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

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Physiogenic or Nonpathogenic Seed Disorders 257of bean and pea showing abnormalities due to drought. The cotyledons showedtransverse cracks resembling mechanical damage (Neergaard, 1979). Affected seedsshowed poor emergence, and the seedlings were very weak. Dickson (1973)attributes this disorder to a combination of factors, e.g., the genotype, the rate ofseed drying at maturation, and imbibition rate at germination. According to Dickson,transverse cracking in cotyledons is common in most Phaseolus bean cultivars andcan occur as much as 100% in some cultivars.Jain (1984) observed cracking of cotyledons in chickpea. The disorder wasobserved both in desi and kabuli types. Cultivars of desi chickpea had higherincidence of seeds with cracked cotyledons than those of kabuli. Cracks are transversebreaks extending partially or completely across the cotyledons and usuallyoccur in both the cotyledons and rarely in one of them only.9.2.2 HIGH HUMIDITY EFFECTSThe high air humidity in the field at the time when the crop is close to maturity orhigh humidity during storage adversely affect the viability of the seed and also causeseedling abnormalities (Neergaard, 1979). In Denmark, such environmental conditionscause gray discoloration in seeds of radish (Neergaard, 1945) and white mustard(Jørgensen, 1967). From a comparative anatomical study of normal and gray discoloredwhite mustard seeds, Jørgenson (1967) demonstrated that the direct causeof the discoloration is swelling of the subepidermal parenchyma of the seed coat.This swelling of subepidermal layers results in the distortion and breaks in theepidermis.9.3 CONCLUDING REMARKSThe deleterious effects of disorders, such as marsh spot, hollow heart, cracking ofcotyledons, and physiological necrosis in cotyledons, are well known. They resultin delayed germination, poor root system, reduced size of seedlings, and poorseedling growth (Löhnis, 1936; Pethybridge, 1936; Perry and Howell, 1965; Perry,1967; Harrison and Perry, 1973; Gane and Biddle, 1973; Jain, 1984; Dempsey andHarrington, 1951; Bass, 1970; Smith, 1989). Effects of marsh spot causing deadplumule tips are more severe than those of hollow heart (Perry and Howell, 1965).The histology of marsh spot-affected cotyledons shows distinctive features of disturbedbiochemical or metabolic functioning. Loosening of the cell wall, dissociationand discontinuity of plasma membrane, degradation of cytoplasm, fretted proteinbodies, corroded starch grains, and secretion of oily pigmented material, whichaccumulate in enlarged intercellular air spaces, reveal profound differences in structuresin normal cotyledons (Singh and Mathur, 1992). The degradation of the membranesystem and macromolecules might account for the poor performance ofaffected seed. What triggers the new biochemical activity in peas and how is itrelated to manganese deficiency needs to be understood.The histology of hollow-hearted cotyledons shows poor cell contents, particularlyreserve materials. These cells seem to lose more water during drying and onimbibition and take more time to recover, causing concavity. Don et al. (1984)
258 Histopathology of Seed-Borne Infectionsconcluded that irrespective of germination conditions, hollow heart is related to thequantity of deteriorated tissues near the center of the adaxial surface of cotyledonsin ungerminated seeds. The question whether these cells are dead as suggested byMoore (1964) and Perry and Howell (1965) has not been confirmed by Singh (1974).The seed development and maturation, including the development of the embryo,comprise an early phase of cell division, expansion, and differentiation, and a periodof physiological maturation in which the deposition of reserve materials takes place.During the last few days or hours, this is a simple drying process, not accompaniedby accumulation of nutrient reserves. The deposition of reserve materials, whichtakes place in the endosperm or embryo, or both, is usually progressive fromperiphery to center. In the case of pea seed, where the endosperm is negligible, itoccurs in the embryo. Premature drying of seeds will imply a cut in the storagephase, which is bound to affect cells of the adaxial central region of the cotyledonsmore than those of other parts.The anatomical study of necrotic lettuce cotyledons (Smith, 1989) has alsobrought out a visible manifestation of membrane damage, such as breaks, myelinlikefigures adjacent to the plasmalemma, withdrawal of the plasmalemma from thecell wall, irregular nuclei with dilated membrane of the envelope, fretted nature ofprotein bodies, and fusion of lipid droplets. Smith (1989) remarked that the subcellularmechanism of deterioration in cotyledons appears no different from thatreported in root tips of aging seeds (Villiers, 1973; Smith, 1978). However, accordingto Smith (1989), it needs to be established if lipid peroxidation is involved in thisprocess.The histological studies of symptomatic parts of structures showing physiogenicdisorders, particularly ultrastructural observations, provide clear evidence of structuralchanges and disturbed biochemical functioning. Histochemical and experimentalstudies may provide further evidence on the mechanism of these deteriorations.REFERENCESAgrios, G.N. 1988. Plant Pathology, 3rd ed. Academic Press, San Diego.Allen, J.D. 1961. Hollow heart of pea seed. N.Z. J. Res. 4: 286–288.Bass, L.N. 1970. Prevention of physiological necrosis (red cotyledon) in lettuce seed (Lactucasativa L.). J. Am. Soc. Hort. Sci. 95: 550–553.Bennett, W.F. 1993. Nutrient Deficiencies and Toxicities in Crop Plants. The AmericanPhytopathological Society, St. Paul, MN.Cuddy, T.F. 1959. Marsh spot in peas. Proc. Assoc. Off. Seed Analysts N. Am. 49: 156–158.Cuddy, T.F. and Lyall, L.H. 1959. Spotted cotyledons of lettuce. Proc. Assoc. Off. SeedAnalysts N. Am. 49: 103–106.Darley, E.F. and Middleton, J.T. 1966. Problems of air pollution in plant pathology. Ann. Rev.Phytopathol. 4: 103–118.De Bruijn, H.L.G. 1933. Kwade harten van der erwten. Tijdschr. Plant Ziekt. 39: 281–318.Dempsey, W.H. and Harrington, J.F. 1951. Red cotyledons of lettuce. Calif. Agric. 5: 4.Dickson, M.H. 1973. Selection of transverse cracking resistance in beans. Ann. Rep. BeanImprovement Coop. 16: 21–22.
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Histopathology ofSeed-Borne Infecti
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PrefaceThe book deals with only one
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The AuthorsDalbir Singh, Ph.D., for
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ContentsChapter 1 Introduction ....
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4.3.2 Routes for Infection from Ova
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8.4 Survival in Seed ..............
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2 Histopathology of Seed-Borne Infe
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4 Histopathology of Seed-Borne Infe
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2Reproductive Structuresand Seed Fo
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Reproductive Structures and Seed Fo
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14 Histopathology of Seed-Borne Inf
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3Structure of SeedsThe seed habit i
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Structure of Seeds 49(1971) of usin
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Structure of Seeds 51is found in as
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Structure of Seeds 53dry seed of Ly
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Structure of Seeds 553.3.2 SEED COA
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Structure of Seeds 57Endosperm: One
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Structure of Seeds 59scendembepsmuA
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Structure of Seeds 61Endosperm: Sca
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Structure of Seeds 63Chalaza simple
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Structure of Seeds 65stystypmerA Bc
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Structure of Seeds 67epsent endCADB
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Structure of Seeds 69perhscendembpe
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Structure of Seeds 71eposgepi′epo
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Structure of Seeds 73(Zea and Sorgh
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Structure of Seeds 75Baker, D.M. an
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Structure of Seeds 77Maheshwari Dev
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Structure of Seeds 79Vries, M.A. de
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TABLE 5.6Location of Mycelium of St
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7Seed Infection by VirusesViruses a
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Seed Infection by Viruses 201TABLE
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Seed Infection by Viruses 203TABLE
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Seed Infection by Viruses 205Unlike
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Page 232 and 233: Seed Infection by Viruses 2177.4.2
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