Ice accommodation in plant tissues pinpointed by cryo-microscopy in reflected-polarised-light

Matthias Stegner,Johanna Wagner,Gilbert Neuner

doi:10.1186/s13007-020-00617-1

Abstract

BackgroundFreezing resistant plant organs are capable to manage ice formation, ice propagation, and ice accommodation down to variable temperature limits without damage. Insights in ice management strategies are essential for the fundamental understanding of plant freezing and frost survival. However, knowledge about ice management is scarce. Ice crystal localisation inside plant tissues is challenging and is mainly based on optical appearance of ice in terms of colour and shape, investigated by microscopic methods. Notwithstanding, there are major uncertainties regarding the reliability and accuracy of ice identification and localisation. Surface light reflections, which can originate from water or resin, even at non-freezing temperatures, can have a similar appearance as ice. We applied the principle of birefringence, which is a property of ice but not of liquid water, in reflected-light microscopy to localise ice crystals in frozen plant tissues in an unambiguous manner.ResultsIn reflected-light microscopy, water was clearly visible, while ice was more difficult to identify. With the presented polarised cryo-microscopic system, water, including surface light reflections, became invisible, whereas ice crystals showed a bright and shiny appearance. Based on this, we were able to detect loci where ice crystals are accommodated in frozen and viable plant tissues. In Buxus sempervirens leaves, large ice needles occupied and expanded the space between the adaxial and abaxial leaf tissues. In Galanthus nivalis leaves, air-filled cavities became filled up with ice. Buds of Picea abies managed ice in a cavity at the bud basis and between bud scales. By observing the shape and attachment point of the ice crystals, it was possible to identify tissue fractions that segregate intracellular water towards the aggregating ice crystals.ConclusionCryo-microscopy in reflected-polarised-light allowed a robust identification of ice crystals in frozen plant tissue. It distinguishes itself, compared with other methods, by its ease of ice identification, time and cost efficiency and the possibility for high throughput. Profound knowledge about ice management strategies, within the whole range of freezing resistance capacities in the plant kingdom, might be the link to applied science for creating arrangements to avoid future frost damage to crops.

Highlights

Freezing resistant plant organs are capable to manage ice formation, ice propagation, and ice accommodation down to variable temperature limits without damage
Knowledge of ice accommodation in plant tissues will be essential for the fundamental understanding of frost survival of plants within the whole range of freezing resistance capacities (< 0 °C to − 196 °C)
Distinction between water and ice in reflected light To verify whether liquid water can be reliably distinguished from frozen water, experiments were performed with water in different aggregation states at different temperatures (Fig. 1)

Summary

Introduction

Freezing resistant plant organs are capable to manage ice formation, ice propagation, and ice accommodation down to variable temperature limits without damage. Knowledge of ice accommodation in plant tissues will be essential for the fundamental understanding of frost survival of plants within the whole range of freezing resistance capacities (< 0 °C to − 196 °C) This might be the basis for the applied sciences to create improved arrangements for frost protection of crops in the future [2]

Methods

Results

Discussion

Conclusion