Figure 1.

The Ultrastructure of Intact and Exuding Hypocotyl Phloem of Castor Bean.

(A) Control. SE with peripheral organelles, sieve plates (arrows), and a CC strand with five CCs, the cytoplasm of which is slightly denser than that of neighboring cells and contains spherical nuclei with prominent nucleoli.

(B) to (D) Exuding phloem.

(B) SE 1.15 mm from the exudation cut with distal sieve plate (arrow) and three CCs bulging out toward the SE and showing retraction of the plasma membrane from the cell wall (asterisk).

(C) Proximal sieve plate of the SE in (B) with moderate callose (white cell wall areas) and open pore channels, loosely filled with P-protein filaments (arrowhead). Bar = 1 μm.

(D) CC 0.25 mm from the exudation cut, with dense cytoplasm, vesiculating endoplasmic reticulum (er), mitochondria (m) that are less dense than the cytoplasm, and an elongated and invaginated nucleus (n). Bar = 1 μm.

All experiments were performed with 5-d-old castor bean seedlings. The cotyledons, but not the rest of the seedling, were incubated in buffered sucrose for 4 h. This preincubation provides a steady state in internal sucrose concentration important for phloem loading and export from the cotyledons. Controls: 10-mm longitudinal hand sections of hypocotyls were taken at the hook and fixed with paraform and glutaraldehyde according to Orlich et al. (1998). To exclude preparation artifacts, the apical and basal 2.5-mm ends were discarded before embedding. Exuding seedlings: The hypocotyls were cut in the hook and exudate collected with a microcapillary every 5 min. Well-exuding hypocotyls were fixed like the controls, but without discarding the basal end at the exudation cut.