AOBPreview published online on December 7, 2006
Annals of Botany, doi:10.1093/aob/mcl246
© The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Desiccation Tolerance in the Moss Polytrichum formosum: Physiological and Fine-structural Changes during Desiccation and Recovery
1 School of Biological Sciences, University of Exeter, Geoffrey Pope Building, Stocker Road, Exeter EX4 4QD, UK
2 Dipartimento di Scienze ambientali, Seconda Università di Napoli, via A. Vivaldi 43, 81100 Caserta, Italy
3 School of Biological Sciences, Queen Mary and Westfield College, University of London, Mile End Road,London E1 4NS, UK
* For correspondence. E-mail j.g.duckett{at}qmul.ac.uk
Received: 12 May 2006 Returned for revision: 8 August 2006 Accepted: 9 October 2006
BACKGROUND AND AIMS: This study explores basic physiological features and time relations of recovery of photosynthetic activity and CO2 uptake following rehydration of a desiccation-tolerant moss in relation to the full temporal sequence of cytological changes associated with recovery to the normal hydrated state. It seeks reconciliation of the apparently conflicting published physiological and cytological evidence on recovery from desiccation in bryophytes.
METHODS: Observations were made of water-stress responses and recovery using infrared gas analysis and modulated chlorophyll fluorescence, and of structural and ultrastructural changes by light and transmission electron microscopy.
KEY RESULTS: Net CO2 uptake fell to zero at approx. 40 % RWC, paralleling the fluorescence parameter 
PSII at 200 µmol m–2 s–1 PPFD. On re-wetting the moss after 9–18 d desiccation, the initially negative net CO2 uptake became positive 10–30 min after re-wetting, restoring a net carbon balance after approx. 0·3–1 h. The parameter Fv/Fm reached approx. 80 % of its pre-desiccation value within approx. 10 min of re-wetting. In the presence of the protein-synthesis inhibitors chloramphenicol and cycloheximide, recovery of Fv/Fm (and CO2 exchange) proceeded normally in the dark, but declined rapidly in the light. Though initial recovery was rapid, both net CO2 uptake and Fv/Fm required approx. 24 h to recover completely to pre-desiccation values. The fixation protocols produced neither swelling of tissues nor plasmolysis. Thylakoids, grana and mitochondrial cristae remained intact throughout the drying–re-wetting cycle, but there were striking changes in the form of the organelles, especially the chloroplasts, which had prominent lobes and lamellar extensions in the normally hydrated state, but rounded off when desiccated, returning slowly to their normal state within approx. 24 h of re-wetting. Sub-cellular events during desiccation and re-wetting were generally similar to those seen in published data from the pteridophyte Selaginella lepidophylla.
CONCLUSIONS: Initial recovery of respiration and photosynthesis (as of protein synthesis) is very rapid, and independent of protein synthesis, suggesting physical reactivation of systems conserved intact through desiccation and rehydration, but full recovery takes approx. 24 h. This is consistent with the cytological evidence, which shows the thylakoids and cristae remaining intact through the whole course of dehydration and rehydration. Substantial and co-ordinated changes in other cell components, which must affect spatial relationships of organelles and metabolic systems, return to normal on a time span similar to full recovery of photosynthesis. Comparison of the present data with recently published results suggests a significant role for the cytoskeleton in desiccation responses.
Key words: Bryophyta, chlorophyll fluorescence, chloroplasts, CO2 exchange desiccation tolerance, electron microscopy, metabolic inhibitors, mosses, Polytrichum formosum
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