Strategies to Cope with Temperature Fluctuations and Temperature Extremes

Seasonally varying temperature extremes can be avoided through various escape strategies. Annual plants can endure unfavourable conditions as dry seeds that are far less frost or heat sensitive. A new life cycle then starts with germination only after a cold winter or a hot, dry summer, depending on the habitat. Alternatives for perennial plants are the shedding of cold-sensitive organs such as the leaves or the overwintering only of the subterranean plant parts (rhizomes, bulbs). Tropical alpine plants have developed mechanisms of short-term freezing avoidance, as the daily periods of sub-zero temperatures are usually shorter than 12 h.

Besides such escape strategies, every plant needs to be able to adjust metabolic fluxes and membrane fluidity to fluctuating temperatures and to protect cellular structures during short periods of temperature extremes. In the absence of homeostatic temperature regulation, most of the mechanisms involved operate at the cellular level and protect the tissues directly exposed to an unfavourable temperature.

In the case of temperatures below the optimum, adjustments result in cold acclimation or cold hardening. Many plant species acquire frost tolerance by exposure to low yet non-freezing temperatures (frost hardening) (the temperature ranges depicted in Table 4.4 take the acclimation into account). Plant organs (e.g. evergreen leaves) become tolerant of freezing of a considerable portion of their tissue water. Such acclimation offers effective protection in most habitats because decreases in temperature are usually slower than the kinetics of cellular responses.

Table 4.4. Temperature tolerance of the leaves of vascular plants from various climate zones. (Modified from Larcher (2003))

Moreover, frost hardening is often initiated even before the onset of cold temperatures, owing to the ability to anticipate the coming winter on the basis of environmental cues such as decreasing day length (Beck et al. 2004). In addition, nondamaging cold can also induce or, as in the case of evergreen conifers, boost an ongoing coldhardening process. Since frost hardening is associated with a change or even a halt of metabolic activity (Nagler et al. 2015), acclimated plants must undergo a de-hardening process at the beginning of the warm season.

Acclimative changes in heat tolerance (= acquired thermotolerance), albeit not nearly as pronounced as cold hardening, are also well established. The underlying mechanisms are only partly specific to the stressor heat; rather, they provide general stress tolerance. This is important, as heat stress (a) occurs in a temperature range of high metabolic activity and (b) is almost always accompanied by or associated with other stresses such as drought, high light intensity and generation of ROS.