Understanding organisms’ capabilities of sensing environmental changes such as increasing or declining temperature is becomes ever more important. Deciduous woody trees and shrubs growing in cool temperate and sub-arctic regions enter quiescent or dormant states as protection against freezing temperatures.

These plants pass through a two-stage process. Firstly, they gradually acclimatise (or 'acclimate', in the USA) where lowering temperatures encourage capacities for withstanding cold. This is a reversible process and if there is a spell of milder weather the acclimatisation state is lost. This can happen, for instance, with a fine spell of 'Indian summer' in October or even early November.

Winter weather and dormant trees. All images by Geoff Dixon

Where acclimation is broken, plants become susceptible to cold-induced damage again. If acclimation continues, however, plants eventually become fully dormant. This is not a reversible state and only ends after substantial periods of warming weather and increasing day-length. Some plants will require an accumulation of 'cold-units' – ie, temperatures below a specific level before dormancy is broken.

Detailed research information is accumulating to describe how acclimatisation develops. Changes take place that strengthen cell membranes, possibly by increasing the bonding in lipid molecules, and causing alterations in respiration rates, enzyme activities and hormone levels.

Non-acclimatised azalea (front), acclimatised azalea (back).

Leaves in a non-acclimated state will leak cellular fluids when they are chilled, whereas acclimated leaves are undamaged. These processes result from an interaction between genotype and the environment. Cascades of genes come into play during acclimation and dormancy. 

The genus Rhododendron offers a model for studies of these states. Some species originate from alpine environments, such as R. hirsutum coming from the European Alps and one of the first English garden 'rhodos'. By contrast, plants of R. vireya come from tropical areas such as the East Indies.        

Comparing the leakage of cellular fluids in acclimatised and non-acclimatised rhododendron leaves subjected to -7°C

Practical outcomes from studies of acclimation and dormancy are twofold. Firstly, are there substances that could be sprayed onto cold susceptible crops, eg potatoes or cauliflowers, that prevent damage? This is so-called 'anti-freeze chemistry'. Some studies suggest that spraying seaweed extracts will dimmish damage. The downside of this approach is that rain washes off the application. Secondly, identifying genes which increase cold hardiness offers possibilities for their transfer into susceptible crops. Gene-editing techniques may offer means of tweaking existing cold-hardiness genes in susceptible crops. 

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Professor Geoff Dixon is the author of Garden Practices and their Science, published by Routledge 2019.