
intracellular freezing point (Charrier et al., 2013a), plasma
membrane composition (Uemura et al., 2006), and cell wall
thickness (Takahashi et al., 2021).
Depending on the species, cold hardiness per se is not easily
measurable and is sometimes based on the consequences of low
temperature stress after the winter period. This is particularly true
in herbaceous species (e.g. winter cereals and other annual crops),
exhibiting cold damage as a disturbance (sensu Grime, 1973), that is
partial or total loss of biomass. In these species, a good cold
hardiness index thus relates to the survival (or the lack of recovery)
of the whole individual, the destruction of plant tissues, and the
resulting yield (generally referred to as ‘winter kill’).
In woody perennial species, cold hardiness can be measured
regularly during the winter period through different techniques
such as lethal temperature for 50% of the individuals, the
temperature of intracellular exotherm by thermal analysis (LTE),
or progressive damage by relative electrolyte leakage, or visual
scoring. Depending on the measured phenotype, the cold hardiness
index can constitute a threshold that, once exceeded, marks the
development of damage and jeopardizes survival (survival, biomass
loss, LTE). A more quantitative cold hardiness index, as measured
by the electrolyte leakage or visual scoring methods, may allow a
more gradual assessment of damage intensity, therefore predicting
cold damage with a thinner resolution, still missing the link with
survival.
One peculiar case is freezing avoidance, that is the ability to resist
cold temperatures by maintaining supercooled water. The process
of ice nucleation depends on the plant tissue and developmental
stage, as well as the nature of the nucleus (e.g. ice-nucleation-active
bacteria), other biophysical conditions in plants (e.g. solute
concentration, wettability, cell wall properties, and physical
barriers), and atmospheric conditions (Kirchhof et al., 2025).
However, current cold hardiness models consider ice nucleation to
be a deterministic process that occurs at a fixed temperature (e.g. 0,
2, or 4°C), whereas experimental evidence has shown that its
stochasticity depends on both biotic and abiotic factors (Kirchhof
et al., 2025).
Although plant mortality is complex and difficult to measure (see
e.g. Leopold, 1978; Filip et al., 2007; Anderegg et al., 2012, for
drought stress), the ability of meristematic cells, particularly those
in the shoot apical meristem, to divide and form new vegetative
organs is crucial for predicting resilience to stressful events
(Thomas, 2013). Current methods cannot easily determine the
sequence of damage leading to plant mortality; they only allow
measurements after the stressful event has occurred. Nondestruc-
tive monitoring techniques, such as micro-dendrometers or
acoustic emission analysis (Charrier et al., 2017,2021; Lamacque
et al., 2022), are therefore needed to assess low-temperature
damage. This would allow a continuous assessment of damages,
required to elucidate the dual role of freeze intensity and duration as
simulated by the death time model (Faber et al., 2024).
As the mechanisms driving cold hardiness continue to be
elucidated, the effects of variables continue to be quantified
through experimental evidence or modeling efforts. Depending on
the biology of the species and the purpose of the model, cold
hardiness can be modeled at a single point in time (static models,
e.g. Charrier et al., 2013a), dynamically over a short period (less
than a year; Cannell et al., 1985), or over multiple years
(Leinonen, 1996). In perennial species, simulations may require
reinitialization at fixed dates (Cannell et al., 1985; Timmis
et al., 1994). In winter cereals and crops, the dynamics typically
begin at the sowing date and continue until harvest (Bergjord
et al., 2008; Byrns et al., 2020).
Predicting cold hardiness
In static models, cold hardiness is predicted using variables
measured on the same date (Charrier et al., 2013a; Jones
et al., 2024), a few days later (Proebsting, 1963; Andrews &
Proebsting, 1986), or up to several months earlier (Anisko
et al., 1994; Poirier et al., 2010; Rapacz et al., 2022). In multiple
regression analysis, a subset of variables is predefined and measured,
and the most informative index is selected through a calibration
process that requires it to significantly improve prediction accuracy
(stepwise procedure; Anisko et al., 1994). Over the past decade,
partial least squares regressions and machine learning algorithms
have significantly increased the number of variables included in
analyses, even when there is no prior knowledge of their effect. For
example, the NYUS.2 model uses 117 features in its training
process, including cultivar features and hourly temperature-based
features such as daily and cumulative temperature descriptors, as
well as exponential and reverse exponential weighted moving
averages (Wang et al., 2025). While these models generally lead to
highly accurate predictions (RMSE lower than 2°C, and even 1°C
in some cases), they lack realism, which can restrict their validity to
the climate range in which they were developed. Empirical models
also usually lack reversibility, that is the ability to allow
deacclimation and reacclimation throughout the modeled period;
this is only achieved through the fluctuation of driving variables.
Based on a large series of experimental measurements and field
observations, Repo et al.(1990) developed a simple, cold hardiness-
based, dynamic model that integrates reversibility through the
concept of stationary cold hardiness in order to predict realistic cold
hardiness mechanisms (Kalberer et al., 2006). In order to enable
reversibility in simulations, this model uses stationary cold
hardiness and the rate of change in cold hardiness. The prevailing
air temperature determines stationary cold hardiness, and the rate
of change is proportional to the difference between prevailing and
stationary cold hardiness. Based on these assumptions: (1) under a
constant air temperature, cold hardiness attains the stationary level
determined by that temperature; and (2) under fluctuating
temperatures, cold hardiness follows changes in air temperature
(which determine the stationary level). To mitigate the impact of
rapid air temperature fluctuations on cold hardiness, a time
constant (s) governs the rate of change. The higher the value of s,
the slower the predicted changes in cold hardiness. The first-order
model predicts a rapid change that slows for small differences
between the prevailing and stationary cold hardiness levels (Repo
et al., 1990; Leinonen, 1996). Leinonen et al.(1995) introduced a
second-order model using two time constants corresponding to two
stationary levels of cold hardiness to address the exceptionally
complex air temperature response of cold hardiness in Douglas-fir
Ó2026 The Author(s).
New Phytologist Ó2026 New Phytologist Foundation.
New Phytologist (2026) 249: 2668–2682
www.newphytologist.com
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