The low temperature limit of tree xylem

Xylem parenchyma cells (XPCs) are usually the least hardy stem tissue and therefore determine frost survival of trees and their northern distribution limit. Based on differential thermal analysis (DTA), two mechanisms for frost survival of XPCs have been described: Less frost-hardy XPCs are killed by lethal intracellular freezing, called deep supercooling, which occurs between −24 and −50°C. Most frost-hardy XPCs (−196°C) are thought to survive by freeze dehydration and were termed freezing tolerant. However, recent evidence suggests that superimposed freeze dehydration may be also involved in deep supercooling. The underlying mechanisms of frost hardiness of XPCs remain largely unknown. Therefore we aim to use a new, high resolution differential scanning calorimeter (DSC) to quantify the extent and temperature-dependent dynamic of supercooling and freeze dehydration of XPCs. Additionally attention is paid to specific freezing responses that originate from intraspecific differences in xylem anatomy, XPC architecture and function. Quantitative cell parameters of XPCs including pit traits of vessel associated cells will all be related to the specific freezing behavior measured by DSC. In this context, specific molecular components inside XPCs (anti ice nucleation substances) and of cell walls that affect their porosity and stiffness, and of the black cap (lipids) associated with the pits that act at the symplast-apoplast interface, will be analyzed by microscopic techniques including Raman micro-spectroscopy and Atomic force microscopy. The mechanisms of frost hardiness of XPCs are poorly understood, and, most strikingly, still unknown for most European tree species. In this context, the aspect of differences in XPC construction types and xylem anatomy have not particularly been investigated by so far. Mechanistic involvement of molecular components in XPC frost survival is – except for some recent studies – an understudied topic. In view of climate change, the regrowth dates are rapidly advancing, which increases the overall probability of devastating frost events. Therefore, the results will yield much needed improvement in our predictions of tree fitness response to climate change, which is economically relevant in forestry but also for the cultivation of fruit trees and ornamental plants.