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

Structure of Seeds 49(1971) of using an occular micrometer in a stereoscopic microscope or a Lufkinpocket slide caliper appear useful. Apparently, large seeds provide more exposedspace for microorganisms as compared to small seeds, but there are no data to supportsuch a conjecture.3.2.4 SURFACEThe surface of the seed coat may be smooth or sculptured. Various types of ornamentationsuch as wrinkles, ridges and furrows, striations, reticulate, punctate,tuberculate, hairy, or spinescent, have been recorded. The surface of the seed coatis covered by the cuticle, a waxy, fatty hemicellulose or pectinaceous substance. Itis thin or thick. The waxy coating may appear as irregular particles, crystalloidrodlets, filaments, flakes, or plates (Barthlott and Wollenweber, 1981). The gloss ofthe seed surface is due to the waxy coating, and the thin, uniform wax coveringmakes the seed surface shiny in fabaceous seeds.The micromorphology of the seed coat surface described using SEM providesgreater detail of sculpturing pattern, including the presence of minute hairs,micropores, cracks, and deposits (Figures 3.1 A through E). It has been studiedextensively for soybean seeds (Wolf, Baker, and Bernard, 1981; Yaklich, Vigil, andWergin, 1984). The surfaces of seed coats of different cultivars of soybean havebeen classified into three types: smooth, with pores (Figure 3.1A), and with a distincttype of waxy deposit called blooms. The pores of seed coat surfaces in the cultivarsWilliams, Guelph, Hoosier, and Jogan vary in number, size, and shape among thefour cultivars. The seed coat surfaces in the cultivars Old Dominion, Laredo, Barchet,and Sooty are without pores (Figure 3.1B) (Kulik and Yaklich, 1991). The surfacesin Barchet and Sooty haves wax deposits called bloom (Figure 3.1C, D) believed tobe derived from the endocarp layer of the pod (Newell and Hymovitz, 1978; Wolf,Baker, and Bernard, 1981).Stomata on the seed surface have been reported in seeds of 19 families, mostwith endotestal or exotegmic seed coats. The presence of stomata is not known tobe a constant feature of any family. Stomata are absent when the exotesta is madeup of a compact palisade layer as in Fabaceae. Among the families with crop plants,stomata are found on the seed surface of Malvacae, Euphorbiaceae, and Papaveraceae.In cotton seed there is greater aggregation of stomata in the chalazal region.The pericarp is photosynthetic and commonly bears stomata. Cochrane and Duffus(1979) observed stomata in the pericarp epidermis on the ventral side at the apicalend in some cultivars of wheat and barley. The lemma and palaea in Oryza (Azegami,Tabei, and Fukuda, 1988), wheat, and barley (Fukuda, Azegami, and Tabei, 1990),bear stomata in outer as well as inner epidermis.Some of the above features of seed surface such as pores, stomata, and fissuresare directly correlated with the functional attributes of seeds. Seed coats with poresare generally permeable (Calero, West, and Hinson, 1981; Wolf and Baker, 1980;Wolf, Baker, and Bernard, 1981). Kulik and Yaklich (1991) report that seeds ofsoybean cultivars that lack pores on the seed coat have a low rate of infection ofPhomopsis phaseoli as compared to those with multiple pores.