SYMPLASMIC CONSTRICTION AND ULTRASTRUCTURAL FEATURES OF THE SIEVE ELEMENT COMPANION CELL COMPLEX IN THE TRANSPORT PHLOEM OF APOPLASMICALLY AND SYMPLASMICALLY PHLOEM-LOADING SPECIES

The ultrastructural features of the sieve element/companion cell compl exes were screened in the stem phloem of two symplasmically loading (s quash, [Cucurbita maxima L.] and Lythrum salicaria L.) and two apoplas mically loading (broad bean [Vicia faba L.] and Zinnia elegans L.) spe cies. The distinct ultrastructural differences between the companion c ells in the collection phloem of symplasmically and apoplasmically phl oem-loading species continue to exist in the transport phloem. Plasmod esmograms of the stem phloem showed a universal symplasmic constrictio n at the interface between the sieve element/companion cell complex an d the phloem parenchyma cells. This contrasts with the huge variation in symplasmic continuity between companion cells and adjoining cells i n the collection phloem of symplasmically and apoplasmically loading s pecies. Further, the ultrastructure of the companion cells in the tran sport phloem faintly reflected the features of the companion cells in the loading zone of the transport phloem. The companion cells of squas h contained numerous small vacuoles (or vesicles), and those of L. sal icaria contained a limited number of vacuoles. The companion cells of broad bean and Z. elegans possessed small wall protrusions. Implicatio ns of the present findings for carbohydrate processing in intact plant s are discussed.