| ||||||||||||||||||||
Rapid responses rule out repair
Many mosses have a remarkable capacity to withstand extreme desiccation and to recover rapidly when re-watered. How they do this is, as discussed by Proctor et al. (Exeter and London, UK and Napoli, Italy, pp. 75–93), a matter for debate. Do cellular components survive desiccation intact, thus remaining ready for reactivation when plants are re-watered (the 'constitutive' model) or do cell membranes and organelles suffer desiccation-related damage and thus need repairing when cells are re-hydrated (the repair model)? The authors set out to resolve this debate with their very thorough work on Polytrichum formosum; here, space permits discussion of only the main features. Chlorophyll fluorescence ratios were found to return to 80 % of control values within 10 min and to 95 % within 1 h. Full recovery took about 24 h. Initial recovery of overall quantum yield and CO2 fixation was slightly slower. Nevertheless, CO2 uptake re-started within 10 min of re-wetting; net carbon gain was achieved within 20–60 min. However, as with chlorophyll fluorescence, CO2 uptake did not fully recover for approx. 24 h. Nevertheless, the rapidity and extent of the first recovery phase suggests there is no need for repair. The authors'; careful light microscope and EM study led to the same conclusion. Taking great care to avoid fixation artefacts, they showed that the internal structures of chloroplasts and mitochondria remained intact during desiccation and re-wetting, consistent with the rapid recovery of photosynthesis (and respiration). However, the numerous lobes and lamellar extensions of the chloroplasts were lost during drying and thus the organelles became much more rounded (although with little or no change in overall volume). Upon re-wetting, the chloroplasts regained their complex morphology over a period of about 24 h. These results also tend to rule out the repair hypothesis. Nevertheless, the correlation between the slower, later phase of photosynthetic recovery and the 're-modelling' of the chloroplasts suggests that reconstruction of certain aspects of chloroplast morphology is needed for a full restoration of photosynthetic capacity.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
| ||||||||||||||||||||