but not in placenta or peach mesocarp. Pectin deesterification and wall softening during ripening may in part be factors that control the presence and amount of CW–ML Ca 2+ .