Winter is Coming: How Grasses Conquered the Long Freeze

Tundra (By Education Specialist)

There are vast areas of the world where cold temperatures create problems for plants.

Such "cold stress" happens when temperatures that are below what are optimal for plants creates physiological problems for the individual plants. Depending on the species, optimal growing conditions usually hovers between 10℃ and 30℃. 

There are two types of cold stress, depending on the temperatures involved.

Chilling refers to cold stress where the temperatures range between 0℃ and 15℃, whereas freezing refers to cold stress where the temperatures are below freezing (0℃). 

In the case of freezing, plants can respond via two different methods. Freezing avoidance is when the plant is able to delay or even prevent freezing altogether in their tissues. In contrast, in freezing tolerance, the plants do not prevent ice crystal formation, but they avoid damage by controlling the size of the crystals and/or where the crystals are formed. Freeze-tolerant plants can survive even freezing of their tissues, and represent the extreme adaptation to conquering the cold over long time periods.

Deschampsia antartica (c) Lomvi2 - Wikipedia

Grasses initially evolved as tropical plants that loved the warmth. But over time, the grasses have managed to adapt to the cold and have become the most widespread vascular plant family, and present in all continents. Indeed, the grass Deschampsia antartica is only one of two vascular plants that are native to Antarctica, and its optimal growing temperature is an amazing 10℃!

Freezing can kill grasses via the formation of ice crystals. At first, ice crystals form in the extracellular spaces between cells, which causes water to be pulled from the cell interior. The loss of intracellular water (and later solutes) results in dehydration and osmotic stress. As ice crystals continue to form into the interior of the cells, the cell membranes start to degrade and programmed cell death occurs.

The evolution of the ability to tolerate freezing in the Poaceae is fascinating, and is confined mostly to species in two different subfamilies of the Poaceae. Grasses in the subfamilies Pooideae (in the Northern Hemisphere) and Danthonioideae (in the Southern Hemisphere) live in cold temperate continental regions, and they have solved the problem in two different ways. 

Hordeum jubatum of the subfamily Pooideae (Northern Hemisphere)

The grasses in subfamily Pooideae in the Northern Hemisphere all use the C3 mode of photosynthesis, and are commonly called the cool season grasses. They include many turf grasses (such as Poa pratensis or Kentucky Bluegrass), as well as food staples such as Triticum aestivum (wheat), Avena sativa (oat), and Hordeum vulgare (barley). 

The ancestors of the Pooideae evolved about 67 million years ago in small niches during the formation of the Eurasian mountain ranges. In this group, ice binding proteins evolved which decreased the rate of the formation of ice, as well as altering the shape of the ice crystals being formed. These anti-freeze proteins prevent the formation of large crystals that can physically damage cells and tissues.

Cortaderia selloana from the subfamily Danthonioideae (Southern Hemisphere)

In contrast, cold-acclimated grasses from the subfamily Danthonioideae in the Southern Hemisphere have evolved a different method for conquering the cold. The members of this group are also C3 grasses, and include the large ornamental grass Cortaderia selloana (Pampas grass). The freeze tolerant members of this subfamily produce ice-nucleation proteins that allow them to control where ice crystals form. Thus, they can prevent damage by limiting crystal formation in the extra-cellular spaces and the leaf surfaces, but not within the cell interiors.

The independent evolution of such mechanisms by the Danthonioideae and Pooideae to tolerate freezing have enabled grasses to dominate large areas where there are extreme seasonal fluctuations in temperature, and contributed to the grassy world we live in today. 

As an interesting aside, at the other end of the temperature spectrum, C4 grasses such as those in the grass subfamilies Chloridoideae and Andropogoneae - which include corn/maize (Zea mays), the bluestems (Andropogon spp), sugarcane (Saccharum officinarum), and cogon grasses (Imperata cylindrica) - have extended the range of environments conquered by the Poaceae to much warmer and drier climes. 

For a quick look at the difference between C3, C4 and CAM photosynthesis in grasses, click here.

