Induction of frond abscission by metals and other toxic compounds in Lemna minor

Fronds of the duckweed Lemna minor L. clone St form colonies of different sizes on the basis of stipes connecting mother and daughter fronds for some time after the development of daughter fronds. All the metals (AsO43−, AsO2−, Cd2+, CrO42−, Co2+, Cu2+, Ni2+, Hg2+, Tl+ and Zn2+) and one non-metal (SeO42−, SeO32−) tested here induced frond abscission, thus decreasing the colony size on the basis of a novel mechanism of abscission described recently. Concentration–response curves were created based on percentages of frond abscission after 7 and 24 h of toxic compound application, and response concentrations were calculated accordingly. The following conclusions could be drawn: (1) in most cases the response demonstrates less sensitivity than the bio test based on the ISO protocol 20079. (2) Even applying 1 mM of the metals, AsO43−, CrO42−, Co2+ and Zn2+ did not reach the half-maximal effects. (3) The concentration–response curves are bell-shaped with AsO2−, Cd2+, Hg2+, SeO32− and Tl+, which demonstrates that abscission is induced by lower but not by higher concentrations. (4) Frond abscission shows fast and sensitive effects (24 h) for Ag+, Cu2+, AsO2−, SeO42−, SeO32− and Tl+. The mechanisms and responses described here quantitatively for the first time complement and explain observations within the frame of the ISO protocol. Therefore, frond abscission should be regularly reported in the standard test protocols as abscission always indicates massive physiological effects.