Microscopic Interpretation and Taxonomic Value of Iodine Staining Reaction in Fungal Spores

The iodine reaction of fungal spores is a microscopic identification method that uses iodine-containing reagents to evaluate the chemical properties of spore walls, spore ornamentation, or related structures. The key point in interpretation is not the intensity of background staining, but whether specific structures show amyloid, dextrinoid, or inamyloid reactions, thereby providing auxiliary characters for fungal classification and identification.

Keywords: fungal spores; iodine reaction; amyloid reaction; dextrinoid reaction; Melzer’s reagent; Lugol’s iodine; IKI; spore wall; spore ornamentation; fungal microscopic identification

1 Basic Positioning of Iodine Staining Reaction in Fungal Spores

1.1 Reaction Targets

(1) Spore wall

The spore wall is one of the most important observation targets in iodine reactions. Fungal spore walls vary in composition, wall-layer thickness, maturity, and surface structure. After iodine-containing reagents are added, they may appear blue-black, reddish brown, or show no specific color reaction. Such differences can serve as microchemical characters in taxonomic identification.

(2) Spore ornamentation

In some fungi, the iodine reaction of spores occurs mainly in the surface ornamentation rather than across the entire spore wall. If verrucose, echinate, ridged, or reticulate ornamentation shows a clear amyloid reaction in iodine-containing reagents, it may become an important basis for genus- or species-level identification.

(3) Spore sheath and external structures

Some spores possess a gelatinous sheath, mucilaginous layer, or appendages. These external structures may show weak coloration or background staining. During interpretation, reactions of the spore wall, ornamentation, and external sheath should be distinguished to avoid misinterpreting staining of non-wall structures as a reaction of the spore body itself.

1.2 Reaction Types

(1) Amyloid reaction

An amyloid reaction usually appears blue, gray-blue, blue-black, or violet-blue. This reaction is commonly observed in the spore walls of some basidiomycetes, spore ornamentation of Russulaceae, and certain ascomycete structures. During interpretation, it is necessary to confirm whether the blue coloration corresponds to the spore wall, ornamentation, or target structure.

(2) Dextrinoid reaction

A dextrinoid reaction usually appears reddish brown, brick-red, brownish red, or wine-red. This reaction is commonly used in microscopic differentiation of certain agaricoid groups. Because ordinary iodine solutions may also produce a yellow to brownish-yellow background, a dextrinoid reaction must show clear structural localization and a stable reddish-brown tone.

(3) Inamyloid reaction

An inamyloid reaction means that the spore or related structure does not show clear blue or reddish-brown specific coloration in iodine reagents. Negative results also have taxonomic value, especially in taxonomic keys for distinguishing amyloid and inamyloid groups.

(4) Hemiamyloid reaction

Hemiamyloid reactions are more commonly observed in the apical structures of asci rather than being a general interpretive category for all fungal spores. This reaction is strongly affected by Lugol’s iodine, Melzer’s reagent, and KOH pretreatment. Therefore, the reagent system and treatment conditions must be clearly recorded.

Table 1. Iodine Reaction Types of Fungal Spores and Related Structures

Reaction type	Typical color	Main observation site	Taxonomic significance	Key interpretive point
Amyloid reaction	Blue, gray-blue, blue-black, violet-blue	Spore wall, spore ornamentation, ascus apical structure	Positive taxonomic character used to distinguish genera, sections, or species groups	Confirm that blue coloration corresponds to the target structure
Dextrinoid reaction	Reddish brown, brick-red, brownish red, wine-red	Spore wall, entire spore, or local wall layer	Microscopic differentiation character in some basidiomycetes	Distinguish from the background color of ordinary iodine solution
Inamyloid reaction	No specific blue or reddish-brown coloration	Spore wall or related structures	Negative taxonomic character used for exclusion or grouping	Exclude reagent failure and immature spores
Hemiamyloid reaction	Varies with reagent and pretreatment	Mostly ascus apical structures	Special reaction type in ascomycete taxonomy	Reagent system and treatment sequence must be specified

2 Common Iodine Reagent Systems and Reaction Conditions

2.1 Melzer’s Reagent

(1) Composition

Melzer’s reagent usually consists of iodine, potassium iodide, chloral hydrate, and water. It is one of the most commonly used reagents for observing iodine reactions in basidiomycete spores. Iodine participates in color development, potassium iodide improves iodine solubility in the aqueous phase, and chloral hydrate helps clarify tissues, making spore walls, spore ornamentation, and hymenial structures easier to observe.

