Global warming could mean that plants are more likely to be damaged by frost. In order to prevent this, it is necessary to understand the mechanisms behind the frost tolerance of plants. Postdoc Majken Pagter from Aarhus University has recently completed a study on the cold hardiness of plants, which shows, among other things, that plants are more sensitive to temperature swings in late winter and early spring than in the middle of winter.

That global warming may lead to more frost damage may sound somewhat paradoxical, but there is an explanation. Climate change leads to more unstable winter temperatures where cold periods are interspersed with shorter warmer periods. This may disturb the plant’s normal rhythm.

- Even transitory temperature rises can lead to perennials losing some of their frost tolerance and being more vulnerable to subsequent frost damages. The consequences of global warming can thus increase the risk of frost damages in plants. To maintain a high plant production in an altered climate it is important to understand the mechanisms behind the loss of frost tolerance, explains postdoc Majken Pagter. 

Vulnerability highest in spring

In the autumn perennial plants prepare for the approaching cold period – they cold acclimate – which enables them to cope with freezing temperatures. They reach their maximum frost hardiness in the middle of winter.

In the spring the process is reversed. The plants wake up – they de-acclimate – and lose their tolerance to frost.  This process is driven mainly by temperature.

The climate changes can disturb the de-acclimation in two ways. As de-acclimation is a relatively rapid process that lasts only days or a few weeks, significant temperature rises can reduce the cold hardiness in only a few days. This has the consequence that vulnerable plant parts risk exposure to frost if the temperature again changes and goes below the freezing point. Secondly, the mild spring weather has a long-term effect on plant resistance to the cold by promoting bud burst.).

In an ideal world, perennial plants would de-acclimate slowly if the winter is unusually warm, if the spring is unusually early and/or if the winter and spring temperatures are unstable and fluctuate between cold and relatively warm periods. This would reduce their risk of frost damage. The prospects for developing plants that can tolerate fluctuating temperatures in critical periods are better if we have a better understanding of the mechanisms behind the de-acclimation. 

Majken Pagter examined the timing and rapidity of de-acclimation in perennial woody plants and some of the biochemical and physiological mechanisms involved. She used two species of hydrangea for her investigation to represent woody perennial plants.

The results show that in hydrangea the risk of frost damage is not linked to the timing and rate of de-acclimation. The fact that one of the species of hydrangea is more vulnerable to frost damage in the spring is because the other species has a higher maximum frost hardiness.

The experiment also showed that the timing and rate of de-acclimation changes during the winter. The plants are thus more sensitive to unstable temperatures at the end of the winter and in early spring than in the middle of winter.

From a physiological approach, de-acclimation is due to changes in the content and composition of carbohydrates and proteins in the plant and an increasing water content. Changes to the carbohydrates are driven by changes in temperature. The protein changes are driven by changes in stress proteins that help the plant survive stress factors such as frost, snow, water deficits and a combination of low temperature and bright sunshine which you can get on clear, frosty days.

- We have improved our understanding of how climate change in the winter half-year affects the frost tolerance of plants and overwintering. A greater insight enables us to develop strategies to reduce the risks caused by climate change, says Majken Pagter. 

Further information:  Postdoc Majken Pagter,  majken.pagter@agrsci.dk, telephone: +45 8715 8325

Tekst: Janne Hansen, e-mail: janne.hansen@agrsci.dk