Is the electrolyte leakage assay an unequivocal test of membrane deterioration during leaf senescence?

The main symptoms of leaf senescence are the degradation of chlorophyll and proteins (which may be accompanied by ammonium accumulation), and an increase of electrolyte leakage (EL), which has been traditionally attributed to disruption of cell membranes. The aim of this study was to determine if ammonium efflux contributes to the increase EL in senescing barley leaves. During senescence of detached leaves the increase of EL correlated with ammonium leakage (r(2) = 0.82) and ammonium content in tissues (r(2) = 0.73), but not with K(1+) leakage (r(2) = 0.23). Although lower amounts of ammonium accumulated in senescing attached leaves, again changes in EL paralleled ammonium accumulation. EL increased early during senescence even though ion leakage was selective (leaves leaked proportionally more ammonium than K(1+)), and membranes appeared intact as judged from staining with the cell impermeant stain propidium iodide. Detached leaves maintained their capacity to regreen after 3 days of senescence-acceleration in darkness, i.e., membrane integrity was not severely compromised. During the early stages of senescence, EL increases due to ammonium accumulation (possibly resulting from protein degradation) even if there is no massive disruption of cell membranes. Therefore, increased EL in senescing leaves is not an unequivocal symptom of cell membrane damage.