(2) Reaction types

Melzer’s reagent is mainly used to observe amyloid and dextrinoid reactions. Amyloid structures may appear blue, blue-gray, or blue-black in this system. Dextrinoid structures may appear reddish brown or brick-red, whereas inamyloid structures do not show these specific color changes.

(3) Applicable groups

Melzer’s reagent is most widely used in microscopic identification of basidiomycetes. Whether the spore ornamentation of Russulaceae is amyloid, whether the spore wall of Amanita is amyloid, and whether spores of certain agaricoid fungi are dextrinoid are often evaluated using Melzer’s reagent as an important basis.

(4) Observation points

When using Melzer’s reagent, mature, dispersed spores with clear outlines should be prioritized. Areas with spore accumulation, abundant tissue fragments, or a dark background may cause generalized color deepening and are not suitable as the basis for interpreting weak reactions.

(5) Limitations

Melzer’s reagent contains a clearing component and provides good visualization of some structures, but it is not suitable for interpreting all iodine reactions in ascomycetes. In particular, for analysis of hemiamyloid structures, results obtained with Melzer’s reagent cannot directly replace those obtained with Lugol’s iodine.

2.2 Lugol’s Iodine and IKI Systems

(1) Composition

Lugol’s iodine and IKI systems usually consist of iodine, potassium iodide, and water, without chloral hydrate. Their staining background is relatively direct, and they are commonly used to observe iodine reactions of ascomycete structures, spore walls, spore sheaths, and some gelatinous structures.

(2) Concentration differences

Different concentrations of Lugol’s iodine affect color intensity and background coloration. Low-concentration systems are suitable for observing weak reactions and fine structures, with relatively light background staining. Higher-concentration systems produce stronger staining but are also more likely to generate yellow-brown background interference. Therefore, when comparing results across samples or literature, differences in reagent concentration should be considered.

(3) Application in ascomycetes

In ascomycetes, the key targets of iodine reactions are often the ascus apical ring, apical plug, or apical apparatus. Lugol’s iodine is commonly used to determine whether these structures are iodine-positive. If the object of study involves hemiamyloid reactions, Lugol’s iodine usually has greater interpretive value than Melzer’s reagent.

(4) Application to spore structures

Lugol’s iodine can also be used to observe iodine reactions in certain spore walls, spore sheaths, gelatinous envelopes, or appendages. In such cases, the reaction site must be clearly specified; a positive reaction in the ascus apical structure should not be incorrectly recorded as a spore reaction.

(5) Interpretive limitations

Lugol’s iodine itself can give the field of view a yellow or brownish-yellow color. If a structure shows only ordinary yellow-brown adsorption without clear blue, blue-black, or reddish-brown structural coloration, it should not be interpreted as amyloid or dextrinoid.

2.3 Differences between IKI and Melzer Results

(1) Differences in reagent composition

IKI and Lugol systems mainly rely on iodine-potassium iodide aqueous solutions for color development. In addition to iodine and potassium iodide, Melzer’s reagent contains a clearing component. Therefore, the two systems differ in their effects on structural transparency, background color, and reaction appearance.

(2) Differences in reaction type

The same structure may show a certain reaction in Lugol’s iodine but appear weaker or less obvious in Melzer’s reagent. This difference is particularly important for hemiamyloid structures in ascomycetes. Mixing results from different reagent systems may lead to taxonomic misinterpretation.

(3) Recording requirements

Results should clearly state the reagent used rather than simply recording “iodine reaction positive.” For example, “amyloid in Melzer’s reagent,” “I+ in Lugol’s iodine,” and “Lugol-positive after KOH treatment” represent different experimental conditions and should not be simplified as the same result.

2.4 KOH Pretreatment and Iodine Reaction

(1) Purpose of treatment

KOH is commonly used to rehydrate dried specimens, soften tissues, clarify structures, and reduce pigment background. For dried basidiomata, dark tissues, or hymenial structures that are difficult to spread, KOH pretreatment can improve the visibility of microscopic structures.

(2) Effects on reaction

KOH treatment not only changes tissue transparency but may also affect the intensity and color expression of iodine reactions. In some structures, the reaction may be enhanced, weakened, or changed in reaction type after KOH treatment. Therefore, KOH should not be regarded as a completely neutral mounting step.

(3) Treatment sequence

Direct addition of Melzer’s reagent, direct addition of Lugol’s iodine, addition of Lugol’s iodine after KOH treatment, and addition of Melzer’s reagent after KOH treatment are all different observation conditions. In formal identification, the procedure should be kept consistent, and the reagent sequence should be recorded.

