PHYSIOLOGICAL AND ULTRASTRUCTURAL EFFECTS OF FUMONISIN ON JIMSONWEED LEAVES

The physiological and ultrastructural effects of fumonisin B1, a phytotoxin obtained from Fusarium moniliforme, on jimsonweed (Datura stramonium L.) were studied. When fumonisin B1 was applied at concentrations of 3 to 1000 mug/mL (4.1 to 1388 muM) to jimsonweed leaves, effects such as electrolyte leakage, autolysis, and photobleaching of tissues were noted in less than 12 h after exposure to light (480 mumol . m-2 . s-1, PAR) at 25-degrees-C. The degree of damage was directly proportional to the concentration and duration of exposure in all cases. Chlorophyll reduction from 5 to 73% occurred after 48 h in the light. In darkness, electrolyte leakage above control levels was noted only in treated samples after 72 h. However, when these dark-treated samples were subsequently placed in the light, electrolyte leakage occurred in 12 h. Ultrastructural damage in mesophyll palisade cells exposed to 500 mug/mL fumonisin B1 began at 6 h and intensified after 12 h. Cytoplasmic degeneration occurred first, followed by disruption of the chloroplasts and accumulation of intracellular debris. The close structural analogue, AAL-toxin, has previously been reported to act by inhibiting nucleotide synthesis. The phytotoxic activity of both AAL-toxin and fumonisin was not influenced by supplying supplemental aspartic or orotic acid, indicating that neither toxin acts by inhibiting nucleotide synthesis. These results indicated that fumonisin B1 caused rapid, light-dependent cytoplasmic and chloroplast disruption, through an unknown mechanism.