Lugol's solution
Lugol's solution (noun,no plural, syn.: Lugol's iodine) – IKI, i. e., a mixture of crystalline iodine (I) and potassium iodide (KI) in water. Recipes and recommendations for the best suited concentrations vary. Strongly concentrated solutions (ca. 1.5-3%) are more reliable for spot test reactions on thalli or hyphae. These concentrated or even saturated solutions can be stored without soon loosing their staining capabilities. They are not suited, however, to examine ascus apical structures that are easily obscured by vivid staining reactions. Better suited to resolve ascus apical structures are lower concentrations (e. g., 0.3 %), but these reagents need to be made up freshly because their iodine concentration rapidly decreases. Unlike Meltzer's reagent, Lugol's iodine by itself has no clearing effect on fungal plectenchyma and thus is often used after pre-treatment with 5-10% potassium hydroxide (KOH). However, a colour change from reddish to blue that is characteristic for a hemiamyloid reaction, can only be observed, if plectenchyma are first observed in Lugol's without pre-treatment, then treated with KOH and again treated with Lugol's. This sequence of test reagents will induce the shift from reddish to blue characteristic for hemiamyloid reactions. In contrast, euamyloid reactions occur as deep blue even without pre-treatment.
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