RELATIONSHIP BETWEEN FUSARIC ACID UPTAKE AND ITS BINDING TO CELL STRUCTURES BY LEAVES OF EGERIA-DENSA AND ITS TOXIC EFFECTS ON MEMBRANE-PERMEABILITY AND RESPIRATION

Fusaric acid, at concentrations from 10-4 M to 10-3 M, induced an early hyperpolarization, followed by a marked depolarization of transmembrane electrical potential difference, inhibited H+ extrusion, caused an increase in leakage of K+ and of other electrolytes, and inhibited O2 uptake in leaves of Egeria densa Planch. These effects are similar to those previously reported for other plant tissues. Measurements of fusaric acid uptake and of cell sap pH show that the toxin behaves like a weak acid, passively permeating the cell in the uncharged form and accumulating in the cytosol in the charged form. The accumulation in tissues was greater than expected on the basis of the pH and fusaric acid concentration of the ambient solution and the additional uptake could be accounted for by binding to walls and to insoluble components, mainly membranes. The amount of fusaric acid bound to the membrane fraction markedly increased when the pH of the ambient solution was increased from 5·3 to 6·7. Fusaric acid inhibited O2 uptake by leaves of E. densa in the dark. At concentrations between 1 and 15 mM it strongly inhibited oxidation of malate and succinate in a mitochondrial preparation from cauliflower inflorescences and this inhibition was also greater at the high than at the low pit. The results suggest that there are two components to fusaric acid toxicity; the first is aspecific, and involves penetration into the cell in the undissociated form followed by accumulation in the cytosol in the dissociated form, while the second involves specific interactions with particular cell targets. © 1993 Academic Press.