Histopathology of Seed-Borne Infections - Applied Research Center ...

Physiogenic or Nonpathogenic Seed Disorders 251A comparative account of ultrastructure of asymptomatic and symptomatic cotyledonsis given by Singh and Mathur (1992). The cell walls are uniformly thickand more or less smooth in asymptomatic cotyledons. The fibrillar material is clear,and fine plasmodesmatal connections occur across the cell wall. The plasma membranelies close to the primary wall, and the cytoplasmic net is well organized,showing cell organelles such as endoplasmic reticulum, mitochondria, vacuoles,ribosomes, protein bodies, and starch grains. The aggregation of endoplasmic reticulum,mostly rough endoplasmic reticulum, and mitochondria is much more in theperipheral cytoplasm. The protein bodies vary in size and are deeply stained(Figure 9.2A).In symptomatic cotyledons the cell walls become broad, showing looseningof fibrillar material, and they are prominently sinuate with well-developed plasmodesmatalchannels connecting adjoining cells and the intercellular spaces(Figure 9.2B). The plasma membrane is deeply invaginated. The ribosomes andmitochondria are disfigured. The protein bodies are membrane bound, but the depositionof reserve material is poor and vacuolation in these bodies produces bizarreshapes (Figure 9.2B). Initially, the cell contents do not reveal the occurrence of oildroplets, but their appearance in the intercellular spaces is indicative of secretion.The starch grains are greatly corroded.Increased derangement leads to severe subcellular effects. The cell walls showfurther loosening of fibrils, broad plasmodesmatal channels, and accumulation of alarge amount of granular material beneath the cell wall (Figure 9.2C). Oily materialoccurs interspersed among these granules. The plasma membrane is dissociated fromthe cell wall and shows prominent breaks. The cytoplasmic net shows disintegration,small fragments of endoplasmic reticulum randomly dispersed, weakening or breakdownof the unit membrane around the cell organelles, and protein bodies. Theremains of protein bodies show great variation in shape (Figure 9.2B). Air spacesare greatly enlarged and show accumulation of a large amount of pigmented oilysecretion (Figure 9.2D).9.2 HUMIDITY EFFECTSClimatic stresses, particularly humidity, low or high, cause visible symptoms inseeds of crop plants (Table 9.1). The symptoms may be exomorphic, e.g., gray seedsof radish and white mustard, or endomorphic, discoloration (lettuce), transversecracks (pea and chickpea) and cavitation on adaxial face (pea and chickpea) ofcotyledons. Histology of hollow heart in pea and gray discolored seeds in whitemustard is known (Perry and Howell, 1965; Singh, 1974; Jørgensen, 1967).9.2.1 LOW HUMIDITY EFFECTS9.2.1.1 Hollow HeartHollow heart or cavitation (Figure 9.3A) on the adaxial face of cotyledons of pea(Pisum sativum) was first observed by Myers (1947, 1948) in seeds from NewZealand, Australia, and the United States. Perry and Howell (1965) examined more