Structure and Histochemistry of Embryo Envelope Tissues in the Mature Dry Seed and Early Germination of Phacelia tanacetifolia

Abstract

The embryo envelope tissues in both mature dry seed and during early germination of Phacelia tanacetifolia were investigated by bright-field and fluorescence light microscopy and scanning electron microscopy. The ruminate seed had an irregularly reticulate surface owing to the presence of polygonal areas, corresponding to the cells of the seed coat. The raised margins of these cells joined at the lobe tips, where radially arranged thickenings occurred. The unitegmic seed coat was made up of three distinct layers: the frayed outer layer, the middle layer with portions rising outwards to form the radial thickenings, and the inner layer, the thickness of which was greatest in the micropylar zone. The endosperm tissue had two regions, the micropylar and the lateral endosperm, which differed in polysaccharide composition, thickness and metachromasy intensity, and presence (in the lateral endosperm) or absence (in the micropylar endosperm) of birefringence of the cell walls. Moreover, in the micropylar region, where the embryo suspensor remnant was found, Ca-oxalate crystals were scarce or absent. The presence of a partially permeable cuticle covering the seed endosperm was observed. Incubation of seeds in Lucifer Yellow CH indicated that water was able to penetrate quickly into the seed coat along the pathway formed by the radial thickenings, the raised margins of the polygonal cells and the middle layer. Afterwards, LY-CH readily infiltrated the apical portions of the seed lobes and then the whole endosperm. Following imbibition, morphological changes were found in the micropylar endosperm, such as the initial digestion of protein bodies. In addition, both in the seed coat and in the endosperm, a weaker fluorescence, probably due to leaching of polyphenolic substances, was observed. Once the seed coat was broken at the micropylar end of the seed, the endosperm cap surrounding the radicle tip had to be punctured by it so that complete germination could occur. Weakening and rupture of the micropylar endosperm are briefly discussed.