Ice barriers and supercooling in alpine plants

Freezing temperatures are an important filter on plant recruitment, survival, productivity and geographic distribution. Knowledge of the mechanisms of plant freezing, ice formation, ice propagation and protective ice barriers against ice penetration can help to understand frost survival of plants and is a key for developing rational strategies for conferring freeze tolerance in plants. Some plant species, organs, tissues and cells rely on the mechanism of persistent supercooling for frost survival. Supercooling is found in bud tissues but also in xylem parenchyma cells of many temperate woody plants. Supercooling is only efficient down to a temperature of -40C, restricting their geographic distribution to places with temperature minima higher than -40C. Ice, once nucleated, usually spreads unhindered and rapidly throughout a plant at a rate of up to 27 cm.s-1 via the xylem and from there into the apoplastic spaces of other tissues. Hence, a prerequisite for supercooling are ice barriers. The structural nature of ice barriers in plants is only poorly understood. Infrared differential thermal analysis (IDTA) was recently developed in the lab of G. Neuner. IDTA allows monitoring the spread of ice in plants and localizing ice barriers. After localization in a first step it is intended to investigate the anatomical features and the build-up of ice barrier tissues in buds. As far as known by now ice barriers can be tissues that lack intercellular spaces and show special cell wall architecture. A crucial point is the pore diameter of cell wall capillaries as water in narrow capillaries (<6 nm) remains unfrozen. Cell wall pore diameter is sensitively affected by various molecular cell wall components such as suberin, lignin but also the swelling degree of pectins. Low ion concentrations and high ph induce swelling of pectins. Such conditions are typically present in the xylem sap of woody alpine plants during winter. The xylem sap of alpine conifers additionally contains an increased amount of sugars (e.g. Glucose 150x) that could further influence cell wall features. We hypothesize that via changes of the apoplastic milieu the cell wall porosity is reduced by swelling of pectins which in turn increases the ice barrier properties of cell walls. In a methodological innovation we further intend to develop a new instrumentation where we employ tiny infrared micro sensors in IDTA mode to determine supercooling in buds at temperatures lower than -25C where the currently available methods fail to work or give too little resolution. Working with alpine plants offers the opportunity for studying supercooling along an altitudinal profile reflecting the response to different natural winter scenarios. Additionally, alpine plants are likely to be even more sensitively affected by climate change than lowland plants due to loss of snow cover and untimely frost dehardening in winter. Under the proposed climate change the understanding of supercooling and ice barriers which are key players with respect to frost survival will get even more important.

Freezing temperatures are an important filter on plant recruitment, survival, productivity and geographic distribution. Knowledge of the mechanisms of plant freezing, ice formation, ice propagation and protective ice barriers against ice penetration can help to understand frost survival of plants and is a key for developing rational strategies for conferring freeze tolerance in plants. Some plants rely on the mechanism of persistent supercooling for frost survival. Supercooling is found in bud tissues but also in xylem parenchyma cells of many temperate woody plants. Supercooling is only efficient down to a temperature of -40C, restricting their geographic distribution to places with temperature minima higher than -40C. Ice, once nucleated, usually spreads unhindered throughout a plant at a rate of up to 27cm.s-1 via the xylem and from there into the apoplastic spaces of other tissues. Hence, a prerequisite for supercooling are ice barriers. The structural nature of ice barriers in plants is only poorly understood. Infrared differential thermal analysis (IDTA) was recently developed in the lab of G. Neuner. IDTA allows monitoring the spread of ice in plants and localizing ice barriers. After localization in a first step it is intended to investigate the anatomical features and the build-up of ice barrier tissues in buds. As far as known by now ice barriers can be tissues that lack intercellular spaces and show special cell wall architecture. A crucial point is the pore diameter of cell wall capillaries as water in narrow capillaries (<6 nm) remains unfrozen. Cell wall pore diameter is sensitively affected by various molecular components such as suberin, lignin but also pectins. Low ion concentrations and high ph induce swelling of pectins. Such conditions are typically present in the xylem sap of woody plants during winter. The xylem sap of onifers additionally contains an increased amount of sugars that could further influence cell wall features. We hypothesize that via changes of the apoplastic milieu the cell wall porosity is reduced which in turn increases the ice barrier properties of cell walls. Working with alpine plants offers the opportunity for studying supercooling along an elevational gradient reflecting the response to different natural winter scenarios. Additionally, alpine plants are likely to be even more sensitively affected by climate change than lowland plants due to loss of snow cover and untimely frost dehardening in winter. Under the proposed climate change the understanding of supercooling and ice barriers which are key players with respect to frost survival will get even more important.