Seedlings of the white mustard (Sinapis alba L.) are sensitive to the cell-free extracts of a toxigenic strain of Microcystis aeruginosa and to microcystin-LR. Fresh mass of plants, plant length, including hypocotyl and root length and lateral root formation is inhibited in microcystin-LR treated seedlings. The decrease of anthocyanin content is obtained in microcystin treated mustard cotyledons. The tissue necrosis of cotyledons is a characteristic consequence of microcystin treatment. Microcystin-LR induces an increase in single stranded deoxyribonucleases (ssDNases) activity of S. alba seedlings as shown by spectrophotometric assays and by ssDNase activity polyacrylamide gels. The significance of this phenomenon is discussed in relation to general stress responses in plants. We conclude that microcystin-LR affects the whole physiology and the growth of plants.

Introduction

Bloom-forming cyanobacteria in eutrophic surface waters have caused increasing concern over the last decade due to their toxic potential. The cyanotoxins are chemically diverse and there have been rapid advances in the study of cyanobacterial toxins at cellular and molecular level (Carmichael, 1989; Cohen, 1989; Carmichael, 1992; Falconer, 1993; Skulberg et al., 1993; Carmichael, 1994; Codd et al., 1994). Most if not all investigations of cyanotoxins in aquatic environments have been stimulated by the cases of poisonings of wild animals, birds, livestock. Several of the common species of scum-forming cyanobacteria are extremely harmful including Microcystis aeruginosa(Codd and Carmichael, 1982; Falconer et al., 1983; Nishiwaki-Matsushima et al., 1992; Falconer, 1993; Bell and Codd, 1994; Carmichael, 1994; Codd et al., 1994). M. aeruginosa produces hepatotoxins, called microcystins, cyclic heptapeptides (Pearson et al., 1990). The antibiosis of cyanobacteria has usually been studied using mouse bioassay for acute toxicity.

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