Involvement of Abscisic Acid and Proline in Cold Acclimation of Winter Wheat

Abstract

During cold hardening of several winter wheat genotypes under growth chamber and field conditions transient increases of the abscisic acid (ABA) contents occur, which are positively correlated with their frost tolerance. Moreover, longlasting increases in the content of free proline are observed which are also correlated with frost tolerance.

In order to get more information about a possible role of ABA in the process of cold hardening, the ABA biosynthesis inhibitor norflurazone was applied at the beginning of cold hardening to seedlings of two winter wheat varieties with different frost tolerance. After this treatment the transient ABA peak disappeared and the degree of frost tolerance was reduced. The extent of this reduction was variety-specific. Moreover, application of norflurazone prevented the formation of a set of high molecular weight proteins, which are formed in response to low temperature treatment. These results provide evidence that ABA is involved in the variety-specific development of frost tolerance.

A completely different approach was used to investigate a possible function of proline in freezing tolerance. Proline overproducing mutants were selected by means of in viro-technique with embryogenic calli of a winter wheat variety. The proline analog hydroxyproline (Hyp) was used as a selection agent. It was found that plants which were regenerated from Hyp-resistant calli possessed significantly higher proline levels and significantly higher frost tolerance. These traits proved to be heritable. In F₃ plants the expected correlation between increased proline content and increased frost tolerance was confirmed. These results are regarded as evidence for a close relationship between the two parameters. Moreover, they open the possibility of improving frost tolerance by in vitro-selection technique.