Ultrastructure of the cuticular membranes of the developing wheat grain

The distribution of cuticular membranes within the mature wheat ovule (Triticum aestivum L. cv. Heron) and the subsequent changes in these layers during grain development were investigated at the ultrastructural level. At anthesis, the inner epidermis of the pericarp and the outer integument are separated by two, thin, osmiophilic, cuticle-like lamellae. Similar osmiophilic boundaries, also double in nature, are evident between the outer and inner integuments and between the inner integument and the nucellar epidermis. By 7 days after anthesis, only the outer layer of cells of the inner integument and the nucellar epidermis are bounded by cuticularized layers, which, by this time, are recognizable as the developing outer and inner cuticles, respectively, of the mature caryopsis. Between 7 and 14 days after anthesis, the outer cuticle assumes a highly reticulate appearance in section and is delineated on the outside by a cutin layer, 90–120 nm in thickness. By 17 days both the outer and inner cuticles have attained maximum thickness, varying from 3.0 to 3.6 μm and 0.8 to 1.1 μm, respectively. Unlike the outer cuticle, the inner cuticle has no distinct internal substructure. The two are closely appressed by 4 weeks after anthesis and show no further ultrastructural changes.