The anatomy of the pathway of sucrose unloading within the sugarcane stalk

Abstract

The physical path of sucrose unloading in the sugarcane stalk is described. About 50% of the vascular bundles in the internodes were located within 3 mm of the outside of the stalk. These bundles were inactive in long distance sucrose transport, as assessed by dye tracers of phloem flow. A sheath of fibres isolates the phloem apoplast from that of the storage parenchyma. In bundles associated with long distance transport (i.e. in the central region), the fibre sheath is narrowest to either side of the phloem fibre cap, and consists of living cells with plasmodesmata within pits in the secondary wall. Plasmodesmata were also arranged into pit fields between cells of the storage parenchyma. Since the vascular apoplast is isolated from the apoplast of the storage parenchyma, sucrose must move through the symplast of the fibre sheath. The calculated flux of sucrose through plasmodesmata of this cell layer was at the low end of reported values in the literature. Sucrose unloading within the storage parenchyma may also follow a symplastic route, with unloading into the apoplast of the storage parenchyma occurring as part of a turgor mechanism to increase sink strength.

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…Therefore, Suc must take an entirely symplastic route from the phloem into the storage parenchyma cells, where it is exported to the apoplast. Fluorescent dye tracer studies support this transport path (Rae et al, 2005;Walsh et al, 2005).…”

“…To prevent apoplastic backflow of Suc to the phloem, the stem veins are surrounded by sclerenchyma cells, which contain suberin and lignin in their cell walls (Walsh et al, 2005). This produces an apoplastic barrier to solute movement, as shown by the fact that xylem sap contains no Suc (Welbaum et al, 1992).…”

Non-Invasive Monitoring of Sucrose Mobilization from Culm Storage Parenchyma by Magnetic Resonance Spectroscopy

“…12,13) Sugarcane vascular bundles are surrounded by an impermeable sclerenchymatous cap that limits the apoplasmic movement of solutes between storage parenchyma and vascular bundles. 14) Hence, the movement of sucrose through these compartments is facilitated by symplasmic connections.…”

An investigation of sugarcane nitrogen physiology: sources, uptake, and metabolism

“…Sucrose moves from phloem to storage cells via a symplastic path (Walsh et al 1996). While one-half of the vascular bundles are located in the outer 3 mm of stem cross section, it is only the vascular bundles of the cenfral region that are involved in long distance sucrose fransport (Walsh et al 1996).…”

“…Sucrose and other sugars are stored in stem parenchyma cells (Welbaum et al 1992;Walsh et al 1996) and in the intercellular spaces (Dong et al 1994). …”