Figure 5.

Light and epifluorescence photomicrographs of phloem plugging and collapse. Phloem plugging by callose deposition was marked by the light blue arrows with the dark blue or black densely staining spots lying toward the inner side of sieve tubes. The darkly stained strip resembling a solid barrier oriented tangentially, radially, or irregularly between adjacent phloem cells represented phloem collapse (indicated by yellow asterisks). (A,B) Valencia sweet orange, through 40× objective lens, (C,D) Sugar Belle mandarin, through 20× objective lens. Note a high accumulation of starch in the parenchyma cells and phloem cells. Scale bars = 20 and 50 μm.