Ultrastructures of phloem and its surrounding parenchyma cells of cassava fibrous and tuberous root

The transmission electron microscopy technology was used to investigate the ultrsastructure of cassava(Manihot esculenta Crantz) fibrous and tuberous root.This research aims to provide cytological evidences for cassava phloem unloading.The results show that there are a few plasmodesmata(PD) at the primary fibrous root stage,with bigger intercellular space,plasma membrane invagination and the other characteristics of apoplasmic unloading pathway.Companinon cells of tuberous root often have plenty of organelles and contain heavily fragmented vacuoles.The number of PD increases evidently,and plenty of starch granules are accumulated in the parenchyma cells,and it is the densest in the maturity stage.The starch granule is also accumulated in the secondary fribrous root,but it is very little,which may be due to the break off of its swelling ability.The PD found in this stage is similar to that in the primary fribrous root.It can be concluded that the PD is associated with starch accumulation.