Iowa State researcher studies relationship between freezing and ‘natural thermometer’ of some plants

AMES — The mystery of how plants survive freezing temperatures and rebound after winter has long fascinated scientists, including Iowa State University professor of horticulture, Rajeev Arora, an expert in the cold physiology of plants.

Arora is particularly interested in the “fascinating phenomenon” of cold-induced “thermonasty,” characterized by the curling and drooping of leaves, which some plants use to adapt to cold temperatures.

“More thermonasty means more cold-hardiness,” Arora said. “Though it is not common, it does happen in several species of broad-leaved Rhododendron, a large family of woody evergreens popular as landscape plants.”

Could the extent of thermonasty under freezing temperatures be employed by breeders as a screening tool to identify more freeze-tolerant genotypes in their efforts to breed stress-resilient cultivars? Arora wondered.

To better understand this complex and poorly understood process, he wanted to study the way these changes take place in a natural setting in real time. To do so, Arora worked with David Livingston, a USDA-Agricultural Research Service scientist, located at North Carolina State University. They conducted detailed time-lapse infrared thermography of Rhododendrons under natural freezing episodes to track the progressive dynamics of the nighttime freezing process and thermonasty.

Because energy in the form of heat is released during freezing, infrared thermography can pinpoint subtle changes that occur precisely when and where ice forms in plant tissues.

Their results showed that, as the soil froze, ice formation was initiated at different locations within the stems and trunks of the rhododendron shrubs. It started predominantly at about 25 to 28 degrees F, affecting the upper portions of the stem and then spread in both directions.

In the leaves, ice formed initially in the tissues of the midrib, then moved through the plant’s vascular, or circulatory, system via veins within the leaves. The recordings also captured the point — at one or two degrees colder than the freeze-initiation — when leaves began to roll inward from the midrib.

As air temperatures dropped, the leaves continued to roll and curl downward into a more vertical position. In the morning, as air temperatures rose above freezing, the leaves slowly unfurled, regaining their pre-frozen configuration. Short videos of the process can be viewed in a publication in the scientific journal Physiologia Plantarum, where the researchers share their findings.

For the first time, their study demonstrated that freezing precedes thermonasty, supporting earlier theories by Arora and others that it serves as a “natural thermometer.”

“We propose that thermonasty functions as a photo-protective strategy to reduce light absorbance, preventing cell damage from frigid conditions. Otherwise, the large leaf surface area of these Rhododendrons would harvest radiation beyond their capacity to utilize it when freezing temperatures cause sluggish photosynthetic biochemistry. Such surplus energy can release undesirable free radicals and damage tender plant cells from freezing” he said. “We hypothesize that thermonasty can also prevent damage from too-rapid thawing.”

The researchers also wanted to better understand what controls the rolling and movement of leaves during thermonasty. For this, they examined the cellular dynamics of movement in the anatomy of the plants’ leaf blades and stalks and devised additional tests, including a cellulose-based paper experiment that allowed them to simulate the rolling effect.

The results suggest that, along with a physiological cause, thermonastic movement is also driven by mechanical processes caused by the non-uniform contraction of cellulose fibers of cell walls in the upper versus lower cells of leaf blades.

“Cells contract as they lose water, which changes to ice crystals inside the vascular system and outside the cells,” Arora said. “This leads to a more pronounced contraction of the lower cells relative to the upper ones, causing the leaves to roll with the stiff midrib serving as a brace. Similar behavior — but from uneven expansion of cellulose fibers — can lead to warping of wooden planks under humid conditions.”

Arora and his co-authors believe it may be possible to use the larger and more easily observed leaves of Rhododendron as a model to study thermonasty in crop plants, including maize, wheat, rye, oats and barley, which behave similarly under freezing and drought stress, with hardier crops (like rye) rolling significantly more when frozen than less hardy plants (like oats).

This work was supported primarily by USDA Hatch Act and State of Iowa funding.