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

Physiogenic or Nonpathogenic Seed Disorders 2499.1 MINERAL NUTRIENT DEFICIENCYThe deficiency in soil of a number of major or micronutrients leads to the developmentof visible symptoms in seeds (Table 9.1). The symptoms may be exomorphic,caused externally, e.g., yellow berry of wheat, tapering seeds in cucumber, anddistorted seeds of Vicia faba due to the deficiency of nitrogen, potassium, and zinc,respectively, or endomorphic (internal), mostly seen on the adaxial surface of cotyledons,namely marsh spot of peas and beans, and hollow heart in groundnut causedby the deficiency of manganese and boron, respectively. The use of the term hollowheart for the disorder in groundnut is a misnomer since this term is widely used foran anomaly caused in peas due to low atmospheric humidity at the time of thematurity of the crop. Detailed histopathological information is available only on peaseeds showing marsh spot.9.1.1 MARSH SPOT IN PEASMarsh spot is characterized by discolored brown areas in the center of the adaxialface of the cotyledons (Figure 9.1A, affected cotyledons [right] and normal [left]).The disorder was first described by Mansholt in 1894 and later observed in theNetherlands (De Bruijn, 1933), Britain (Lacey, 1934), and Finland (Jamalainen,1936). It is common in peas grown in Romney Marsh, England, and in newlyreclaimed sea silt soils of the Netherlands (Noble, 1960). Singh (1974) detectedmarsh spot in seeds of smooth seeded cultivars, namely Diktron, Marrow (theNetherlands) and Feltham First (Scotland). Manganese deficiency in soil is shownto be the cause of this disorder (De Bruijn, 1933; Löhnis, 1936; Pethybridge, 1936;Heintze, 1938, 1956; Cuddy, 1959), and manganese sulphate as a measure of controlwas established by these authors.Wallace (1951) has figured brown lesion on adaxial surface of cotyledons, similarto marsh spot, in broad bean, harricot bean and runner bean. He also attributes thisdisorder to manganese deficiency of soil.9.1.1.1 HistologyAnatomically the asymptomatic cotyledons consist of parenchyma cells, which, apartfrom the epidermis, are all polygonal (Figure 9.1B). None of the mesophyll layersare of the palisade type. Air spaces occur throughout the mesophyll (Figure 9.1B,C). The size and shape of the nuclei show great variations depending upon the sizeof the cell and the amount of the starch grains. The nuclei are hypertrophied, havingdiffused and granular intranuclear contents. An organized nucleus is rarely seen. Thecells contain abundant simple starch grains (Figure 9.1B, C).The affected cotyledons on the adaxial side reveal that the subhypodermal layers,three to four cells deep, show more serious symptoms of derangement. The cellsand intercellular space are enlarged and the cell contents stained deeply with safranin.The cell walls become broad with granular material beneath. The cell contents,including starch grains, show corrosion and secretion of glistening dark brown oilydroplets, which accumulate in intercellular air space (Figure 9.1D, E).