Phytotoxicity of cobalt ions on the duckweed Lemna minor - Morphology, ion uptake, and starch accumulation.

Cobalt (Co2+) inhibits vegetative growth of Lemna minor gradually from 1 µM to 100 µM. Fronds accumulated up to 21 mg Co2+ g(-1) dry weight at 10 µM external Co2+ indicating hyperaccumulation. Interestingly, accumulation of Co2+ did not decrease the iron (Fe) content in fronds, highlighting L. minor as a suitable system for studying effects of Co2+ undisturbed by Fe deficiency symptoms unlike most other plants. Digital image analysis revealed the size distribution of fronds after Co2+ treatment and also a reduction in pigmentation of newly formed daughter fronds unlike the mother fronds during the 7-day treatment. Neither chlorophyll nor photosystem II fluorescence changed significantly during the initial 4d, indicating effective photosynthesis. During the later phase of the 7-day treatment, however, chlorophyll content and photosynthetic efficiency decreased in the Co2+-treated daughter fronds, indicating that Co2+ inhibits the biosynthesis of chlorophyll rather than leading to the destruction of pre-existing pigment molecules. In addition, during the first 4d of Co2+ treatment starch accumulated in the fronds and led to the transition of chloroplasts to chloro-amyloplasts and amylo-chloroplasts, while starch levels strongly decreased thereafter.

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 (AsO(4)(3-), AsO(2)(-), Cd(2+), CrO(4)(2-), Co(2+), Cu(2+), Ni(2+), Hg(2+), Tl(+) and Zn(2+)) and one non-metal (SeO(4)(2-), SeO(3)(2-)) 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 24h 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 1mM of the metals, AsO(4)(3-), CrO(4)(2-), Co(2+) and Zn(2+) did not reach the half-maximal effects. (3) The concentration-response curves are bell-shaped with AsO(2-), Cd(2+), Hg(2+), SeO(3)(2-) and Tl(+), which demonstrates that abscission is induced by lower but not by higher concentrations. (4) Frond abscission shows fast and sensitive effects (24h) for Ag(+), Cu(2+), AsO(2-), SeO(4)(2-), SeO(3)(2-) 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.

Growth rate based dose-response relationships and EC-values of ten heavy metals using the duckweed growth inhibition test (ISO 20079) with Lemna minor L. clone St.

The duckweed Lemna minor L. clone St was used to investigate the effect of 10 heavy metals under the standardised test conditions of the ISO protocol 20079. By using growth rates derived from frond number (FN), fresh weight (FW), dry weight (DW), chlorophyll and carotenoid (Car) contents, concentration-response curves for all heavy metals and all growth parameters were classified. In addition, all data were fitted to obtain the inhibitions of growth rates (E(r)C(x)) at the level of 10%, 20% and 50% (E(r)C(10), E(r)C(20) and E(r)C(50), respectively) then used to evaluate the phytotoxicity of the different heavy metals. On the basis of the E(r)C(50) values (average ranking of all five growth parameters), the following series of phytotoxicity was detected by using molar concentrations: Ag(+)>Cd(2+)>Hg(2+)>Tl(+)>Cu(2+)>Ni(2+)>Zn(2+)>Co(2+)>Cr(VI)>As(III)>As(V).

Sensitivity of different growth inhibition tests--just a question of mathematical calculation? Theory and practice for algae and duckweed.

Hazard assessment often needs to compare inhibition values of different test species and different test durations. But the three different methods to calculate inhibition in growth inhibition tests (final biomass, growth rate, area under the growth curve) may lead to very different and sometimes contradicting numerical sensitivities of the test species. This paper will depict why there are these different results and what consequences this has for the evaluation of results. Comprehensive discussion of different aspects will show that using growth rate may eliminate most of the problems occurring for comparisons between test species, different test times and different laboratories. The use of growth rate and the adaptation of toxicity levels maximises reproducibility, comparability and biological sensitivity of biotests.