Intranet

Introduction and purpose

In temperate perennials cold hardiness is a seasonal process. In autumn plants cold acclimate, whereby they become increasingly tolerant to subzero temperatures. Maximum hardiness is reached midwinter, and in spring plants loose acclimated cold hardiness by deacclimation /1/. Due to increasing atmospheric CO2 concentrations the global mean temperature is projected to increase by 3-5°C over a 100 year period /2/. However, climate warming may actually increase the risk of plant frost damage. Although temperate winters have become a little milder, the temperature patterns have become very irregular with many warm spells, during which plants tend to loose cold hardiness, thereby increasing the risk of subsequent freezing injury /3/. Additionally, shifting phenological patterns such as an earlier start to the growing season and earlier flowering /4;5/, consistent with climate warming, may enhance the risk of frost injuries caused by increasing temperature variation. Therefore, understanding the mechanisms behind loss of plant cold hardiness is important to ensure the sustainability of food and plant sources under changing climatic conditions.
Evolutionary responses to climatic changes will take time, particularly in woody perennials due to their long generation time. The ability of woody perennials to cope with climatic stress will therefore, to a large extent, depend on physiological plasticity /6/. Regulation of development of cold hardiness in the autumn and the underlying physiological, biochemical and molecular responses have been extensively studied /7;8;9/. However, much less is known about the process of deacclimation, defined as a loss of acclimated cold hardiness, both in terms of regulation by environmental signals and particularly in terms of associated physiological and biochemical changes /10/.

Questions addressed

The project is addressing the following two main questions:

1. Which characteristics of deacclimation kinetics are important for woody perennials to resist premature deacclimation under increasing temperature instability?

Deacclimation is driven mainly by temperature, and since it is a relatively fast process (days-weeks), substantial increases in temperature can directly decrease hardiness within a few days, resulting in susceptibility to subsequent frost events /11/. Ideally, to reduce the risk of subsequent frost injuries, perennials should deacclimate slowly or late during unseasonably warm spells in winter and early spring. Recent studies indicate that the rate of deacclimation is not a linear response but may change as deacclimation progresses. Additionally, there may be a lag-phase during which exposure to warm temperatures does not result in deacclimation /12;13/. The kinetics (timing and rate) of deacclimation should therefore play an important role for the ability of woody perennials to resist premature deacclimation during unseasonable warm spells.

2. Which physiological and biochemical mechanisms characterise deacclimation in woody perennials and hence may serve as markers for the progress of deacclimation?

The physiological and biochemical analyses will focus on plant-water status, carbohydrate metabolism and qualitative and quantitative protein changes. Variability in water status and accumulation of soluble carbohydrates contribute to observed differences in cold hardiness among related genotypes during cold acclimation /14;15;16;17/. However, the importance of alterations in water status and carbohydrate metabolism in controlling the rate and degree of deacclimation is less clear. Little is known about qualitative and quantitative protein changes during deacclimation. Most extensively documented is seasonal changes in the expression of dehydrins, a group of cryoprotective proteins /15;18;19/. Identification of proteins that appear or disappear during deacclimation will provide new insights into strategies that protect the plant from cold temperatures or prepare the plant for future development /10/. Proteomics is a recent addition to the molecular tools used to analyse stress responses in plants, which provides a direct assessment of proteins involved in stress responses /20/. In addition to providing novel information about the biological functions in a cell during deacclimation, the use of proteomics may help to separate causal factors of deacclimation from correlative relationships /10/. Currently, almost all proteome studies are carried out on herbaceous model plants, but the recent publication of the poplar genome has increased the potential of proteomics in studies involving woody species by allowing more precise polypeptide identification /20;21/