Research

Theoretical framework: Survival of overwintering buds is critical for reproduction, growth, species distribution, and crop productivity. Buds have diverse architectures, but their role in cold hardiness remains poorly understood. Cold hardiness is a dynamic trait. Climate warming increases frost risk as buds fail to develop sufficient cold hardiness without cold cues. Molecular and phenological studies exist, but mechanistic understanding of freezing at the bud level is limited. Understanding how architecture governs ice formation, accommodation, and injury is essential to predict frost risk. Hypotheses: We hypothesize that bud survival depends on bud architecture, freeze-induced dehydration, and biochemical traits. We will reassess freezing survival typologies, identify structural and chemical features enabling supercooling or ice segregation, and determine whether injury arises from intracellular ice or dehydration. This will improve understanding of cold hardiness and frost vulnerability under climate change. Approach: Woody species will be grown at three locations with distinct temperature regimes under the same photoperiod. Cold hardiness will be linked to structural and functional traits. Freezing dynamics (ice formation, supercooling vs. freeze dehydration, water frozen per temperature) will be studied with differential scanning calorimetry and psychrometry. Sublethal and lethal ice masses will be visualized with thermography and cryomicroscopy. Regions where ice forms or is inhibited will be analyzed with quantitative microscopy, Raman spectroscopy, and atomic force microscopy. Seasonal carbohydrate deposition will be mapped to assess its role in supercooling. Differential scanning calorimetry and cold hardiness tests will determine whether intracellular freezing or critical dehydration causes injury. Transmission electron microscopy of frozen samples will reveal membrane or organelle dysfunction. Innovation: This project is innovative because it addresses how buds remain ice-free despite ice in surrounding tissues. Integrating thermal, structural, and biochemical analyses will clarify whether injury is driven by intracellular ice or dehydration. Outcomes will identify protective metabolites and structural traits for breeding resilient crops and trees, and improve models predicting frost damage, biodiversity shifts, and forest stability under climate change.

The move out of the water was an evolutionary milestone of early land plants. Streptophyte algae and bryophytes belong to these early settlers and are ideal models that range in complexity from filaments to multicellular 3-D bodies. They adapted on different hierarchical levels (tissues, cells, interfaces, cell wall) and life cycles (vegetative organs, rigid spores). In-depth understanding of the different life forms from organ to micro to nano level and their in-situ reactions upon desiccation are missing, although necessary to understand adaptations under changing environments. Three flagship species groups will be investigated during realistic desiccation scenarios: 1) streptophyte algae with different abilities to tolerate desiccation like Klebsormidium sp. and Zygnema circumcarinatum (Zygnematopyhceae), 2) the liverwort Riccia fluitans (Ricciaceae) with terrestrial as well as water-prone representatives and 3) the moss Physcomitrium patens (Funariaceae). Our integrative approach is combining cell biological, biochemical and biophysical methods to study the properties and rearrangements of cells in early land plants. The data will be used to develop a predictive biophysical model for cellular reactions to dehydration/desiccation scenarios. In-situ analysis of cell walls and surfaces of the flagship species under desiccation will give insights into adaptations in different life cycles as well as remodeling and microstructural changes upon drying. We will gain a better view on the properties of distinct layers from the cortical cytoplasm, the plasma membrane, the cell wall and the cuticle and by finite element models on the role of cell-shape, cell wall thickness and mechanical properties on buckling and wrinkling of the cells. With innovative multidisciplinary approaches we will tackle a common research question from various angles. We will gather essential information on adaptation to desiccation and climate change in algae and mosses. These early land plants are leading the way as they managed to move out of the water to thrive in a changing environment. We will learn from evolution for the present and future.

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.