Figure 1: A snow covered maple may look peaceful to us but is very stressful to a plant. (Credit: Layla Dishman)
Embedded Files
Frosty Flora: Exploring the Ecophysiology of Frost Tolerance
Frosty Flora: Exploring the Ecophysiology of Frost Tolerance
What may look like a charming Christmas card cover to us is a plant fighting for its life.
Plants have evolved various mechanisms to avoid the worst forms of botanical frostbite. These mechanisms are quite complex, but researchers are still able to quantify frost tolerance within plants by using elegant experiments. The results from these experiments can then be used with ancient and modern methods to improve the hardiness of various plants we rely on, which will be an ever increasing important economic and societal issue due to climate change.
From decades of research, we have learned that there are various ways plants fight against frostbite, yet they all share the same goal of keeping the cell from shutting down. When exposed to cold temperatures, any cell will start to undergo changes: membranes will congeal like grease that is put in the fridge, enzymes deform and cease to function as their atoms huddle together, cytoplasm will turn into slush causing all cellular transport to grind to a halt, and the flow of electrons through the machinery of photosynthesis and cellular respiration short-circuit (Theocharis et al., 2012).
To avoid this catastrophic cellular fate, plants have two main strategies to mitigate the effects of cold weather. The first strategy is that of frost avoidance; many plants who utilize this strategy sew their progeny, in the form of spores or seeds, and let the overwinter deep within the soil, away from frigid weather (Ambroise et al., 2019; During, 1979; Haufler et al., 2016). The other method of frost acclimation takes a more direct approach by battening down the hatches until warmer weather returns (Grossman, 2022). A good example of this is when deciduous trees lose their leaves in the fall and go dormant for the winter. The underlying mechanisms of frost acclimation are a complex interaction of various metabolic factors, which starts with detecting a drop in temperature .
Plants can detect changes in temperature by using a physiological thermometer. When exposed to chilly air, the cell wall, plasma membrane, and cytoskeleton (all crucial structural components of the plant cell) deform. This deformation kicks off a biological chain reaction, known as an activation cascade (Guo et al., 2018), summarized in Figure 2.
From this elaborate chain reaction, we get two broad types of physiological response. The first among these is the activation of genes, such as the ones responsible for making heat shock proteins (Ambroise et al., 2019). Although this may seem a bit odd, as these proteins are normally made to relieve high heat stress, these proteins can help “warm” up a cold plant cell. They do this by wrapping around enzymes and other biological molecules to help them maintain their structure and function within the cell, acting almost like a molecular coat (Ambroise et al., 2019). The other major response is the creation of a chemical cocktail—containing various sugars, simple proteins, and antioxidants. (Ambroise et al., 2019). As these chemicals, called osmolytes, flow through the plant, they act like antifreeze in a car, by lowering the freezing point of the plant’s cytoplasm and other liquids, keeping everything running smoothly (Ambroise et al., 2019). However, these responses have their limits, and once breached, it results in lethal damage to the plant.
Figure 2: A graphical summary of frost stress and related mechanisms in plants. Positive signs symbolize an increase in a particular component of the stress response while negative signs indicate a decrease.
There are various events that can cause this deadly damage, but the worst among these is the formation of extracellular ice. When ice forms in the pockets surrounding plant tissue, it causes water from within cells to rush out into the extracellular space (Ambroise et al., 2019). This is dangerous as it causes water to leave a specific plant tissue called the xylem, which normally carries water up through the plant in an uninterrupted column of pure water (Ambroise et al., 2019). When the water is drawn out of the xylem, it causes air bubbles to form within the tube of tissue. This causes the xylem to lose all its power to move water up through the plant, which causes the plant to further dehydrate and ultimately die (Suissa et al., 2022). The consequence of this can be seen in Figure 3, where a cold snap in Houston killed a local family's mango tree. This can spell disaster for many farmers who plant crops with little to no frost tolerance and have major economic repercussions.
Figure 3: A mango tree killed by extreme cold weather. Notice how the leaves droop down, a sign that the xylem has embolized and lost all of its structural support (Credit: Layla Dishman).