(4) Risk control

Excessively high KOH concentration or prolonged treatment may cause structural swelling, wall-layer alteration, or weakened reactions. For samples with weak iodine reactions, strong alkaline clearing should not be overused; observation quality should preferably be improved by preparing thin sections, selecting mature spores, and controlling background interference.

Table 2. Common Iodine Reagent Systems and Applications

Reagent system	Main compositional features	Main observation target	Application	Main limitation
Melzer’s reagent	Iodine, potassium iodide, chloral hydrate, etc.	Basidiomycete spore wall, spore ornamentation	Observation of amyloid and dextrinoid reactions	Weak reactions are easily affected by background and sample thickness
Lugol’s iodine	Iodine, potassium iodide, water	Ascus apical structures, spore wall, gelatinous structures	Observation of iodine reactions in ascomycetes and some hemiamyloid reactions	Yellow to brownish-yellow background may interfere with weak reactions
IKI system	Iodine-potassium iodide aqueous solution	Ascomycete structures, some spore structures	Standardized observation of iodine reactions	Concentration and mounting conditions must be specified
Iodine after KOH	Clearing or softening before iodine reagent addition	Dried specimens, darkly pigmented material, ascus structures	Structural expansion and reaction confirmation	May alter reaction intensity or reaction type

3 Microscopic Interpretation of Spore Iodine Reactions

3.1 Spore Maturity

(1) Prioritize mature spores

Mature spores have fully developed wall layers and relatively stable ornamentation and external structures. They are the primary targets for interpreting iodine reactions. Immature spore walls are not fully developed and may show weak reactions or false-negative results.

(2) Confirm across multiple fields

Coloration in a single spore cannot represent the entire specimen. Multiple mature spores should be observed across several microscopic fields to confirm whether the reaction is stable and consistent.

(3) Avoid areas of spore accumulation

Spore aggregation, tissue fragments, or overly thick preparations may cause local color deepening. Interpretation should be based on dispersed, intact spores with clear outlines.

3.2 Structural Localization

(1) Localization to the spore wall

If coloration is distributed along the spore outline or wall layer and corresponds to the spore wall structure, it may be interpreted as a spore wall-related reaction. If the color is located in the spore contents, it should not be directly classified as a spore wall reaction.

(2) Localization to ornamentation

If coloration is concentrated on surface warts, spines, ridges, or reticulate structures, it should be interpreted as an ornamentation reaction. In Russulaceae spores, ornamentation coloration is especially important as the main observation target.

(3) Localization to external sheath structures

If coloration occurs outside the spore in a gelatinous sheath or appendage, it should be recorded separately as a reaction of the external structure. A sheath reaction should not be simply equated with a spore wall reaction.

3.3 Color Interpretation

(1) Blue-series reactions

Clear blue, blue-black, or violet-blue structural coloration supports an amyloid reaction. If blue coloration occurs only in background precipitates or overlapping areas, it should not be used as evidence of a positive reaction.

(2) Reddish-brown-series reactions

Stable reddish-brown, brick-red, or brownish-red structural coloration supports a dextrinoid reaction. Ordinary yellow-brown background staining or pigment darkening should not be misinterpreted as a dextrinoid reaction.

(3) Negative reaction

If no clear blue or reddish-brown coloration is observed, the reaction may be interpreted as inamyloid or negative, provided that reagent activity, spore maturity, and correct observation site have been confirmed.

4 Key Applications in Different Fungal Groups

4.1 Russulaceae

(1) Ornamentation reaction

Spores of Russulaceae, including Russula and Lactarius, often have verrucose, ridged, or reticulate ornamentation. Melzer’s reagent can be used to demonstrate amyloid reactions in these ornamentations. Ornamentation height, connectivity, and staining intensity all have taxonomic value.

(2) Interpretive focus

For Russulaceae spores, the key point is usually not the overall color of the spore, but whether the surface ornamentation is amyloid. If only the spore outline is examined while the ornamentation layer is overlooked, key identification characters may be missed.

4.2 Amanita and Related Groups

(1) Spore wall reaction

In Amanita and related fungi, whether spores show an amyloid reaction in Melzer’s reagent is an important character for group delimitation and taxonomic keys. Some groups have distinctly amyloid spores, whereas others are inamyloid.

(2) Integrated identification

Identification of Amanita cannot rely solely on spore iodine reactions. It should also integrate the volva, annulus, pileus surface remnants, spore morphology, and ecological information. Iodine reaction is an important microscopic character, but not an independent basis for identification.

