Allelopathic effects of cyanotoxin Microcystin-LR in greater duckweed, Spirodela polyrhiza (L.) Schleid
Microcystin-LR is the most toxic microcystin produced by the cyanobacteria Microcystis aeruginosa Kützing. Although it has widely been known that this toxin is hepatotoxic to animals, its effect on plants is still unclear. Given that this toxin acts as a protein phosphatase inhibitor, an allelopathic effect on aquatic plant growth is also possible. For our study, greater duckweed, Spirodela polyrhiza (L.) Schleid was grown in a gradient of Microcystin-LR concentrations (control, 0.1, 1, 2, 3, 4, and 5 µg/L) for four weeks. Although the results showed that final frond numbers and plant growth rates were significantly different (P= 0.0017 and P=0.0138 respectively) among concentrations, no clear concentration-dependent effects of Microcystin-LR were observed on final frond number and plant growth rate. Some plants exposed to higher concentrations had chlorotic spots and marginal necrosis. However, no significant difference (P=0.9679) was noticed for the final dry biomass of plants among different concentrations. Therefore, it appears that the growth of S. polyrhiza is not regulated by long-term exposure to Microcystin-LR at concentrations typically observed in cyanobacteria bloom.