Ultrastructural studies of protophloem sieve elements in Gossypium hirsutum

Abstract

The differentiation of sieve element protoplasts was studied in the root protophloem of Gossypium hirsutum (cotton). The nuclei degenerate, but frequently remain into late stages of sieve element maturation as darkly stained amorphous aggregates. At the beginning of nuclear degeneration the chromatin aggregates, usually near the envelope, and becomes a composite of darker and lighter stained granular material. Membranous material of unknown origin is also frequently associated with the chromatin. The protophloem sieve element nucleolus is smaller than nucleoli in adjacent parenchyma cells and persists into the intermediate stages of nuclear degeneration. When the nuclear envelope is still intact, the endoplasmic reticulum (ER) cisternae form a small number of stacks, some of which may lie near the nucleus. The peripheral regions of protophloem sieve elements are conspicuously deficient in aggregated ER. Convoluted membranous material, apparently of ER origin, accumulates near the nuclear envelope. Protophloem sieve element plastids lack the characteristic starch grains in early stages of differentiation, but usually develop one or more grains at a somewhat later stage in differentiation. Mitochondria decrease in density as the cell matures and frequently become associated with ER. Plastids and mitochondria become incorporated in the stacked and convoluted ER that accumulates near the nuclear envelope. In mature sieve elements the plastid starch grains become fissured and coarsely granular.