Freezing cytorrhysis and critical temperature thresholds for photosystem II in the peat moss Sphagnum capillifolium
Leaflets of Sphagnum capillifolium were exposed to temperatures from −5°C to +60°C under controlled conditions while mounted on a microscope stage. The resultant cytological response to these temperature treatments was successfully monitored using a light and fluorescence microscope. In addition to the observable cytological changes during freezing cytorrhysis and heat exposure on the leaflets, the concomitant critical temperature thresholds for inactivation of photosystem II (PS II) were studied using a micro fibre optic and a chlorophyll fluorometer mounted to the microscope stage. Chlorophyllous cells of S. capillifolium showed extended freezing cytorrhysis immediately after ice nucleation at −1.1°C in the water in which the leaflets were submersed during the measurement. The occurrence of freezing cytorrhysis, which was visually manifested by cell shrinkage, was highly dynamic and was completed within 2 s. A total reduction of the mean projected diameter of the chloroplast containing area during freezing cytorrhysis from 8.9 to 3.8 μm indicates a cell volume reduction of approximately −82%. Simultaneous measurement of chlorophyll fluorescence of PS II was possible even through the frozen water in which the leaf samples were submersed. Freezing cytorrhysis was accompanied by a sudden rise of basic chlorophyll fluorescence. The critical freezing temperature threshold of PS II was identical to the ice nucleation temperature (−1.1°C). This is significantly above the temperature threshold at which frost damage to S. capillifolium leaflets occurs (−16.1°C; LT50) which is higher than observed in most higher plants from the European Alps during summer. High temperature thresholds of PS II were 44.5°C which is significantly below the heat tolerance of chlorophyllous cells (49.9°C; LT50). It is demonstrated that light and fluorescence microscopic techniques combined with simultaneous chlorophyll fluorescence measurements may act as a useful tool to study heat, low temperature, and ice-encasement effects on the cellular structure and primary photosynthetic processes of intact leaf tissues.
KeywordsChlorophyll fluorescence Cytorrhysis Freezing tolerance Heat tolerance Light microscope temperature-controlled chamber Sphagnum capillifolium
This work was enabled by the Austrian Science Found (FWF-project 17992-B06 to G. Neuner) as a preliminary feasibility study in context with the general examination and visualisation of freezing phenomena in plant tissues at the tissue and the cellular level. We are thankful to Dr. Georg Gärtner for providing taxonomic expertise.
Conflict of interest
The authors declare that they have no conflict of interest.
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