ANATOMICAL, MORPHOLOGICAL AND HISTOCHEMICAL FEATURES OF MONARDA DIDYMA L. LEAVES IN RELATION TO FUNGAL PATHOGEN INFECTION
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This study investigates the anatomical, morphological, and histochemical responses of Monarda didyma L. leaf blades to natural fungal infections under introduction conditions on the Southern Coast of Crimea. Plant material was sampled from collection plot of the Nikita Botanical Gardens in July 2024–2025. Healthy and infected leaves were examined using stereomicroscopy, light microscopy and histochemical tests. Morphometric parameters of the leaf lamina were quantified and statistically compared between healthy and diseased leaves across two seasons. Pathogen identification was performed via morphological analysis and PCR.
The overall dorsiventral leaf structure (single-layered epidermis, one-row palisade and 3–4-row spongy mesophyll, collenchyma, and collateral vascular bundles) remained conserved regardless of infection. However, the character and extent of anatomical changes strongly depended on the dominant pathogen’s trophic strategy. In 2024, under dominance of the necrotrophic pathogen Alternaria alternata (Fr.) Keissl., infected leaves showed pronounced compensatory hypertrophy, with significant increases in central vein height and lamina thickness, epidermal height, palisade and spongy mesophyll height, and the number of vascular elements. No additional accumulation of lignin was detected histochemically, consistent with the opportunistic nature of this pathogen.
In contrast, in 2025, when biotrophic powdery mildew fungi (Golovinomyces biocellatus (Ehrenb.) V. P. Heluta) predominated, anatomical alterations were more localized and moderate, affecting primarily lower epidermal height and palisade mesophyll width. Histochemical analysis revealed localized lignification of individual mesophyll cells, indicating activation of targeted structural defense responses against haustorial pathogens.
These results indicate that (1) the trophic strategy of the dominant pathogen is associated with distinct anatomical and histochemical response patterns in M. didyma, (2) the observed tissue hypertrophy may represent a compensatory response aimed at maintaining leaf structural integrity and photosynthetic function under necrotrophic infection, and (3) localized lignification of individual mesophyll cells constitutes a targeted structural defense against biotrophic pathogens. Overall, M. didyma exhibited relatively high structural resilience to the opportunistic necrotroph, while displaying more specific inducible defenses against the obligate biotrophic pathogen.
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