Ultrastructural features of differentiating protophloem sieve elements in adventitious roots of Salix viminalis

The differentiation of the protophloem in 9- to 14-day-old adventitious roots of Salix viminalis was studied. Ultrastructural observations were mainly made on longitudinal serial sections through an uninterrupted file of 32 differentiating sieve elements. The first cell in the file was located about 50 μm from the apical meristem. At an early stage the nucleus was lobed in outline, and in older cells the nucleoplasm became electron lucent. In the first or second cell from the first mature sieve element the nuclear envelope broke open. The nucleoli decreased gradually in size and disappeared finally. From the 9th cell the plastids contained starch and grew somewhat in size. ER increased in amount and began to form stacks in the 20th cell. These stacks moved to a peripheral position. Callose platelets were first observed on the transverse walls in cell 18. Flattened ER-cisternae covered the sieve pore sites. Gradually the middle lamella was dissolved and the callose aggregations formed cylinders around the pores of the sieve plate. Aggregations of tubular P-protein were present from cell 15. P-protein bodies were also present in parenchyma cells adjoining mature sieve elements. The only cell components remaining in mature sieve elements were plastids, mitochondria, stacked ER, the plasmalemma, remnants of other membranes and bodies consisting of P-protein and of an unidentified granular material. The sieve elements had no ontogenetically related companion cells. At a level where both metaphloem and metaxylem had matured the first formed protophloem sieve elements remained intact.