The mechanism of freezing resistance in cold-acclimated winter wheat and rye crowns

The ability of cold-acclimated crowns to survive freezing is an important environmental factor limiting winter wheat (Triticum aestivum L.) expansion in Western Canada. Coldacclimation refers to the physiological and biochemical processes by which plants acquire freezing resistance. Cold-acclimation establishes an ice segregation freezing survival mechanism. The shoot apical meristem (SAM) cold-acclimates through the accumulation of dehydrins, vernalization-responsive and cold shock proteins. Modifications to the vascular transition zone (VTZ), are centered on increases in pathogenesis-related, antifreeze proteins, sugar hydrolyzing enzymes and increased cell wall glucuronoarabinoxylans. In cold-acclimated winter wheat and rye (Seacale cereale L.), magnetic resonance microimaging revealed a desiccative intermediate zone between the SAM and VTZ with reduced water mobility. Slow cooling rates (≤ 2°C h-1) are required to establish freezing injury in the VTZ prior to injury in the SAM. Infrared thermography was used to observe in the absence of an extrinsic ice nucleator that freezing initiated from the base of the crown. The first freezing event (-3°C to -5°C) corresponded with warm ice nucleation temperatures (-3.5°C) and ice formation in the leaf sheath. Removal of the leaf sheath prior to cooling reduced cold hardiness and resulted in injury to the SAM prior to the VTZ. The second freezing event (-8°C to -12°C) corresponded with non-lethal injury to the VTZ. The third freezing event (-21°C in Norstar, -27°C in Hazlet and -30°C in Puma rye) corresponded with injury to the SAM and the plant’s killing temperature based on whole plant survival tests. It is hypothesized that cold-acclimated crowns survive sub-zero temperatures through the following mechanism. The leaf sheath is established as an ice sink relative to the SAM. An intermediate zone acts as a barrier to ice propagation, allowing the SAM to supercool and avoid freezing. The VTZ acts as an ice sink within the crown. Possible differences in SAM freezing survival between rye and wheat could explain their contrasting cold hardiness