Nematode Paste in Horticultural Crops and their Management

Alimentary Canal

Alimentary canal of plant parasitic nematodes can be distinguished into 3 zones; the stomodaeum (fore-gut), the mesenteron (mid-gut) and the proctodaeum (hind-gut).
Stomodaeum and proctodaeum are internally lined with cuticle
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(I)Stomodaeum

• It starts at the oral aperture or the mouth which is located terminally and is surrounded by 6 lips. (2 sub-dorsal, 2 sub-ventral and 2 lateral in position).
• The oral aperture leads internally into a cavity known as stoma or buccal cavity.
• Among plant parasitic nematodes, the stomal walls fuse together to form a hypodermic needle-like structure called stylet (stomatostylet or spear) having a fine lumen of less than 1µm.
• The stylet is a protrusible organ and when pushed inside the plant cell, helps in ingestion of the cell cytoplasm through its lumen.
• The movement of the stylet is regulated by stylet protractor muscles.

Figure 5.8 Structure of stomatostylet

The lumen of the stylet is continuous with that of the oesophagus/ pharynx, the next part of the stomodaeum.

• Oesophagus/pharynx is basically a pumping organ which sucks in and pushes the ingested food material into the intestine.
• The structure of the oesophagus is also variable in nematodes of different feeding habits. It may be cylindrical throughout (1 part or uni-partite) as in predatory mononchids or narrow anteriorly and broader posteriorly (2 part or bottle like or bipartite) as in Dorylaimids, or it may have one cylindrical procorpus and two bulbous structures (3 part or tripartite) as in case of most plant parasitic nematodes.

Figure 5.9 A. Unipartite, B. Bipartite, C. Tripartite oesophagi

• The various parts of a tripartite oesophagus are procorpus, meta corpus /median bulb and a narrow isthmus leading to the terminal/ basal bulb. Variations within 3-part oesophagus are seen among different groups of plant parasitic nematodes.
• Procorpus is a narrow anterior portion which enlarges to form a muscular median bulb (meta corpus). Meta corpus is highly muscular and armed with valves. The pulsation of median bulb helps in the ingestion of cell cytoplasm through the stylet into the oesophagus, and further pushing of the ingested food material into the intestine.
• Following this is a narrow zone, the isthmus which is linked to the basal bulb in which three oesophageal glands (1 dorsal and 2 sub-ventrals) are located.
• In plant parasitic tylenchids, the dorsal oesophageal gland opens near the base of stylet in the anterior part of pro corpus.The sub-ventral glands join the oesophageal lumen in the median bulb.
• These glands secrete the ‘saliva’ for the digestion of food.
• In certain nematodes the oesophageal glands are more developed and cannot be accommodated within the basal bulb. In such cases ,these extend (overlap) over the anterior part of the intestine to some distance on any of the various sides i.e. dorsally, ventrally or laterally.
• The oesophageal lumen is triradiate and is lined by cuticle.
• The junction of oesophagus with the intestine is guarded by an oesophageo-intestine valve referred as cardia which ensures the uni-directional flow of food from oral aperture to intestine without regurgitation.

Figure 5.9a The Stomodaeum

Mesenteron:

• It consists of a simple straight tube i.e.the intestine running most of the length of the body.
• The intestine is composed of a single layer of endodermal epithelial cells.
• Internally, these cells bear a layer of finger-like projections (microvilli), which are absorptive and secretory in function.

Proctodaeum

• It consists of a short flat tube called rectum which is lined internally by cuticle.
  
• Rectum opens to the outside ventrally through anus in females.
• In case of males, the reproductive system joins the rectum to form a common tube called cloaca, which opens to the outside through cloacal aperture on the ventral side.