RESUMO
Lemna minor L. (duckweed) forms colonies through vegetative propagation because mother fronds remain connected for some time with their daughter fronds by stipes. The colony size is controlled by abscission of stipes at a specific preformed abscission zone. Application of silver ions (Ag(+) ) enhances the rate of frond abscission, thus resulting in smaller colonies. The mechanism behind this process has not yet been identified. Silver caused an abscission response that saturated after 7 h of treatment. The half-maximal effective concentration was 0.72 µm Ag(+) for the standard clone, L. minor St. Other clones of the same species show sensitivities that differ by one order of magnitude. Transmission electron microscopy revealed: (i) large numbers of vesicles close to the plasmalemma in cells adjacent to the abscission zone, which proves a vesicular type secretory activity; and (ii) a moderately electron-dense secretion accumulated in the enlarging intercellular spaces, and seemed to flow from the adjacent cells towards the abscission zone. We assume that increasing pressure causes this material to push apart the cells, thereby causing the break in the abscission zone of the stipe. This is a novel mechanism of abscission that has not previously been described. The same mechanism occurs in stipes of both control and Ag(+) -treated samples. Silver ions only accelerate the process leading to abscission of stipes, without affecting the mechanism involved.