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

48 Histopathology of Seed-Borne Infections3.2 EXOMORPHIC FEATURESVariations in exomorphic seed characters are considerable and many of them areimportant with respect to the functional attributes of seeds. With the developmentof seed technology during and after the 1940s, several publications with characteristicsand drawings of seeds have appeared (Korsmo, 1935; Murley, 1944, 1946,1951; Isley, 1947; Martin, 1954; Brouwer and Stahlin, 1955; Heinisch, 1955;McClure, 1957; Dobrokhotov, 1961; Musil, 1963; Breggren, 1969). The U.S. Departmentof Agriculture (USDA) and some state agriculture departments have publishedseed handbooks (USDA, 1948, 1952, 1961; Bellue, 1952; Britton and Fuller, 1957).In the Seed Identification Manual, Martin and Barkley (1961) included 824 photographsof seeds of more than 600 plant species occurring in the United States. Gunn(1970a,b, 1971) provided seed characters and drawings of seeds of native andnaturalized Vicia spp.The common external features of seeds concern seed color, shape, size (length,width, and thickness), surface features, and the size, shape, and position of hilum,micropyle, and raphe. Other recognizable features, when present, are the appendages,e.g., wings, pappus, aril, caruncle, elaisome, spines, tubercles, and hairs. The mainfeatures of seed are fairly constant for a species; however, variations occur in seedsof different cultivars or clones, but such variations are not marked within the cultivar.3.2.1 COLORSeeds may be monochrome or marked by points, mottles, and streaks. White, brownand brown derivatives, and black are by far the most common seed colors. Othercolors, such as red, green, yellow, and double colors (e.g., the red and black of Abrusprecatarious), are infrequent.Seed pigment is reported to impart slight resistance to fungal pathogens. Singhand Singh (1979) have reported that the white seeds of sesame are most susceptibleto Macrophomina phaseolina, the brown seeds showed weak incidence, and noneof the black seed samples carried the infection. Glueck and Rooney (1978) foundthat kernel pigmentation in sorghum provides resistance to head mold caused byCurvularia lunata and Phoma sorghina. Stasz, Harmon, and Marx (1980) also foundthat fewer Pythium ultimum hyphae developed on the surface of colored seeds of pea.3.2.2 SHAPECommon seed shapes are spherical, subspherical, oblong, oval, ellipsoid, sublenticular,subpyramidal, cuboid, subcuboid, and reniform. Seeds may be turgid or compressed.3.2.3 SIZESeeds vary immensely in size from dustlike particles to large coconuts and doublecoconuts. The feature is not very reliable, but seed length, width, thickness, and theratio between length and width may be used in seed identification at the species andsubspecies levels. It is difficult to determine the size of small seeds; Gunn’s methods