CLSM-images of living actually translocating sieve
elements
The direction of fluorochrome translocation is from right to left in (A) to (F).
(A) and (B) Translocation of CFDA through intact phloem. In (A) the dye has already passed the image frame in one sieve tube. (B) shows the specimen in (A) 72 sec later. A second sieve tube has become highly fluorescent. The distance traveled by the dye and the intensity of the fluorescence are indicative of the rate of mass flow.
(C) and (D) High magnification of translocating sieve elements obtained by use of a 63X water immersion objective. In (C) CFDA remains restricted to the sieve elements (asterisks) and the companion cells (diamonds). Apparently, CFDA moving through the sieve elements strongly accumulates in the companion cells. The sieve plates (arrows) demarcate the sieve elements. (D) is a deeper optical section through the same phloem bundle. The dye is evenly distributed over the sieve element. The dye accumulates in the vacuoles of the companion cells, exposing darker central (nucleus) and parietal (cytoplasm) regions. Starch grains in the P plastids are visible as minute bright spots.
(E) Intact phloem tissue after 15-min staining with RH-160. No fluorescence-labeled substances have shown up inside the sieve element. Sieve plate pores (arrows) are not occluded. Pore plasmodesm units (arrowheads) between the sieve elements and the adjoining companion cells are stained clearly. Chloroplasts and probably mitochondria are visible in the companion cells.
(F) Intact phloem tissues after 1-hr staining with RH-160. P plastids (arrowheads) are located along the plasma membrane of the sieve element and are evenly distributed. Parietal proteins line the plasma membrane of the sieve element with local thickenings (arrows). A thick layer of parietal protein is deposited onto the sieve plate.
CC = companion cell; SE = sieve element; SP = sieve plate.