Comparative study of direct and indirect evaluations of frost tolerance in barley

The aim of this paper was to compare different field, laboratory and physiological methods for the evaluation of frost tolerance in barley; as well as to show both the possibilities and limitations of these individual methods and approaches. The tolerances of 39 barley cultivars and breeding lines were evaluated by four direct methods (based on the exposure of plants to frost) and three indirect (based on diagnostic traits and markers). The direct methods included the evaluation of: (1) field survival after five winters 1999–2004; (2) winter survival in a provocation pot test under natural conditions; (3) lethal temperature (LT50) of plants taken from a field in winter; and (4) LT50 of plants grown and hardened in a growth chamber. The indirect methods were based upon: (1) endogenous levels of abscisic acid (ABA); (2) water content (WC); (3) osmotic potential (OP) of the plants cultivated and hardened in growth chambers. All four direct methods correlated well with one another, and resulted in similar sequences of frost tolerance of the tested barley cultivars and lines. While it was only possible to distinguish individual barley cultivars based on field survival after the 2002/2003 winter, by using the provocation method it was possible to distinguish between barley cultivars after all of the winters. Moreover, the average winter survival of barley from multi-year pot tests was in correlation with the minimal LT50, evaluated in plants hardened in both the field or in growth chambers. From indirect indicators, the levels of ABA increased slightly, while WC and OP clearly decreased in all cultivars during cold hardening. No correlation was observed between LT50 and the ABA content. A significant correlation between LT50 and WC, and between LT50 and OP was found, but only after 1 or more weeks of cold acclimation of the plants had been completed, since both the duration and the rate of decline of these parameters differed in individual cultivars during the cold hardening. The LT50-values of barley cultivars not only accurately characterized their field tolerance to frost, but also were indicative of changes associated with the induction of frost tolerance in plants under the given conditions, when comparing the direct and indirect methods.