Muhlenbergia sericea of the subfamily Chloridoideae, a C4 grass

Finally, to read more about the fascinating topic of cold adaptation in grasses in depth, check out the references below.

Note: In addition to the specialized mechanisms of freezing tolerance above, grasses can deploy other additional mechanisms for coping with the cold that are beyond the scope of this shirt post, such as accumulating fructan in their vacuoles, using supercooling (for shorter cold spells), and increasing anti-oxidant enzymes.

Phalaris arundinacea of the subfamily Pooideae

References:

Timothy J. Gallaher, T. J. Gallaher, Paul M. Peterson, P. M. Peterson, Robert J. Soreng, R. J. Soreng, Fernando O. Zuloaga, F. O. Zuloaga, De-Zhu Li, D. Li, Lynn G. Clark, L. G. Clark, Christopher D. Tyrrell, C. D. Tyrrell, Cassiano A.D. Welker, C. A.D. Welker, Elizabeth A. Kellogg, E. A. Kellogg, & Jordan K. Teisher, J. K. Teisher. (2022). Grasses through space and time: An overview of the biogeographical and macroevolutionary history of Poaceae. Journal of systematics and evolution, 60, 522-569. doi: 10.1111/jse.12857

Schubert M, Humphreys AM, Lindberg CL, Preston JC, Fjellheim S. 2020. To coldly go where no grass has gone before: A multidisciplinary review of cold adaptation in Poaceae. In: Annual Plant Reviews online.Wiley.523–562.

A mossy life in the clouds

The Cascade Mountain Trail near Lake Placid in New York State is a moderate hike that takes you above the treeline to bald rocky outcrops 1250 meters above sea level. At this height, alpine vegetation can be seen among the plant life, their low slung bodies hugging the ground in order to shelter from any strong winds.

I was eating lunch next to large boulders, when I noticed some vegetation hidden in the shadow of the rocks. When I looked more closely, I was delighted to find some grasses growing from a luxuriant carpet of moss.

The grasses had very fine leaves, and dried seedheads rose from the clumps like tiny white flags. A quick investigation showed the panicles contained the remnants of extremely tiny spikelets.

I took macro shots of the spikelets, which were probably only 1-2 mm in size, and based on that and the leaves tentatively identified the species as Deschampsia flexuosa.

The association of this species with mosses immediately brought to mind the habits of its sister, the noteworthy Deschampsia antartica, which is only one of two vascular plants that have managed to colonize deep into Antartica.   That species is also frequently seen growing from mosses, and researchers have found that specimens in such an association are larger and healthier than those individuals living on bare rocks (Casanova-Katny and Cavieres, 2012).

They surmised that mosses help the grass by modifying the surrounding environment, perhaps by increasing soil moisture and soil nutrients content, or by also buffering temperature compared to bare areas.

Whatever the reason for their association, my surprise find at the summit of Cascade Mountain certainly added to the enjoyment of the hike!

Literature Cited

Casanova-Katny M. A., Cavieres L. A. (2012). Antarctic moss carpets facilitate growth of Deschampsia antarctica but not its survival. Pol. Biol. 35 1869–1878

Deschampsia flexuosa (Crinkled Hairgrass) in Shenandoah National Park

During hikes at Shenandoah National Park in mid-July, one of the more notable grasses I saw was a somewhat short species with very airy panicles. Some had only a few flower spikes, but a few looked fantastic, as if a cloud of midges was hovering above them, and I have to admit I spent time during hikes paying attention to specimens of this species that I saw by the side of the trail.

The spikelets themselves were also quite notable, with 2 sinuous awns forking out from the 2 florets that made up each spikelet. But they were very tiny, and I must have been a sight kneeling by the side of the path trying to take macro pics of it!

I later identified the species as Deschampsia flexuosa with the help of Zihao Wang from a grass identification group in facebook.

If you ever hike Shenandoah National Park, then keep your eyes open for this very pretty little grass as you make your way along one of the many gorgeous trails of this national park.

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