Abstract
Main conclusionAn extremely high resolution infrared camera demonstrated various freezing events in wheat under natural conditions. Many of those events shed light on years of misunderstanding regarding freezing in small grains.Infrared thermography has enhanced our knowledge of ice nucleation and propagation in plants through visualization of the freezing process. The majority of infrared analyses have been conducted under controlled conditions and often on individual organs instead of whole plants. In the present study, high-definition (1280 × 720 pixel resolution) infrared thermography was used under natural conditions to visualize the freezing process of wheat plants during freezing events in 2016 and 2017. Plants within plots were found to freeze one at a time throughout the night and in an apparently random manner. Leaves on each plant also froze one at a time in an age-dependent pattern with oldest leaves freezing first. Contrary to a common assumption that freezing begins in the upper parts of leaves; freezing began at the base of the plant and spread upwards. The high resolution camera used was able to verify that a two stage sequence of freezing began within vascular bundles. Neither of the two stages was lethal to leaves, but a third stage was demonstrated at colder temperatures that was lethal and was likely a result of dehydration stress; this stage of freezing was not detectable by infrared. These results underscore the complexity of the freezing process in small grains and indicate that comprehensive observational studies are essential to identifying and selecting freezing tolerance traits in grain crops.
Highlights
The response of plants to freezing temperatures has been studied for over a century and has revealed that freezing in plants is a complex process
The observed temperature turbulence above the soil and within the plant canopy must be taken into account when considering how radiative cooling affects the freezing process in plants
In this study, using a significantly higher definition camera than has been used before, we identified several misunderstandings regarding freezing in small grains that have been accepted as fact for some time
Summary
The response of plants to freezing temperatures has been studied for over a century (reviewed by Wisniewski et al 2003) and has revealed that freezing in plants is a complex process Environmental variables, such as soil moisture and temperature fluctuations, as well as physiological and genetic factors within plants, including the presence or absence of intrinsic and extrinsic nucleating agents and structural barriers that inhibit the propagation of ice, are all important factors determining whether a plant survives freezing conditions. These variables all interact continuously in a seemingly stochastic manner making the improvement of freezing tolerance in plants a considerably challenging endeavor. While a high level of resolution of free and bound or frozen water in internal tissues can be achieved using MRMI analyses, several aspects of this technology, including logistical considerations, access to equipment, and expense make it impractical in many circumstances
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