Translocation of mal secco toxin in lemons and its effect on electrolyte leakage, transpiration, and citrus callus growth

A bioassay test was developed for quantifying mal secco toxin based on electrolyte loss from treated carrot discs. A linear relationship was established between toxin concentration and electrolyte leakage as measured by conductivity of the bathing solution. The lowest amount of toxin which could be detected by this method was 32 μg/ml. The toxin decreased transpiration rate of tomato cuttings. A hyperbolic plot was obtained when reduction in transpiration rate was followed as a function of time. C¹⁴-labeled toxin was obtained by growing the fungus in the presence of radioactive amino acids. The purified radioactive toxin had a specific activity of 5.5 x 10⁴ cpm/mg toxin. Radioactivity was readily translocated in lemon cuttings. The detection of radioactivity in the leaves was correlated with symptoms' appearance. Callus growth of a lemon cultivar highly susceptible to mal secco (Eureka) was inhibited by 0.3 mg toxin/ml, whereas an orange cultivar (Shamouti) was not affected by the toxin.