Abstract

Abstract The value of frost tolerance in wheat is increased with decreasing the temperature in late autumn and/or early winter (phase I, acclimation), then shows plateau state for a period with the coldest temperature (II), finally appears to decrease with warming the temperature (III, de-acclimation). This study was aimed to determine the threshold frost hardiness in wheat for avoiding the winter-kill events, and estimating the probability of occurrence of winter-killing across the each phase in five locations of Iran. The model of wheat acclimation/de-acclimation to temperature was recoded in QBASIC programming and run for long-term (39-44 years) weather data, the nonlinear functions were used for describing the changes in frost tolerance across the late autumn to early spring (say, for distinguishing the three phases). Results indicated that the threshold frost hardiness for acceptable [probability (P) = 95%] over-wintering survival is -10 o C for Isfahan, -8.5 o C for Shiraz, -16 o C for Kermanshah, -18.5 o C for Tabriz, and -14.5 o C for Mashhad, for fully (P=100%) avoiding the winterkill events, the wheat should be contained frost hardiness -12, -8.5, -16.5, -21, and -16.5 o C for named locations, respectively. The probability of occurrence of named events over phase III for wheat contained frost hardiness lower than the threshold value (P=10% for experiencing the death) was 3% in Tabriz, but zero in other locations, the order of locations for magnitude of this probability over phase II tended to be as Kermanshah (79%)> Isfahan (73%)> Tabriz (56%)> Mashhad (50%)> Shiraz (40%), the rest probabilities appeared to devote to the phase I.