The anatomy of the pathway of sucrose unloading within the sugarcane stalk
Kerry B. Walsh A C , Russell C. Sky A and Sharon M. Brown A BA Plant Sciences Group, School of Biological and Environmental Sciences, Central Queensland University, Rockhampton, Qld 4702, Australia.
B Current address: School of Land and Food, University of Queensland, Brisbane, Qld 4072, Australia.
C Corresponding author. Email: k.walsh@cqu.edu.au
Functional Plant Biology 32(4) 367-374 https://doi.org/10.1071/FP04102
Submitted: 2 June 2004 Accepted: 9 March 2005 Published: 26 April 2005
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.
Keywords: apoplastic movement, plasmodesmata, sucrose unloading, sugarcane, symplastic movement.
Acknowledgments
Funding support from SRDC (grant UCQ 1S) and CQU, and the input of Dr V Shepherd with light microscopy, and Dr S Stowe, RSBS EM Unit, ANU with freeze–fracture work is gratefully acknowledged. We thank J Wilson, now retired, from CSIRO, for editorial input. An abridged version of this study was presented at the SUGAR2000 Symposium (Brisbane, 1996), and published within the symposium proceedings, ‘Sugarcane: research towards efficient and sustainable production’ (Eds JR Wilson, DM Hogarth, JA Campbell, A Garside). pp. 105–107. (CSIRO Division of Tropical Crops and Pastures).
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