High-definition infrared thermography of ice nucleation and propagation in wheat under natural frost conditions and controlled freezing

Authors: Livingston, David; Tuong, Tan; MURPHY, J - North Carolina State University; GUSTAL, LARRY - University Of Saskatchewan; WILLICK, IAN - University Of Saskatchewan; Wisniewski, Michael

Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2017
Publication Date: 12/9/2017
Citation: Livingston, D.P., Tuong, T.D., Murphy, J.P., Gustal, L., Willick, I., Wisniewski, M.E. 2017. High-definition infrared thermography of ice nucleation and propagation in wheat under natural frost conditions and controlled freezing. Planta. 247:791-806.

Interpretive Summary: When water freezes it gives off heat which can be observed using an infrared camera. Because freezing is a continuous process, a computer attached to the camera can record any freezing process over time. This technology was first used to study freezing in plants in 1995 and cameras since then have improved to the point that individual plant cells can be observed as they freeze. Most freezing studies are conducted under laboratory conditions but scientists always wonder if those conditions accurately reflect what happens under natural conditions, especially with regard to freezing in plants. In this study a high-definition infrared camera was used to observe wheat plants freezing under natural condition during the winter and spring of 2016 and 2017 respectively. A major freeze event that occurred in many parts of the US in March, 2017, while wheat was in the boot stage, was recorded. Several observations contradicted long accepted principles of freezing in plants. One general observation was that plants froze one at a time throughout the night beginning just after dark and ending just before sunrise. In addition to plants freezing one at a time and in an apparent random pattern, the leaves on each plant also froze one at a time. In this case they froze in an age-dependant pattern with the older leaves on the outside of the plant freezing first. The most interesting and potentially controversial observation was that plants never freeze from the top down even when the tips of leaves were several degrees colder than the base of the plant. Under both natural and controlled conditions and whether the soil was frozen or not, freezing in wheat, rye and oats always began at the base of the plant and proceeded upwards to the top. Many researchers have assumed that “leaf tip burn” , sometimes observed after a freeze event during winter or in the spring, is a result of only the tips of leaves freezing. In 4 years of research observing many hundreds of plants freezing we never observed only the tips of leaves to freeze. In leaves with tip-burn, the entire leaf always froze. This indicates that the tips of leaves are less freezing tolerant than the base of leaves. Many other findings were reported in this paper that are difficult to describe and must be viewed by video. To that end this paper is submitted with 7 high resolution videos that illustrate these and other findings.

Technical Abstract: Infrared thermography has been used to visualize the freezing process in plants and has greatly enhanced our knowledge of ice nucleation and propagation in plants. The majority of IR analyses have been conducted under controlled rather than natural conditions and often on plant parts instead of whole plants. This is due to the unpredictability and complexity of natural freezing events as well as the limited visual resolution of available infrared video equipment. In the present study, high-definition (1280 x 720 pixel resolution) infrared thermography was used to visualize the freezing process of winter wheat plants in a vegetative and reproductive state during natural frost events in 2016 and 2017. Many of the observed freezing patterns were similar to previous reports of studies conducted under laboratory conditions. Contrary to a general assumption, however, freezing always began at the base of the plant and spread upwards. In addition, the assumption that leaf-tip burn is the result of only leaf tips freezing was found to be false. Freezing of only leaf tips was never observed. Freezing of the entire leaf occurred on all leaves on which leaf-tip burn was observed, indicating that the tips of leaves are less freezing tolerant than the rest of the leaf. Freezing within an individual wheat plant was also observed to occur in an age-dependant manner, with older leaves freezing first. In fact, in many cases the youngest leaves of plants never froze. A difference in the ice-nucleation-activity of apoplastic extracts from crowns and leaves suggested the differential presence of unidentified compounds with the ability to freeze-inoculate tissues, and partially explains the freezing patterns observed in this study. These results underscore the complexity of the freezing process in small grains and indicate that comprehensive observational studies are crucial for developing a logical approach to breeding for increased frost tolerance in grain crops.