Short-term storage of carbohydrate in stem tissue of apple (Malus domestica), a woody perennial: evidence for involvement of the apoplast
Joanna C. McQueen A E , Peter E. H. Minchin B C , Michael R. Thorpe B D and Warwick B. Silvester AA Department of Biology, University of Waikato, Private Bag 3105, Hamilton, New Zealand.
B HortResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand.
C ICG-III Phytosphaere, Forschungszentrum Jülich, D-52425, Jülich, Germany.
D Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
E Corresponding author. Email: mwatton@ihug.co.nz
Functional Plant Biology 32(11) 1027-1031 https://doi.org/10.1071/FP05082
Submitted: 11 April 2005 Accepted: 1 August 2005 Published: 28 October 2005
Abstract
This work investigates the pathway and mechanism for lateral retrieval of carbohydrate into the transport phloem of apple stems (Malus domestica Borkh.). A heat exchanger was set up on the stem, allowing rapid chilling and subsequent re-warming of stem segments while the time course of axial transport of 11C-labelled photoassimilate was measured at a position ∼65 mm downstream of the heat exchanger. Whenever axial transport was blocked by a sudden chill at the heat exchanger, transport 65 mm downstream from the blockage immediately slowed but did not stop, showing that there was retrieval of solutes into the pathway (buffering), within that 65 mm of stem, to help maintain the axial flow. Use of PCMBS, an inhibitor of sugar transporters, showed that the buffering included retrieval of sugar from the apoplast. We concluded that in apple, apoplastic sugar in stem tissue can buffer phloem transport during short-term changes in supply and demand for carbohydrates. Buffering was stronger when mobile reserves in the stem were higher, for example late in the photoperiod, or if carbohydrate demand in the terminal sink was increased. We also suggest that the concentration of sugars in the apoplast is a regulator of carbohydrate storage and re-mobilisation.
Keywords: apoplast, apple, 11C, phloem loading, retrieval, stem buffering, sugar transport, transport phloem.
Acknowledgments
J. McQueen was supported by a Doctoral Scholarship from the Agricultural and Marketing Research and Development Trust. PEHM was supported by PGSF funding: Contract C06X0005. Chris Purcell continued his efficient effective maintenance of the 11C facility of the Institute of Geological and Nuclear Sciences.
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