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

74 Histopathology of Seed-Borne Infections3.5 CONCLUDING REMARKSSeed (sensu lato) includes true seeds as well as one-seeded dry indehiscent fruits.A viable seed has a protective covering, embryo, or germ and reserve food material,which may be stored separately or in the cotyledons of the embryo. The color, shape,size, and surface features of seeds vary considerably. The micropyle may be closedor open; the hilum varies in size, shape, and position. The organization of the hilumprecedes seed maturity and partial abscission observed during seed development.The size, shape, and position of embryo in seed are variable. The axile embryo maybe surrounded by endosperm or lie below the seed coat, while the peripheral embryo,despite copious endosperm, has close contacts with the protective covering. Theseed coat in true seeds and pericarp in one-seeded fruits have thick-walled mechanicaltissue and often in colored seeds pigmented phenolic compounds. The seedstructure is variable in different taxa, but broadly follows a common pattern in aparticular family.The features of seed, particularly the nature of cuticles, position and natureof mechanical tissue, position of embryo, nature of micropyle (open or closed),and features of the seed surface (micropores, stomata, hairs, cracks, and waxydeposits) play an important role in imposing functional attributes of seeds. Theyact as preformed passages or barriers to water absorption and infection by microorganisms.The seeds may show di- or polymorphism with respect to the above microfeatures.Bhattacharya and Saha (1992) have identified two categories of seeds inCassia tora: seeds with (1) smooth surface and closed micropyle and (2) roughsurface with pores and micropyle open. Seeds of the first category are dormant,while those of the second category are nondormant. Similarly, Russi et al. (1992)observed differences in cuticle thickness (thin and thick), which accounts for differencesin seed dormancy. Thickness of the cuticle, light line, palisade cells, andpresence or absence of pores and closed or open micropyle are features that havebeen found to affect water permeability and as infection by microorganisms infabaceous seeds (Marouani, 1990; Kulik and Yaklich, 1991).The presence of pigmented contents, phenols, or phenol-like substances in seedcoat and pericarp may impart resistance to infection by providing biochemicalbarriers (Glueck and Rooney, 1978; Singh and Singh, 1979). In Helianthus annuus,phytomelanin in the pericarp reduces insect predation (Carlson and Witt, 1974;Rogers and Kreitner, 1983).REFERENCESArora, K. 1976. Morphological and Embryological Studies in Umbelliferae. Ph.D. thesis,University of Rajastha, Jaipur, India.Artschwager, E. 1927. Development of flower and seed in the sugar beet. J. Agric. Res. 34:1–25.Azegami, K., Tabei, H., and Fukuda, T. 1988. Entrance into rice grains of Pseudomonasplantarii, the causal agent of seedling blight of rice. Ann. Phytopathol. Soc. Japan54: 633–636.