4.3 Small Agarics and Brown-Spored Groups

(1) Dextrinoid reaction

Spores of some small agarics or brown-spored groups may show dextrinoid reactions. This character is commonly used as an auxiliary criterion for distinguishing morphologically similar groups.

(2) Combined characters

Spores in these groups may already be darkly pigmented. Therefore, iodine reactions should be interpreted together with spore size, shape, wall thickness, germ pore, basidia, and cystidia.

4.4 Ascomycetes

(1) Ascus apical structures

In ascomycetes, the important targets of iodine reactions are often the ascus apical ring, apical plug, or apical apparatus. The reaction type of these structures in Lugol’s iodine or Melzer’s reagent is highly significant for taxonomic identification.

(2) Ascospore structures

In some ascospores, wall layers, gelatinous sheaths, septa, or appendages may also show iodine reactions. The reaction site should be clearly recorded to avoid incorrectly describing an ascus apical reaction as a spore reaction.

Table 3. Observation Priorities for Iodine Reactions in Different Groups

Group or structure	Main observation target	Common reagent	Interpretive focus	Taxonomic significance
Russulaceae	Spore surface ornamentation	Melzer’s reagent	Whether warts, ridges, or reticulate ornamentation are amyloid	Key microscopic character for genus- and species-level identification
Amanita and related groups	Spore wall	Melzer’s reagent	Whether spores are amyloid	Group delimitation and taxonomic keys
Small agarics	Spore wall or entire spore	Melzer’s reagent	Whether dextrinoid or inamyloid	Auxiliary distinction among closely related groups
Ascomycetes	Ascus apical structures	Lugol’s iodine, Melzer’s reagent	Whether the apical ring or apical apparatus is iodine-positive	Taxonomic identification of ascomycetes
Ascospores	Spore wall, gelatinous sheath, appendages	Lugol’s iodine, IKI	Reaction site and color type	Auxiliary microscopic identification

5 Common Errors and Control Points

5.1 False Negatives

(1) Immature spores

Immature spores have insufficiently developed wall layers and may not show typical iodine reactions. If mature spores are insufficient in the sample, material should be replaced or a mature hymenial region should be selected again.

(2) Reduced reagent activity

Long-term storage, volatilization, light exposure, or poor sealing of iodine reagents may weaken reactions. If positive controls or typical materials show abnormal reactions, reagent condition should be checked first.

(3) Excessive pretreatment

High-concentration or prolonged KOH treatment may alter cell wall status and weaken or change iodine reactions. Pretreatment conditions should remain consistent and be specified in experimental records.

5.2 False Positives

(1) Reagent precipitates

Iodine reagent precipitates may form dark granules. If these precipitates adhere near spores, they may cause false-positive interpretation. Such particles usually do not distribute regularly along the spore wall or ornamentation.

(2) Pigment interference

Dark spores, aged tissues, or pigment-rich specimens may become generally darker in iodine reagents. This phenomenon should not be directly interpreted as an amyloid or dextrinoid reaction.

(3) Overly thick preparations

Thick preparations darken the entire field of view and may make spores and tissue edges appear dark. Darkness in thick preparations has no clear taxonomic significance.

5.3 Inconsistent Results

(1) Differences in reagent systems

The same structure may behave differently in Melzer’s reagent, Lugol’s iodine, and IKI systems. In hemiamyloid structures of ascomycetes, reagent differences directly influence interpretation.

(2) Differences in treatment sequence

Direct iodine addition, iodine addition after KOH pretreatment, and water-mounted observation followed by iodine addition may all influence color expression. The treatment sequence should be retained in formal records.

(3) Differences in literature terminology

Terms such as I+, IKI+, and MLZ+ may correspond to different reagents and operating conditions in different publications. When comparing results, the specific reagent system should be confirmed.

