INCREASES IN CYTOSOLIC CA2-CELLS CORRELATE WITH LEAF SENESCENCE( IN PARSLEY MESOPHYLL)

The ability to maintain the cytoplasmic Ca2+ concentration ([Ca2+](cyt )) at homeostatic levels has been examined during leaf senescence in d etached parsley (Petroselinum crispum) leaves. Fluorescence ratiometri c imaging of mesophyll cells isolated from parsley leaves at various s enescence stages and loaded with the Ca2+ indicator fura-2 has reveale d a distinct elevation of [Ca2+](cyt), which was positively correlated with the progress of leaf senescence. This initial increase of [Ca2+] (cyt), which was first observed in cells isolated from 3-d-senescent l eaves, occurred 1 d before or in parallel with changes in two establis hed senescence parameters, chlorophyll loss and lipid peroxidation. Ho wever, the [Ca2+](cyt) elevation followed by 2 d the initial increase in the senescence-associated proteolysis. Whereas the [Ca2+](cyt) of n onsenescent cells remained at the basal level, the elevated [Ca2+](cyt ) of the senescent cells was a long-lasting effect. Experimental retar dation of senescence processes, achieved by pretreatment of detached l eaves with the cytokinin benzyladenine, resulted in maintenance of hom eostatic levels of [Ca2+](cyt) in cells isolated from 3-d-senescent le aves. These observations demonstrate for the first time to our knowled ge a correlation between elevated [Ca2+](cyt) and the process of senes cence in parsley leaves. Such senescence-associated elevation of [Ca2](cyt), which presumably results from a loss of the cell's capability to extrude Ca2+, may serve as a signal inducing subsequent deteriorati ve processes.