Ice mass accumulation in plant tissues

Frosts are highly selective for plants and therefore affect their geographical distribution. Paradoxically, despite global warming, the risk of frost injury is increasing in many places on earth, which can be attributed to insufficient snow protection and mild winters. We are all familiar with regular headlines and reports about harmful frosts and agricultural losses of yield worth millions. If water freezes inside the cells, plants are instantly dead. When frost-hardy plants freeze, harmless ice is formed between the cells, which leads to freeze dehydration of the cells. In particular, the biophysical aspects of how ice growth is controlled and how cell water is segregated to the ice are not yet understood. Since plants consist to a large extent of water, the resulting ice masses must be considerable. Water expands when it freezes, which must create a space problem. Unfortunately, there has been a lack of simple methods that could distinguish ice in plant tissues from liquid water. Only recently a technique has been developed in our laboratory that makes just that possible. This means that completely new insights into freezing and the formation of ice crystals in plant tissues can be expected. In order to understand whether chemical substances have an influence on ice formation, locations of ice formation and control of the growth of ice crystals, the ice is isolated and biochemically analyzed. Places of ice segregation are also chemically characterized using imaging methods. The latest calorimetry technology enables the amount of ice masses to be recorded in the course of a dynamic freezing process with unprecedented accuracy. While reducing water content is part of adaptation to freezing temperatures in winter, spring and alpine plants survive freezing with high water content. A topic that is also highly relevant for many cultivated plants that are at risk of frost injury, especially during the growing season. Hardly anything is known how plants deal with the expectable large ice masses formed in water rich tissues. However, there is recent evidence that ice accumulates in predetermined spaces. Ice accumulation may take place in already existing spaces or spaces formed by tissue rupture at predetermined places. Again, it is not understood how this is controlled. Biophysical and -chemical aspects of ice growth in plant tissues and mechanisms of ice segregation in given spaces are still largely unexplored. In particular, the intended use of a set of new and most innovative methods promises completely new trend-setting insights into the biophysical mechanisms of freezing of plant tissues.

Plants cannot escape freezing temperatures, and the formation of ice inside their tissues poses a major challenge. In cold regions, the ability to tolerate frost is essential for plant survival and for maintaining crop yields. This research project investigated how plants survive ice formation and minimize frost damage. In ice-tolerant plants from temperate regions, ice forms outside living cells, but the exact locations and mechanisms were previously unknown. This study provides novel insights, showing that many leaf regions remain completely ice-free. Because water expands by about 9% when it freezes, ice formation requires space. Ice therefore accumulates in pre-existing cavities or in newly formed tissue gaps, which plants can tolerate without damage. Although extracellular ice formation is relatively harmless, it causes progressive dehydration of nearby cells. The results show that small cells with thicker, chemically modified cell walls and fewer cavities are better able to resist this frost-induced dehydration. These traits promote "supercooling," a state in which cells remain unfrozen despite surrounding ice, thereby preventing frost damage. Rigid cell walls slow water loss and protect cellular integrity. In hemp palm leaves, supercooling was observed, but for the first time it was shown that this mechanism is accompanied by moderate dehydration. In nature, these leaves freeze at 3.3C, while lethal intracellular ice forms only at 15.6C. The study also clarified the causes of frost damage in two major crops. Potato leaves experience frost damage at 3C in two stages: an initial, non-destructive phase where ice forms outside cells while they remain supercooled, followed by a destructive phase in which intracellular ice crystals rupture the cells. As a result, potatoes can survive frost only if freezing is brief, an important consideration for breeding strategies. In wheat, the commonly observed leaf-tip-damage is caused by frost. Leaf tips freeze faster than the base due to differences in freezing behavior linked to cell wall chemistry, particularly the esterification of cinnamic acids. Evergreen leaves freeze more slowly in winter, which contributes to higher cold hardiness. Ivy and boxwood leaves additionally form less ice overall, likely due to reduced water content and effective osmoregulation. Mountain pine needles exhibit a specialized anatomy: lignin in the endodermis confines ice to the vascular cylinder. As a result, photosynthetic tissue remains ice-free and hydrated, allowing gas exchange and photosynthesis even in frozen needles-an apparent key to survival in alpine environments. Overall, adaptations at the cellular, cell wall, and tissue levels regulate ice formation and freeze-induced dehydration. These processes are central to frost survival and offer promising avenues for improving cold hardiness in crops. Such advances are increasingly important for food security, particularly under climate change, where reduced frost-hardening or premature dehardening increases the risk of damaging late frosts.