Leaves Protect From Freezing

close up photograph of a large green plant

Biological Strategy

Giant groundsel

Mary Hoff

Image: Juniperus_scopulorum / Flickr / CC BY NC - Creative Commons Attribution + Noncommercial

Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Insulated outer leaves of the cabbage groundsel protect tender inner leaves from freezing by folding inward to enclose them at night.

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Introduction

The cabbage groundsel (Dendrosenecio keniensis) is a shrub-like plant with clustered leaves found at elevations of around 3,000–4,000 meters (10,000 to 13,000 feet) on the slopes of Mt. Kenya in eastern Africa. Its thick, platter-sized leaves form a rosette, with 50 or more older, tougher leaves encircling a cluster of tender young leaves that form a bud shape at the center.

Image: Dwergenpaartje / CC BY SA - Creative Commons Attribution + ShareAlike

The delicate inner new growth at the center of the cabbage groundsel rosette is easily damaged by freezing temperatures.

cabbage groundsel, Dedrosenecio keniensis

Image: Dwergenpaartje / CC BY SA - Creative Commons Attribution + ShareAlike

The thick, furry outer leaves of the cabbage groundsel fold inward at night, protecting tender new leaves from freezing temperatures.

The Strategy

The rocky, barren places the cabbage groundsel grows are warm and sunny during the day. At night, however, temperatures regularly drop below freezing. The cold could stunt the growth of the newly emerging leaves in the center of the rosettes except for one thing: The insulating power of the older leaves that surround them. The thick, leathery outer leaves have air pockets inside them that slow the movement of heat, much as a styrofoam cooler does.

When temperatures plummet as dusk falls, some of the cells in the large leaves release fluid into the space between cells, causing the inward-facing surface to become less stiff. As a result, the leaves fold upward and inward, forming an insulating layer around the more tender parts of the plant and protecting the newer, clustered leaves from the cold. The outer leaves freeze, but they are hardy enough to thaw out the next day, when they reopen within minutes to catch the morning sun.

The Potential

We’re already quite familiar with the ability of thick layers of material with air pockets to insulate. Consider foam wall insulation and down comforters, for instance. Combining that trait, as the cabbage groundsel does, with a temperature-triggered mechanism for moving insulation could be valuable for everything from protecting landscape plants from frost to improving the energy efficiency of entire buildings.

Last Updated October 18, 2020

References

“A typical feature of the giant rosette plants is the ability of the adult leaves to carry out special movements. The leaves bend upwards and inwards upon cooling and since the rosettes are composed of some 70–200 adult leaves, this movement results in the whole system taking on a globular shape which has been designated a ‘night-bud.’ … The reverse leaf movement takes place upon warming, thus opening the rosette to its day position.The dense layering of the night-bud effectively insulates the inner leaves and especially the core of meristematic leaves from nocturnal frosts.” Beck 1985:83

Book section

Cold tolerance in tropical alpine plants

Tropical alpine environments. Plant form and function. Cambridge University Press | Erwin Beck

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Reference

“Upon onset of the nocturnal frost period, the adult leaves of the afroalpine giant rosette plants nyctinastically bend upwards and inwards, giving the whole rosette a globular shape. This so-called night-bud … provides a more or less sealed and insulated air space around the bud-cone during the night, but rapidly opens upon warming by the first rays of sunshine in the morning. Since Beck & al. … have shown that the developing leaves are almost as frost resistant as the adult ones, protection of the leaf-bud from subfreezing temperatures in the night-bud may be interpreted rather as a means to prevent nocturnal stand-still of leaf growth due to freezing than to avoid cryoinjury.” (Beck 1982: 128)

Journal article

Biology of Afroalpine Dendrosenecio (Asteraceae)

Plant Systematics and Evolution | 17/10/2020 | Erwin Beck

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