Birch leaves and branches as a source of ice-nucleating macromolecules

Birch pollen are known to release ice-nucleating macromolecules (INM), but little is known about the production and release of INM from other tissues of the tree. We examined the ice nucleation activity of tissues from ten different birch trees ( Betula spp. ). Samples were taken from nine birch trees in Tyrol, Austria, and from one tree in a small urban park in Vienna, Austria. Filtered aqueous extracts of 30 samples of leaves, primary wood (new branch wood, green in colour, photosynthetically active), and secondary wood (older wood of a branch, brown in colour, with no photosynthetic activity) were analysed in terms of ice nucleation activity using VODCA (Vienna Optical Droplet Crystallization Analyser), a cryo microscope for emulsion samples. All samples contained ice nuclei in the submicron size range. Concentrations of ice nuclei ranged from 6.7*10 4 to 6.1*10 9 per mg sample. Mean freezing temperatures varied between −15.6 °C and −31.3 °C with the majority of the samples showing freezing temperatures close to those of birch pollen extract, indicating a relationship between the INM of wood, leaves and pollen. Extracts derived from secondary wood showed the highest concentrations of INM and the highest freezing temperatures. Extracts from the leaves exhibited the highest variation in INM and freezing temperatures. Infrared and fluorescence spectroscopy of the extracts suggest that the birch tissues tested contained chemical substances similar to birch pollen.