Table 4. Common Errors in Iodine Reactions of Fungal Spores

Problem type	Common cause	Result pattern	Control point
False negative	Immature spores, reagent failure, excessive pretreatment	Structures expected to be positive show no clear coloration	Use mature material, renew reagents, control pretreatment
False positive	Reagent precipitate, pigment interference, overly thick preparation	Non-structural dark areas	Select clear spores and compare background with structural localization
Difficult weak reaction	Intrinsically weak reaction, unsuitable illumination, thin wall layer	Unstable color change	Observe multiple fields and record cautiously
Inconsistent results	Different reagent systems or treatment sequences	Same structure shows different color patterns	Clearly record reagent and treatment conditions
Localization error	Spore contents or background mistaken for spore wall	Color does not correspond to structure	Confirm reaction site before determining reaction type

6 Reagent Selection for Iodine Staining Reaction in Fungal Spores

Table 5. Basic Iodine Reaction Reagents and Supporting Microscopic Stains

Product category	Product name	CAS No.	Role in the system	Applicable direction
Solubilizing component	Potassium iodide	7681-11-0	Improves iodine solubility in water and stabilizes the system	Preparation of Lugol’s iodine and IKI systems
Pretreatment reagent	Potassium hydroxide	1310-58-3	Softens tissues, reduces pigment background, and improves structural visibility	Rehydration of dried specimens, tissue clearing, pretreatment before iodine reactions
Mounting component	Glycerol	56-81-5	Slows drying of mounts and maintains observation time	Observation of spore morphology and microscopic structures
Structural visualization reagent	Congo red	573-58-0	Enhances visualization of fungal cell wall outlines	Used with KOH to observe hyphae, cystidia, and spore morphology
Mounting/clearing component	Lactic acid	50-21-5	Improves tissue transparency and mounting effect	Lactic acid-based mounting fluids or staining systems
Preservative/clearing component	Phenol	108-95-2	Traditional component of lactophenol cotton blue systems	Fungal microscopic staining; safety protection required

Table 6. Core Reagents and Auxiliary Staining Products for Iodine Staining Reaction in Fungal Spores

Product category	Cat. No.	Product Name	Grade/specification	Role in the system	Applicable direction
Lugol’s iodine	S1506558	Standard Lugol's iodine solution (1%)	BioReagent,Suitable for microbiology,Biological Stain,for microscopy,1%	Provides a low-concentration iodine-potassium iodide color-development system	Observation of fungal spore walls, ascus structures, and weak iodine reactions
Lugol’s iodine	L1508168	Lugol's Staining Solution (5%, Sterile)	BioReagent,Suitable for microbiology,Biological Stain,for microscopy,sterile,5%	Provides a higher-concentration sterile Lugol’s iodine system	Review observation of ascomycete structures, spore walls, and iodine reactions
Melzer’s reagent	M1508164	Melzer's Reagent	BioReagent,Biological Stain,Suitable for microbiology,for microscopy	Used to observe amyloid and dextrinoid reactions	Microscopic identification of basidiomycete spore walls, spore ornamentation, Russulaceae, and Amanita
Routine fungal stain	L774540	Lactophenol Cotton Blue Staining Solution	BioReagent, Biological Stain, for microscopy	Demonstrates fungal hyphae, spores, and sporulating structures	Used with iodine reactions for basic microscopic observation of fungal morphology
Cell wall auxiliary stain	C1516057	Congo Red Staining Solution (1%)	BioReagent,Biological Stain,for microscopy,1%	Enhances outlines of fungal cell walls and certain microscopic structures	Auxiliary observation of hyphae, cystidia, spore outlines, and tissue structures
Cell wall auxiliary stain	M1520403	Methanol Congo Red Staining Solution (0.5%)	BioReagent,Biological Stain,for microscopy,0.5%	Enhances cell wall visualization through the Congo red system	Auxiliary observation of fungal cell walls, spore outlines, and microscopic structures
Mounting reagent	C1508971	Routine Glycerol Mounting Medium	BioReagent,for microscopy,Suitable for Immunofluorescence(IF),for fluorescence analysis	Stabilizes temporary mounts and prolongs observation time	Observation of spore morphology, spore ornamentation, and review of weak reactions
Mounting reagent	P1508974	Polyvinyl Alcohol Glycerol Mounting Medium	BioReagent,for microscopy,Suitable for Immunofluorescence(IF),Suitable for Immunohistochemistry(IHC)	Improves mounting stability and reduces sample drying	Preservation and subsequent review of fungal microscopic structures
Mounting reagent	A1508972	Gum Arabic Glycerol Mounting Medium	BioReagent,for microscopy,Suitable for microbiology,Suitable for Immunofluorescence(IF),Suitable for Immunohistochemistry(IHC)	Provides a relatively stable mounting environment suitable for microbial microscopy	Observation of fungal spore morphology, ornamentation structures, and microscopic image documentation

The iodine reaction of fungal spores is a microchemical character that requires precise structural localization. Reliable interpretation should integrate the reagent system, reaction site, color type, and spore maturity, and should be analyzed together with spore morphology, ornamentation, and the taxonomic context of the fungal group.