For instance, one study from 2017 found that in one month, the European Union lost close to 3.3 billion Euros to frost damage (Faust and Herbold, 2018). One may naively think that such damage will decrease as the Earth warms through global climate change, but some evidence suggests the very opposite.
Normally, there is a sharp temperature gradient between the arctic and the temperate zones with the arctic being colder than the temperate zones. This gradient then creates optimal conditions for a strong, atmospheric jet stream to form, which traps frigid, bone-chilling air in the arctic (Cohen et al., 2021; Cohen et al., 2022). Climate change is disturbing this balanced system. As the arctic warms, the temperature gradient separating the two zones weakens (Cohen et al., 2021; Cohen et al., 2022). This results in a weaker jet stream, which can’t contain the strong winds of polar vortex, allowing freezing temperatures to dip down into lower latitudes (Cohen et al., 2021; Cohen et al., 2022). This means that as the climate warms, it will, almost paradoxically, become more likely for plants to sustain frost damage.
With the looming threat of extreme weather events fueled by global change and the ever-present threat of frost damage to crops and other plants, scientists have come up with many ways to quantify overall frost tolerance. From various attempts and trials, one method has become the Gold Standard, Electrolyte Leakage, due to its ease of application and robustness of results. This method works based on a basic principle of frost stress in plants, when plant cells are exposed to cold temperatures, they leak electrolytes out of their cells (Kovaleski and Grossman, 2021). Researchers can then use the amount of electrolytes leaked out of a sample of plant tissue to quantify a plant’s resistance by subjecting them to different temperatures with a relatively simple procedure (Kovaleski and Grossman, 2021).
The experimental procedure, adapted from work performed by Kovaleski and Grossman in 2021, for electrolyte leakage goes as follows. First, researchers cut up a few small bits of plant tissue, typically a twig or rhizome a few millimeters long. The plant tissue is then submerged in test tubes filled with deionized water. The special water is free of any electrolytes which may otherwise skew the data. Now, the samples are ready to be chilled. The best practice is to take a few samples and gradually lower them to varying temperatures in a programmable freezer. After they are chilled for a given time, the tubes are removed from the freezer and set on a test tube shaker to acclimate to room temperature overnight. Next, researchers measure the amount of electrolytes–also called conductivity– of the test tubes; this is a crucial step as it accounts for the electrolytes leaked from the tissue because of damage to the cold temperatures.
After that step, all the plant tissue must be “killed” or be completely drained of electrolytes; this can be achieved many ways– autoclaving or boiling– but the most used is liquid nitrogen. As the super chilled liquid nitrogen is poured over the tubes, it causes all the electrolytes to leak out of the plant tissue. The tubes are placed on the shaker once more and the tubes acclimate to room temperature overnight. The researcher then takes one final measurement of the sample’s conductivity; this measurement allows the researcher to quantify how much electrolytes leaked out due to cold temperatures out of the total amount of electrolytes in the tissue sample. Finally, the researcher has some usable plant frost tolerance data which can be turned into meaningful ecophysiological values with a bit of statistical leg work (Kovaleski and Grossman, 2021).
Figure 4: Here is an example of the set up for an electrolyte leakage experiment. The Tenney Freezer has been programmed to the specific temperatures the researcher is interested in, and test tubes have been labeled and are ready to receive their plant tissue samples.
Figure 5: A grove of Populus tremuloides in Arizona (Credit: Layla Dishman).
From the electrolyte leakage data, there is one crucial value that ecophysiology researchers are interested in, the temperature at which the plant cells lose fifty percent of their cellular electrolytes– the LT 50 (Kovaleski and Grossman, 2021). The higher this value is the less tolerant a plant is to frost damage, and the lower it is the more frost tolerant this plant is. Additionally, this value can be compared across species, strains, and even to different plant tissues– such as roots, rhizomes, stems, and leaves- and can provide useful economic and ecological insights (Kovaleski and Grossman, 2021; Bucher and Rosbakh, 2021; Savage et al., 2024; Ouyang et al., 2021; Deacon et al., 2019).
