Other than the internal symptom of laticifer browning, no anatomical changes
were observed in the dieback-affected tissues examined using bright field microscopy
(in leaf, Fig. 2), compared with corresponding tissues in healthy plants. The
areas of tissue necrosis that characterize dieback are anticipated to demonstrate loss
of cellular membranes, but these regions were not sampled in the current study. In
the TEM study, roughly spherical membrane-bound structures of up to 1,200 nm
diameter were observed in vacuoles of cells within phloem tissue in sections taken
from two blocks. These cells were either phloem parenchyma, immature sieve elements, or immature laticifers. One block involved secondary veins of leaves with
the “X-Y” patterning symptom (Figs. 3 and 4). The content of the membrane-bound
bodies was variable, with some cells containing granular, ribosome-like electron
dense particles (Fig. 3), while other bodies were empty elements observed appeared healthy (not collapsed).