Botany - B.Ye. Yakubenko 2017

Part Two. The System of the Organic World
Chapter V. The System of the Organic World
Laboratory Session Topic 10.27. Phylum Ascomycota (Ascomycota). Class Ascomycetes (Ascomycetes)

Objects. Order Erysiphales (powdery mildews) (Erysiphales). Representatives: powdery mildew, cereal powdery mildew (Erysiphe graminis DC.). Order Hypocreales (Hypocreales). Representative: ergot fungus (Claviceps purpurea Tul.). Order Pezizales (Pezizales). Representative: common morel (Morchella esculenta (L.) Pers.).

Tasks:

1. Examine and study the developmental features of powdery mildew Fungi.

2. Examine and study the developmental cycle features of clavicipitaceous fungi.

3. Examine and study the developmental cycle features of pezizalean fungi.

4. Sketch the individual Stages of the fungal developmental cycles and label their constituent parts.

Equipment and Materials: MBR-1 or Biolam microscopes, magnifiers, prepared slides, forceps, scalpels, living and herbarium material: wheat leaves infected with powdery mildew; oak leaves infected with Microsphaera; preserved germinated sclerotia of Claviceps purpurea, fruit bodies of the common morel, germinated sclerotia of Claviceps purpurea (herbarium specimens), etc.

Method for preparing cereal powdery mildew slide. To prepare the slide, use leaves of wheat or another cereal infected with powdery mildew (Erysiphe graminis). Grayish-felted cushions formed by the intertwined numerous hyphae of the Erysiphe mycelium are visible on their surface; locate the brown or black spherical bodies—closed fruit bodies called cleistothecia. Using a dissecting needle, scrape some fruit bodies into a drop of Water on a Glass Microscope slide. Distribute them evenly with the needle and cover with a coverslip. Place the finished slide on the microscope stage and secure it with stage clips.

Microscopic examination of the Erysiphe graminis slide. First under low and then under high magnification of the microscope, locate the spherical brownish balls. These are cleistothecia. They feature elongated, thread-like appendages extending in all directions from the lower part. If you gently press the coverslip with a needle, you can then observe crushed cleistothecia. Examine them carefully, and you will notice asci protruding from some, containing ascospores. Eight ascospores in the form of oval bodies are visible in each of them. The surrounding wall of the cleistothecium is called the peridium. It resembles a soccer ball. Make a detailed sketch of the fruit body and indicate its marked constituent parts. Separately, sketch a cereal (wheat) leaf and label the areas infected with Erysiphe, the fruit bodies on the felted mycelial cushions (Fig. 76). In addition, find numerous short, simple, colorless conidiophores on the leaf surface. The conidia are colorless, unicellular, and usually arranged in chains. Sketch and label them.

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Figure 76. Powdery mildew fungi:

a — cereal powdery mildew (Erysiphe graminis); b — cleistothecium of Erysiphe; c — cleistothecium of Sphaerotheca;

d — cleistothecium of Microsphaera; 1 — mycelium; 2 — conidia; 3 — peridium; 4 — appendages; 5 — asci; 6 — ascospores

Macroscopic studies of Claviceps purpurea. Examine an ear of rye infected with Claviceps purpurea. Macroscopic examination reveals dark, horn-like structures—sclerotia—standing out among the grains in the spikelet. The formation of sclerotia is preceded by the infection of the flower by the parasite's ascospores. Adhering to the stigma of the pistil, the spore germinates into a germ tube that penetrates the Ovary and forms a mycelium, which produces spherical, unicellular conidiospores. They become enveloped in a sugary liquid, forming honeydew. Honeydew attracts insects that spread the conidiospores and cause secondary flower infection.

During grain filling, the fungus intercepts nutrients and accumulates them in the mycelium within the ovary. As a result, a sclerotium is formed instead of a grain (Fig. 77).

Figure 77. Claviceps purpurea

In fact, a sclerotium is a dense network of fungal hyphae filled with nutrients, serving for its reproduction. The fungus overwinters in the sclerotium stage in the soil or crop residue. In spring, sclerotia germinate into stromata, which consist of a stipe and a reddish spherical HEAD.

Perithecia containing asci form on the periphery of the head; the maturation of ascospores coincides with the flowering of cereals, and The life cycle of the fungus renews.

Macroscopic Study of the common morel. Using fixed material of the fruit body of the common morel, examine its general appearance carefully and sketch it in your album. Note that individual white strands of residual mycelium stand out among the adhered clumps of soil. The fruit body is divided into a rather massive stipe and a wrinkled cap that appears fitted onto it. Numerous dark-brown depressions—apothecia—are clearly visible on the cap. The latter are open fruit bodies of ascomycetous fungi.

Method for preparing the hymenium slide of the common morel. From a bacteriological Petri dish, take a small piece of pre-crushed outer-layer mass of the apothecium onto the tip of a needle. It is best to take the dark brown mass. It is precisely in it that you will find the hymenium. Place this mass into a drop of water on a prepared glass slide. Slightly crush the material mass with a dissecting needle and cover with a coverslip. Place the slide on the microscope stage and secure it with clips.

Microscopic examination of the common morel hymenium slide. On your self-prepared slide, thoroughly study the hymenium of the common morel apothecium. Using low and high microscope magnification, locate an area in the field of view where its elements are best displayed. The hymenial layer is clearly visible as a palisade layer of elongated, cylindrical asci interspersed with paraphyses (sterile filamentous structures). Eight ellipsoidal ascospores are clearly visible in each of them. Part of the Contents of the asci is represented by epiplasm in which the ascospores are embedded. The lower layer of the fungal hyphal network from which the asci and paraphyses develop is called the subhymenium, or subhymenial layer.

Further down, it transforms into a denser layer of mycelial network—the hymenophore (Fig. 78).

This part of the fruit body resembles the tissue of higher plants. However, this is merely a dense network of Cells not united by a common METABOLISM. Therefore, they form a false tissue known as plectenchyma.

Conclusion. The mycelium of most sac fungi is multicellular and septate. As a result of the Sexual process in Ascomycetes, specific receptacles called asci are formed. They typically contain 8, less frequently 4, ascospores. The asci develop either directly on the mycelium or within specialized fruiting bodies—cleistothecia, perithecia, or apothecia. Asexual reproduction occurs via conidiospores. Many ascomycetes are destructive plant pathogens affecting crops (Erysiphe, Microsphaera, Claviceps, Uncinula, Sphaerotheca).

Self-Assessment Questions

1. What is the Structure and ploidy of the mycelium in most ascomycetes?

2. What is the product of the sexual process in ascomycetes?

3. How does asexual reproduction occur in sac fungi? 4. Name the types of fruiting bodies found in ascomycetes.

5. Name the disease caused by the cereal powdery mildew fungus (Erysiphe).

6. Explain The Nature and stages of the Life Cycle of Claviceps purpurea.

7. What type of fruiting body is characteristic of Pezizales?

8. What is the hymenial layer and how does it form?