For instance, a study in 2019 used electrolyte leakage to determine the frost tolerance of a particular aspen community near the Niobrara River in Nebraska. Specifically, this community consisted of two species of aspen trees, Populus grandidentata and Populus tremuloides, and their hybrid Populus x. smithii (Deacon et al., 2019). Researchers quantified the differences in frost tolerance across these species using the method of electrolyte leakage, and they found that P. tremuloides is more frost tolerant than P. grandidentata and Populus x. smithii (Deacon et al., 2019). This study also provides insight into conservation strategies for this unique aspen community in the face of a changing climate (Deacon et al., 2019). This exemplary study illustrates the versatility and power of electrolyte leakage as being an easy experimental method that can provide important ecological and economic insights.
In sum, plants have many ways of protecting themselves from frigid weather, but as with all life, they have their limits. With the future climate conditions being unpredictable and quite extreme, there is a great need to understand and quantify the physiological tolerances of many agricultural crops, so that we can better prepare for when, like in Game of Thrones, Winter comes.
References
References
Theocharis, Andreas, Christophe Clément, and Essaïd Ait Barka. "Physiological and molecular changes in plants grown at low temperatures." Planta 235 (2012): 1091-1105.
Ambroise, Valentin, et al. “The roots of plant frost hardiness and tolerance.” Plant and Cell Physiology, vol. 61, no. 1, 18 Oct. 2019, pp. 3–20, https://doi.org/10.1093/pcp/pcz196.
During, Heinjo J. "Life strategies of bryophytes: a preliminary review." Lindbergia (1979): 2-18.
Haufler, Christopher H., et al. "Sex and the single gametophyte: Revising the homosporous vascular plant life cycle in light of contemporary research." BioScience 66.11 (2016): 928-937.
Grossman, Jake J. “Phenological Physiology: Seasonal patterns of plant stress tolerance in a changing climate.” New Phytologist, vol. 237, no. 5, 5 Dec. 2022, pp. 1508–1524, https://doi.org/10.1111/nph.18617.
Guo, Xiaoyu, Dongfeng Liu, and Kang Chong. "Cold signaling in plants: Insights into mechanisms and regulation." Journal of integrative plant biology 60.9 (2018): 745-756.
Suissa, J.S., Preisler, Y., Watkins, J.E., and McCulloch, L.A. (2022). Vulnerability Segmentation in Ferns and Its Implication on Their Survival During Drought. American Fern Journal. 112(4), 336–353.
Faust, Eberhard, and Joachim Herbold. "Spring frost losses and climate change–not a contradiction in terms." Munich Re (2018).
Cohen, Judah, et al. “Linking arctic variability and change with extreme winter weather in the United States.” Science, vol. 373, no. 6559, 3 Sept. 2021, pp. 1116–1121, https://doi.org/10.1126/science.abi9167.
Cohen, Judah, et al. "The “polar vortex” winter of 2013/2014." Journal of Geophysical Research: Atmospheres 127.17 (2022): e2022JD036493.
Kovaleski, Alisson Pacheco, and Jake J Grossman. “Standardization of electrolyte leakage data and a novel liquid nitrogen control improve measurements of cold hardiness in woody tissue.” Plant Methods, vol. 17, no. 53, 7 Apr. 2021, https://doi.org/10.21203/rs.3.rs-392368/v1.
Bucher, Solveig Franziska, and Sergey Rosbakh. "Foliar summer frost resistance measured via electrolyte leakage approach as related to plant distribution, community composition and plant traits." Functional Ecology 35.3 (2021): 590-600.
Savage, Jessica A., Sydney J. Hudzinski, and Mady R. Olson. "Use of electrolyte leakage to assess floral damage after freezing." Applications in Plant Sciences (2024): e11569.
Ouyang, Lin, et al. "Dehydrins and soluble sugars involved in cold acclimation of Rosa wichurana and rose cultivar ‘Yesterday’." Horticulturae 7.10 (2021): 379
Deacon, Nicholas J., Jake J. Grossman, and Jeannine Cavender‐Bares. "Drought and freezing vulnerability of the isolated hybrid aspen Populus x smithii relative to its parental species, P. tremuloides and P. grandidentata." Ecology and evolution 9.14 (2019): 8062-8074.
Page updated
Google Sites
Report abuse